Chapter 2 Y2024 Electromagnetic Induction (PDF)

CHAPTER TWO ELECTROMAGNETIC INDUCTION 1|P a g e (1) ELECTRIC, MAGNETIC AND LORENTZ FORCES Q) What happens when a charged particle is thrown into an electric field? Why? If a positively charged particle (+q) moves in a perpendicular direction to the lines of a unifo...

CHAPTER TWO ELECTROMAGNETIC INDUCTION 1|P a g e (1) ELECTRIC, MAGNETIC AND LORENTZ FORCES Q) What happens when a charged particle is thrown into an electric field? Why? If a positively charged particle (+q) moves in a perpendicular direction to the lines of a uniform electric field (E), this particle will be affected by electric force (FE) parallel to lines of electric field, which illustrates electric force which given by the following relation: 𝑭 𝑬 = 𝒒𝑬 Where FE: electric force. E: electric field. q: charge. Q) What happens when a charged particle is thrown into a uniform magnetic field and in a perpendicular direction to it? Why? If the particle moves with a velocity (𝜈⃑) perpendicularly on the lines of a uniform magnetic field, with a flux density (B). It will be affected by a magnetic force (FB) is perpendicular on that flux, the particle will deviate from its original path, it will follow a circular path because the magnetic force affects perpendicularly on the velocity vector (𝜈⃑). The vector formula for magnetic force is given by the following relation: Where FB: magnetic force. v: particle velocity. q: charge. B: magnetic flux density. 2|P a g e  Q) Write the vector formula for the magnetic force, and the mathematical formula for calculating the amount of the magnetic force. Directional (vector) formula for magnetic force: Mathematical formula for calculating the amount of magnetic force: Q) How to determine the direction of the magnetic force affecting a charge entering a uniform magnetic field? By “the right hand rule”, where the right-hand fingers rotate in the direction of velocity (V) towards the magnetic field (B), so, the thumb indicates the direction of the force (FB) in the case that the charge is positive. 3|P a g e  Q) When the magnetic force is: 1) Equal to the maximum. 2) Equal to zero. 1) The maximum value is if the velocity vector (V) is perpendicular to the flux vector (B) and the angle is: 𝜽 = 𝟗𝟎° ∴ 𝑭𝑩 = 𝒒𝒗𝑩 𝒔𝒊𝒏 𝟗𝟎 𝑭𝑩 = 𝒒𝒗𝑩 (𝟏) 𝑭𝑩 = 𝒒𝒗𝑩 2) The zero value is if the velocity vector (V) is parallel to the flux vector (B) and the angle is: 𝜽 = 𝟎 ∴ 𝑭𝑩 = 𝒒𝒗𝑩 𝒔𝒊𝒏 𝟎 𝑭𝑩 = 𝒒𝒗𝑩 (𝟎) 𝑭𝑩 = 𝟎 Q) What is the Lorentz force? In what field it may use? It is the resultant of the electric and magnetic forces acting on an electric charge entering perpendicularly in the two fields: Lorentz force is used in practical applications, like Cathode –ray tube that controls the path electronic band on the monitor, Ministerial Exams: Q.1) What is happening and why? If a positively charged particle (+q) moves in a perpendicular direction to the lines of a uniform magnetic field with a flux density (B)? 4|P a g e Q.2) What is happening and why? If a positively charged particle (+q), when moves with a velocity of (V) in a perpendicular direction to the lines of an electric field? Q.3) Explain how you can practically know whether a magnetic field or an electric field is present in a certain space? Q.4) What is the Lorentz force? In what field it may use? (2) ELECTROMAGNETIC INDUCTION Q) Who is the scientist who is considered the first to create the relationship between electric and magnetic? What is his discovery? The Scientist Oersted He discovered that the electric current flowing into a conductor generates a magnetic field, and the amount of the magnetic field depends on the amount of electric current. Q) Explain, how does an electric current generate in the coil to which the ammeter is connected in the following cases? 1- If the magnetic rod is at rest relative to the coil. 2- If we hold the magnetic rod by hand and direct its north pole to one sides of the coil 3- If the magnetic rod removed from the hollow of the coil and its north pole facing it. 1) When the magnetic rod is at rest relative to the coil, the reading of the ammeter is zero. Because the magnetic flux (Ø) , which penetrates the coil, does not change through time. Moreover, there is no relative motion between the magnet and the coil. Therefore, no current pass through the circuit. 2) when we hold the magnetic rod by hand and direct its north pole to one sides of the coil, push it towards the coil and a parallel to coil axis, we find the ammeter indicates deflection of current in the circuit in a certain direction. The reason for this is increase in magnetic flux (Ø), which penetrates the coil when the magnet approaches to the coil. 3) When the north pole of the magnet moves away, facing the core of the coil, and parallel to its axis, the ammeter will indicate a current flowing in a direction opposite to the approach. This is due to a decrease in the amount of magnetic flux (Ø) penetrating the coil. 5|P a g e  Q) What does the induced current flowing into the coil depends on? Q) What are the factors affecting the induced current? 1) Velocity of relative motion between the magnet pole and the coil. 2) Number of turns for the coil. 3) The amount of magnetic flux penetrates the coil. 4) Magnetic Permeability of Core coil Material. Q) What is Faraday's conclusion? Or What is the phenomenon of electromagnetic induction? An induced current is generated in a closed circuit (like a wire coil or conductive ring) only ∆∅ when there is change in the magnetic flux which penetrates that circuit per unit time ( ). ∆$ Q) Why did the attempts that preceded Faraday's discovery failed to generate an electric current with a magnetic field? This is because all attempts relied on static magnetic fields only. Q) What is the requirement to have an induced current? That the circuit be closed... and that a change occurs in the magnetic flux that penetrates the circuit in the time unit. Q) What is the condition for obtaining an induced electro-motive force? the change in the magnetic flux that penetrates the coil to the unit time Q) What is the condition for obtaining an induced electric motive force and an induced current? That the circuit be closed... and that a change occurs in the magnetic flux that penetrates the circuit in the time unit. 6|P a g e  Q) Explain an experiment illustrating the discovery and deduction of the scientist Faraday? two coils of two wires wrapped around a closed ring of wrought iron. One of the coils is connected in series combination to a battery and a switch the circuit on the left side, which is called the primary coil circuit, while the other coil is connected to a device that detects small currents. It has zero point in the middle (circuit on the right side) this one is called the secondary coil circuit. Faraday observed deflection of the pointer, connected to the secondary coil, to one of the sides at the switch connected to the primary coil is turned off, then it goes back to zero, coil circuit, this current is called "induced current", although there is no battery or voltage source in this circuit. As for the pointer goes back to zero after closing the switch, it was because the current flow in the primary coil circuit was constant. Hence, there was no change in the magnetic flux, which penetrated the ∆∅ secondary coil per unit time ( ) ∆$ Faraday also noticed that another deflection in the pointer of the gauge again, at the moment when the switch is opened, but this time the deflection was opposite to zero, then it went back to zero. Faraday’s observation was this effect (current flow in the secondary circuit) accrued only during the two stages of growth and decay of current in the primary coil circuit. Since both growth and decay of current in the primary coil circuit would cause increase and decrease in the magnetic flux, that penetrates the core of iron that wrapped in the two coils. 7|P a g e Therefore, Faraday pays attention to the necessity of the availability of the basic factor to generate induced current in a closed circuit, the change in the magnetic flux that penetrates the coil to the unit time. According to this, Faraday concluded: An induced current is generated in a closed circuit (like a wire coil or conductive ring) only ∆∅ when there is change in the magnetic flux which penetrates that circuit per unit time ( ) ∆$ Q) Explain an activity (experiment) that demonstrates the electromagnetic induction phenomenon? Activity Tools: Two hollow coils of different diameters (one can be inserted in the other). Galvanometer with zero reading in the middle Magnetic rod. wires. Battery. Electric switch. Activity Steps: (First): One end of the coils is connected to the galvanometer. Put north end of the magnetic rod faces the coil during in activity for the coil. We will find that galvanometer pointer will remain fixed at zero scale. There is no flow of current in the coil circuit. The magnetic rod is pushed towards the coil, then moved away. We find that the galvanometer pointer deflects to one side of zero scale (when the rod is close) and the pointer moves to the opposite direction (when is moved away). This indicates induced current flow in the coil, in both cases. (Second): Connect the two ends of other coil (primary coil) are connected to a battery terminal by wires to create electromagnet. The primary coil connected to the battery is moved in front of the secondary coil which connected to the galvanometer, brought it closer and then moved away from it which is parallel to its axis. What do you notice? 8|P a g e  We note that the pointer of galvanometer is deflected on one side about zero scale and in opposite direction again. This indicates an induced current flow in the secondary coil circuit and then it goes back to zero when there is no relative motion between the two coils. (Third): Connect an electrical switch to the primary coil circuit and make it open. Insert the primary coil is in the secondary coil; the ratio of both is maintained. Will the galvanometer pointer deflect? Close and open the switch in the primary coil circuit. What do you notice? We find the galvanometer pointer deflects by moving on both sides of the zero reading in opposite directions only once the switch is turned on and off in the primary coil circuit respectively. It indicates flow of induced current in the secondary coil circuit in those two ways Conclusion: An electromotive force is induced (ɛind) and an induced current (Iind) flows in a closed circuit (conductive ring or a coil) only when there is change in the magnetic flux that penetrates that circuit per unit time, (although there is no battery in that circuit). The polarity of the induced emf (ɛind) and direction of the induced current (Iind) in the electric circuit, have a certain direction when there is increase in the magnetic flux that penetrates it, and they have opposite direction when the flux decreases. Ministerial Exams: Q) Explain an activity (experiment) that demonstrates the electromagnetic induction phenomenon? (3) MOTIONAL EΜF (𝜺𝒎𝒐𝒕𝒊𝒐𝒏𝒂𝒍 ) Q) What is the motional electromotive force? It is an electric potential difference is generated between ends of the rod by moving a conductor rod in a uniform magnetic field. It is considered as a special case of electromagnetic induction 9|P a g e  Q) Derive the mathematical formula for the motional electromotive force? 𝑭𝑬 = 𝑭𝑩𝟏 𝒒𝑬 = 𝒒𝒗𝑩 𝑬 = 𝒗𝑩 ∆𝑽 ∵𝑬= 𝒍 ∆𝑽 ∴ = 𝒗𝑩 𝒍 ∆𝑽 = 𝒗𝑩𝒍 ∴ 𝜺𝒎𝒐𝒕 = 𝒗𝑩𝒍 Where Emot: motional electromotive force. v: rod velocity. l: rod length. B: flux density. Q) What are the factors, the motional electromotive force depends on? 1) velocity of the rod. (v). 2) density of magnetic flux (B). 3) length of the rod (l). 4) the situation of the rod with respect to the magnetic flux. (The angle between the velocity vector and the magnetic flux density vector). 𝜺𝒎𝒐𝒕 = 𝒗𝑩𝒍 𝒔𝒊𝒏𝜽 10 | P a g e  Q) When the conductor rod moves inside a magnetic flux, the positive charges collect at one end of the rod and the negative charges at the other end, generating (𝜺𝒎𝒐𝒕 ), give a reasons for this? Inducing emf by moving a conductor rod inside a uniform magnetic field, As a result of, the movement of the conductor rod inside the magnetic field, the positive charges of the rod are affected by a magnetic force 𝑭𝑩𝟏 = 𝒒𝒗𝑩 𝒔𝒊𝒏𝜽 When the rod is moved perpendicularly against the magnetic flux, this force is expressed as follows: 𝑭𝑩𝟏 = 𝒒𝒗𝑩 According to the “right-hand rule”, this force works to separate the positive charges on one side and the negative charges on the other side. Charges continue to collect at both ends of the rod, and an electric potential difference called (motional electromotive force) is generated. If the direction of the moving rod is reversed or the magnetic field is reversed, will the polarity of the motional electromotive force ( 𝜺 motional) reverse? Yes... the polarity of the motional electromotive force is reversed. Q) What is the origin of the force impeding to the movement of the conductor rod in a magnetic field? Due to the flow of an induced current in the rod in a perpendicular direction to the magnetic flux, a magnetic force appears affecting this rod, given by the following equation: 𝑭𝑩𝟐 = 𝑰𝑩𝒍 By applying the right-hand rule, we find that the force acts in a perpendicular direction to the rod and is opposite to the direction of the velocity (v) with which the rod moves. And it moves toward the left side. Therefore, this force obstructs the movement of the rod. 11 | P a g e  Q) Is there an induced current flow in the circuit figure? If yes, determine the direction of induced current in it. The induced current does not flow because the velocity direction is parallel to the direction of the flux density and then the angle between (B & v) are equals zero (θ = 0) and from the equation: 𝑭𝑩 = 𝒒𝒗𝑩 𝒔𝒊𝒏 𝜽 𝜽 = 𝟎 𝑭𝑩 = 𝒒𝒗𝑩 𝒔𝒊𝒏 𝟎 𝑭𝑩 = 𝒒𝒗𝑩 (𝟎) 𝑭𝑩 = 𝟎 Q) Mathematically prove that electromagnetic induction obeys the law of conservation of energy. 12 | P a g e  Q) Derive the mathematical equation for the external pull force (Fpull) affecting a conductor in which an induced current pass, moves in a uniform magnetic field and is perpendicular to it. Ministerial Exams: Q.1) What is the motional electromotive force acting on both ends of a conductor rod (which moving according to a uniform magnetic field) depends on? Q.2) What does the kinetic electromotive force acting on both ends of a conducting rod is moving relative to a uniform magnetic field, depends on? (4) MAGNETIC FLUX AND FARADAY'S LAW Q) What are the methods for obtaining a change in the magnetic flux (∅) when there is a relative movement between the magnet and the coil? According to the equation: ∅ = 𝑨𝑩 𝒄𝒐𝒔𝜽 1) Change in angle measurement (θ) between area vector (A) and magnetic flux density vector ( B ). is changed by rotating the ring or coil inside a uniform magnetic field. 13 | P a g e 2) Changing area of the ring facing the uniform magnetic flux (Φ). This is done by pressing the ring or pulling it from opposite sides, thus area (A) reduces. 3) Moving the conductive ring in a plane perpendicular to a uniform magnetic flux: (Pushing the ring to be inserted into a uniform magnetic field or pulling it out of the field). 14 | P a g e  Q) Write the equation for the magnetic flux, and explain when it be with the maximum amount and when it be with the minimum amount? ∅ = 𝑨𝑩 𝒄𝒐𝒔𝜽 Where: (∅): is the angle between a vector (B) and a vector (A). (B): The density of the magnetic field. (A): The area of the ring. The flux be with the maximum amount when the (B) vector be in parallel with (A) vector, as (θ = 0), that: ∅ = 𝑨𝑩 𝒄𝒐𝒔𝟎 = 𝑨𝑩 The flux be with the minimum amount when the (B) vector be in perpendicular direction to the (A) vector, as (θ = 90), that: ∅ = 𝑨𝑩 𝒄𝒐𝒔𝟗𝟎 = 𝟎 Q) What is the measure unit of: 1) Magnetic flux? 2) The time rate of change of the magnetic flux? 3) Magnetic flux density? 1) The magnetic flux (∅) is measured in units of Weber (web). ∆∅ %& 2) The time rate of change of the magnetic flux ( ) is measured in units ( ). ∆$ '&( %& 3) The magnetic field density (B) is measured in units of Tesla (T) and Tesla is equal to ( ! ). ) 𝒘𝒃 Q) Prove that Tesla is equal to ( )? 𝒎𝟐 ∅ = 𝑨𝑩 𝒄𝒐𝒔𝜽 ∴ 𝒘𝒃 = 𝒎𝟐 ∙ 𝑻 𝒘𝒃 𝑻= 𝒎𝟐 15 | P a g e  Q) What is the text of Faraday's law? With mention of the mathematical equation? Amount of induced electro motive force (𝜀-./ ) in a conductive ring is directly proportional to the time rate of change in the magnetic flux that penetrates the ring Faraday's law mathematical equation: ∆∅ 𝜺𝒊𝒏𝒅 = −𝑵 ∆𝒕 Q) What is the (𝜺𝒊𝒏𝒅 ) polarity depends on? It depends on the magnetic flux whether it is increasing or decreasing. Q) What causes an electrical current to flow in a closed circuit? The presence of a source of electric motive force (𝑉011 ) such as a battery or generator. Q) What causes an induced current to flow in a closed circuit? The presence of an induced electromotive force (𝜀234 ), which is generated by a change in magnetic flux in the unit of time. Ministerial Exams: Q.1) What is required in a closed circuit to generate: a) An electric current. b) An induced current. Q.2) What are the physical quantities that are measured in the unit of (𝐰𝐞𝐛𝐞𝐫/𝐦𝟐 )? 16 | P a g e (5) LENZ'S LAW Q) What is the text of Lenz's law? The induced current in a closed electric circuit has an opposite direction (in its magnetic field) to the change in magnetic flux that causes it Q) What is the practical benefit of Lenz's law? a) Lenz’s law is useful for determining the direction of the induced current in a closed electric circuit. b) Lenz’s law also an application of conservation law of energy. Q) How does Lenz's law represent an application of energy conservation law? Or ….. Is Lenz’s Law subject to the energy conservation law? and why? Yes, and the reason: Because in both cases (approaching the magnet or moving it away from the ring) it takes a mechanical work, this work is transformed into another kind of energy in load (when the ring is connected to a load), this is an application conservation law of energy. Q) How can an induced current generate in an induced magnetic field that is opposite by its effect to the cause? The answer is a magnetic rod is moved near the front of a closed conductive ring and parallel to its perpendicular axis on both sides and passes from its center. If the north pole of the rod is facing the ring. 1- When the north pole approaches from the face of the ring, it causes increase in ∆∅ the magnetic flux penetrates the ring ( ∆# > 0 ).The direction of the effective ∆∅ magnetic flux (B) downward and it increases by ( ∆# > 0). Therefore, the direction of the induced current is counterclockwise (according to right-hand rule). It generates an induced magnetic field with density (B ind) its direction upward, so it opposite the affecting magnetic flux itself, in order to resist the increase in the magnetic flux that generated the induced current. Simply, a north pole (N) is created at the front of the ring facing the north pole of the rod, so that it repulses the approaching north pole (N) (according to Lenz law). 17 | P a g e When the North Pole is moved away from the face of the ring there will be decrease in the magnetic flux that penetrate the ring. Direction of the affecting magnetic flux density ∆∅ (B) is downward and decreasing by ( ∆# < 0) The direction of the induced current is clockwise (according to right- hand rule). It generates an induced magnetic field with density (Bind) downward, so it will be in the direction of the affecting magnetic flux itself (B), in order to resist decrease in magnetic flux which generated the induced current. This means, a south pole (S) is created at the face of the ring to attract the north pole (N) which moves away from it. (According to Lenz law) You might wonder, Q) What is the practical advantage of Lenz Law? Lenz law helps us identify direction of the induced current in a closed electric circuit. It is also an application of energy conservation law. Because in both cases (approaching the magnet or moving it away from the ring) it takes a mechanical work, this work is transformed into another kind of energy in convection (when the ring is attached to a convection), this is an application of energy conservation law. Q) When a magnetic rod falls freely downward while it is in an upright position, and below it is a copper ring that is locked and fixed horizontally (by neglecting the influence of air). 1) Does this rod fall with an acceleration equal to the Earth's acceleration? Or larger than it, or smaller than it? 2) Determine the direction of the magnetic force that the ring is acting on the rod as the rod approaches the ring. 1) The rod falls with a smaller acceleration than the acceleration of the Earth's gravity, due to the generation of a repulsive force that resists the increase in the magnetic flux generated by the induced current. 2) The direction of the magnetic force is upward. 18 | P a g e  Q) When the area of one turn in a coil is (A) and the coil rotates with an angular velocity of (ω) within a uniform magnetic field, its flux density is (B). The magnetic flux that penetrates the single turn is given in the form of a cosine function [∅𝑩 = 𝑨𝑩 𝒄𝒐𝒔(𝝎𝒕)] while the electromotive force induced on both ends of this coil is given as a sinusoidal function [𝜺𝒊𝒏𝒅 = 𝑵𝑩𝑨𝝎 𝒔𝒊𝒏(𝝎𝒕)]. Explain this in a mathematical way. Answer: Q) In an alternating current generator (ac). What is the equation for the electromotive force (𝜺𝒊𝒏𝒅 )? and when it reaches: 1) The greatest value. 2) The smallest value (zero). Answer: 19 | P a g e  Q) What does the peak inductive voltage of a coil rotate in a uniform magnetic field at a uniform angular velocity depend? 1) Number of turns of the coil (N). 2) Magnetic field density (B). 3) Coil area (A). 4) Angular velocity (W). and according to the equation of: 𝜺𝒎𝒂𝒙 = 𝑵 𝑨𝑩 𝝎 Q) What does the peak inductive current of a coil rotate in a uniform magnetic field at a uniform angular velocity depend? 1) Number of turns of the coil (N). 2) Magnetic field density (B). 3) Coil area (A). 4) Angular velocity (W). 5) Coil wire resistance (R). and according to the equation of: 𝜺𝒎𝒂𝒙 𝑵 𝑨𝑩 𝝎 𝑰𝒎𝒂𝒙 = = 𝑹 𝑹 Ministerial Exams: Q.1) What is the practical benefit of applying Lenz's law? Q.2) Why Lenz's law is an application of the energy conservation law? Q.3) What is the practical benefit of applying Lenz's law? and how is the law considered as an application of energy conservation law? Q.4) What is meant by Lenz’s law and what is the practical benefit of applying it? Q.5) What is the peak voltage (maximum voltage) generated at both ends of a coil rotating at a uniform angular velocity within a uniform magnetic field depends on? 20 | P a g e (6) EDDY CURRENTS Q) What are eddy currents? How does it arise? What are its disadvantages? How can its damage be minimized? Eddy currents are induced currents that rotate in concentric circles in a plane perpendicular to the lines of magnetic flux. It arises when a change occurs in the magnetic flux in the unit of time according to the law of electromagnetic induction. Eddy current damages cause energy loss in the form of heat in devices or in the steel core of windings according to Joule's law. The damages of eddy currents can be minimized by making the core of the coil in the form of sheets of wrought iron that are arranged parallel to the magnetic flux and are insulated and well pressed, so that the resistance increases, and the eddy currents decrease. Q) What happen? Why? If you pulled a plate of copper horizontally between the poles of an electromagnet, the density of the flux is uniform. Eddy currents are generated on the surface of the plate due to the relative motion between the copper plate and the magnetic flux. Q) How to reduce effect of eddy currents in conductors: Activity tools Two identical pendulums each in the form of plate made of a conductive material with weak magnetization (not ferromagnetic for example aluminum), connected to the end of light rod made of the same material. One of the plates is sliced and the slices are isolated like the teeth of a comb, the other plate (is not sliced), strong magnet (high flux density), holder. Activity steps Displace the two plates with equal displacement to one side of their stabilization site. Both plates are left simultaneously to swing freely between the poles of the magnet. the (unsliced) plate pendulum stops 21 | P a g e  when the gap comes between the magnetic poles, while the sliced plate goes between the magnetic poles and crosses to the other side and keeps swinging back and forth but with slow deceleration. Conclusion Huge eddy currents are generated in the unsliced plate when goes into the magnetic field between the poles, in a specific direction, due to an increase in magnetic flux that penetrates to the unit of time (according to Faraday’s law). Yet, they have an opposite direction when they go out of the field, due to decrease in the magnetic flux, In both cases, a magnetic force is generated which hinders the movement of the plate (according to Lenz’s Law). In conclusion, the swinging amplitude of the plate and finally stops. While the eddy currents in the sliced plate are very small, therefore, they have little effect on the plate. Q) What about the vibration energy in the unsliced plate inside a magnetic field, when it stops vibrating? The mechanical energy of the plate is converted into thermal energy by generating eddy currents. 22 | P a g e  Q) What are the benefits of eddy currents? Or Q) Where are eddy currents invested? 1) Eddy currents are also utilized in metal detectors (currently used in checkpoints and airports) 2) Eddy currents are utilized in modern trains’ brakes. 3) Metal detectors are also used to control traffic lights on some roads. Q) Explain how eddy currents are used in the brakes of some modern trains. Eddy currents are utilized in modern trains’ brakes. Wire coils (acting as an electric magnets) are applied facing the railway rods, In the uniform movement, no electric current flows in these coils, to stop from moving the train, the electrical circuit of these coils is closed, electric current flows in these coils; this current generates a strong magnetic field that pass through the railway rods. Because of the relative movement between the magnetic field and the railway rods, eddy currents are generated in them. According to Lenz law, these currents generate a magnetic field that hinders that movement, so the train stops. Q) Explain how eddy currents are used to detect metals. Eddy currents are also utilized in metal detectors (currently used in checkpoints and airports). The concept of metal detectors depends on electromagnetic induction, which is sometimes called pulse induction. A metal detector has two wire coils, one is a transmitter, and the other is a receiver, An alternating potential difference is forced on the transmitter coil, an alternating current flow in the coil, which, in turn generates an alternating magnetic flux, this time-variable flux induces a current the receiver coil. This current is measured initially in the case where there is no material between the two coils except air. When a conductor material comes in between the emitter and receiver coils (not required to be a plate), eddy currents will generated in that metal object. Induced eddy currents in that object will hinder change in the magnetic flux in the emitter coil, this causes decrease in the initial current measured in the receiver when there is air between them, hence, metal objects can be detected in bags, purses and clothes. Ministerial Exams: Q.1) Mention some of the areas in which eddy currents are invested, explaining one of them. Q.2) How to reduce the amount of energy dissipated by eddy currents in an iron core of coils? 23 | P a g e Q.3) What happen? Why? If you pulled a plate of copper horizontally between the poles of an electromagnet, the density of the flux is uniform. Q.4) Explain how eddy currents are investing in the brakes of some modern trains. Q.5) What is meant by eddy currents, and what is the reason for their emergence? Q.6) Explain an activity to explain how to reduce the effect of eddy currents generated on conductors, and what do you conclude from this activity? (7) ELECTRIC GENERATOR AND ELECTRIC MOTOR Q) What is an electric generator? What are its types? Electric Generator: it is a device that convert mechanical energy into electric energy by effect of a magnetic field. Types: Alternating Current generator (AC) (single or three phase). Direct Current generator (DC). Q) What is the components of an alternating current (single-phase) generator? 1) core coil. 2) Strong magnet that generates a strong magnetic field. 3) two carbon-brushes. 4) two metal rings (sliding rings). Q) What is produced when the generator coil rotates, at a uniform angular velocity within a magnetic flux, the intensity of the flux is uniform, and for a complete cycle? An induced electromotive force is generated, sine wave, with polarity reflected twice in a single cycle. 24 | P a g e Q) What is a three-phase generator? And what is the benefit of it ? It consists of three coils around the core, connected in “star- connection”, separated by equal angles each of which is (120°), their other terminals are connected to a wire called neutral wire or (zero line), the output current is transmitted via three lines. Its benefit: it supplies an alternating current with a larger amount than that of single-phase alternator. Q) What is the components of a direct current generator? 1) core coil. 2) Strong magnet that generates a strong magnetic field. 3) two carbon-brushes. 4) a single metal ring consisting of two halves isolated from each other by electric isolation, called the commutator. Q) How to convert an alternator to a direct current generator? it required to raise the two metal rings (the two slippery rings) and put metal ring consisting of two halves isolated from each other by electric isolation, called the commutator. Q) What is the commutator? And what is the practical benefit of the commutator? it is a single metal ring consisting of two halves isolated from each other by electric isolation, and They are in contact with the carbon brushes in order to connecting the coil with the external circuit, the number of commutator pieces is twice of the generator coils. Its benefit: to make the current out in one direction. Q) In a DC generator, what is the shape of the output current and what is equation, and what is the waveform? The output current from this generator is a pulsed current, The average amount (Iaverage) of this current is as follows: 𝑰𝒂𝒗𝒆 = 𝟎. 𝟔𝟑𝟔 𝑰𝒎 25 | P a g e  Q) How can you make the output current from the DC generator closer to the battery current? By the number of coils around the core is increased, with equal angles between them. AC generator DC generator the coil is connected with a single metal the coil is connected with two metal rings ring consisting of two halves isolated from (sliding rings). each other by electric isolation, called the commutator. Gives a sine wave current of variable Gives a pulsed current of variable amount amount and direction. and nearly constant direction. Q) What is an electric motor? And what does it consist of? Electric motor: It is a device that converts electrical energy into mechanical energy in the presence of a magnetic field. Its components: (the same components of the continuous generator). 1) core coil. 2) Strong magnet that generates a strong magnetic field. 3) two carbon-brushes. 4) a single metal ring consisting of two halves isolated from each other by electric isolation, called the commutator. Q) What is the basis of the engine's work? The flow of an electric current in a coil inside a magnetic field, magnetic forces which affect the ring rotates it by the effect of a torque called a coupled torque inside a magnetic field. Q) Explain the following statement: (the electric motor operate as the electric generator when its core rotates (when operating)). when the core coil rotates inside a magnetic field, change happens in the magnetic flux which penetrates the coil. According to Faraday’s law of electromagnetic induction, an induced electromotive force is generated on both sides of core coil of the motor, it is called back electromotive force ( 𝜺𝒃𝒂𝒄𝒌 ). It is calculated from the following relationship: ∆∅ 𝜺𝒃𝒂𝒄𝒌 = −𝑵 ∆𝒕 26 | P a g e  Q) What does the induced electromotive force (𝜺𝒃𝒂𝒄𝒌 ) depend on? and why is it called back? Depends on: 1) Velocity of core rotation, (the average time for change in magnetic flux of one winding), 2) number of winds of the coil, 3) the winding area. 4) the magnetic flux density. It is called back because it is opposite to the which generated it according to Lenz law. Q) What is meant by the induced electromotive force (𝜺𝒃𝒂𝒄𝒌 ) in the electric motor, and why is it called as back? It is an induced electric motive force generated in the motor as a result of rotating the motor core, so the magnetic flux penetrating the coil changes according to Faraday's law. It is called back because it is opposite to the which generated it according to Lenz law. Q) What determines the amount of current flowing into the electric motor circuit? Or Q) What does the current flowing into the motor circuit depend on? Difference in applied voltage (Vapplied) and back induced electromotive force (ɛback) in the motor circuit determine the amount of flowing current in that circuit, 𝑽𝒂𝒑𝒑 − 𝜺𝒃𝒂𝒄𝒌 𝑰= 𝑹 Ministerial Exams: Q.1) What does the amount of the induced electromotive force (𝜺𝒃𝒂𝒄𝒌 ) depend on, in the DC electric motor? Q.2) What does the amount of current flowing into the DC electric motor circuit depend on? Q.3) What does a three-phase alternative conductor generator consist of? What is the practical benefit of it? Explaining this by drawing. Q.4) What is the purpose of increasing the number of DC generator core coils? 27 | P a g e Q.5) How can you make the output current from the DC generator closer to the battery current? Q.6) What is the practical use of a three-phase alternative conductor generator? Q.7) What is meant by the induced electromotive force (𝜺𝒃𝒂𝒄𝒌 ) in the electric motor, and why is it called as back? (8) SELF INDUCTANCE Q) Explain an experiment with a drawing showing that the change in magnetic flux due to a change in the flowing current can generate an electromotive force induced in the coil. Q) Explain an activity in which you demonstrate the phenomenon of inductance. Two identical lamps are connected in parallel combination to a battery. The alternating resistance R has an equal amount to that of the coil resistance L and connected in series combination to one of the lamps, while the coil is connected to the other lamp in series combination (the coil core is wrought iron to increase magnetic flux density to have clear effect). after closing the switch, for a while both lamps glow equally when the current becomes constant, but they don’t reach that moment at the same time, there is a considerable delay in time for the lamp which is connected in series combination with the coil compared to the other lamp connected in series combination to the resistance Conclusion: There is delay in glow of the lamp connected to the coil is because inductance effect of the coil (or self-inductance of the coil), such a coil is called inductor. Q) What does self-inductance mean? A process that generates an induced electromotive force in a coil, due to change in flowing current to the time unit in the coil itself. ∆𝑰 𝜺𝒊𝒏𝒅 = −𝑳 ∆𝒕 28 | P a g e  𝜺𝒊𝒏𝒅 Q) Derive the following equation: 𝑳 = − ∆𝑰/∆𝒕 Q) Derive the law of the self-induced electromotive force (𝜺𝒊𝒏𝒅 ) in the coil. Q) Define the self-inductance coefficient (L), and what does it depends on? Ratio of induced electromotive force to the time rate of change of flowing current in the coil itself. and is measured in Henry (H) unit. 𝜺𝒊𝒏𝒅 𝑳=− ∆𝑰/∆𝒕 It depends on: 1) The number of turns of the coil. 2) size of the coil. 3) The geometric shape of the coil. 4) the magnetic permeability of the medium inside the coil. Q) what means the Henry -./010 Henry is the unit for self-inductance coefficient of a coil. If the current changes by ( 203456 ) one volt of induced electromotive force (ɛind), is generated on its both sides. 29 | P a g e  Q) Discuss the phenomenon of self-inductance in the following figures. In the following figure: It shows us the flow of a constant amount current through the coil, creating a magnetic flux of constant amount, so it does not cause generation of induced electromotive force (𝜀234 ). Which mean: ∆𝑰 =𝟎 ∆𝒕 ∴ 𝜺𝒊𝒏𝒅 = 𝟎 ∆𝑰 Where 𝜺𝒊𝒏𝒅 = −𝑳 ∆𝒕 In the following figure: ∆? In the figure: shows the flow of an increasing current ( ∆$ > 0), so the increased current generates an increasing magnetic flux, It is generated (𝜀234 ) with an opposite polarity, as it impedes the increase in current. Therefore, the growth time of the current from zero to its constant amount is large, so the net electric potential difference is given by the following equation: 𝑽𝒏𝒆𝒕 = 𝑽𝒂𝒑𝒑 − 𝜺𝒊𝒏𝒅 In the adjacent figure: 30 | P a g e  ∆? < 0) in the coil, the decreasing In the figure: it shows the flow of a decreasing current ( ∆$ current generates a decreasing magnetic flux, It is generated (𝜀234 ) with an opposite polarity as the applied voltage, and then the net voltage in the circuit is given by the following equation: 𝑽𝒏𝒆𝒕 = 𝑽𝒂𝒑𝒑 + 𝜺𝒊𝒏𝒅 Q) Is the time of current growth from zero to its constant amount in a circuit containing a DC coil and source large or small? Why? With mathematical relationship writing? The growth time is large due to the generation of (𝜀234 ) with opposite polarity to the voltage source, which impedes the surge growth in the current. 𝑽𝒏𝒆𝒕 = 𝑽𝒂𝒑𝒑 − 𝜺𝒊𝒏𝒅 Q) Is the time of current fading from its constant amount to zero in a circuit containing a DC coil and source large or small? Why? With mathematical relationship writing? The fading time is small for two reasons: 1) Generating (𝜀234 ) with a polarity similar to the source polarity. 𝑽𝒏𝒆𝒕 = 𝑽𝒂𝒑𝒑 − 𝜺𝒊𝒏𝒅 2) An air gap appears between the two parts of the switch, making the circuit resistance too large. 31 | P a g e  Q) Draw a diagram showing that the current fading time from its constant amount is smaller than the current growth time from zero to its constant amount? Q) Compare in terms of the mathematical relationship between the energy stored in the electric field of the capacitor and the energy stored in the magnetic field of the coil. Energy stored in the capacitor 𝟏 𝑸𝟐 𝑷∙𝑬= 𝟐 𝑪 Energy stored in the coil 𝟏 𝟐 𝑷∙𝑬= 𝑳𝑰 𝟐 Where Q: charge. C: capacitance of capacitor. L: self-inductance factor. I: current. 32 | P a g e  Q) Explain an activity demonstrating the generation of a self-induced electromotive force at both ends of a coil? Activity Tools: Battery( 9 v ). Electric switch. A wire coil with wrought iron core. Neon lamp (80 V to glow). Activity Steps: 1) The coil, switch, and battery are connected in series combination. 2) The neon lamp is connected in parallel combination with the coil. 3) The coil and battery circuit is closed by the switch, no glow in the lamp. 4) The coil and battery circuit is opened by the switch; the neon lamp will glow brightly for a while, although the battery is disconnected from the circuit. Conclusion: First: the neon lamp doesn’t glow when the switch is closed, because the voltage applied on its ends is not sufficient to glow it. The growth of current from zero to a constant amount is slow because of generate an induced electromotive force in the coil hinders the cause of that force according to Lenz’s law. Second: the neon lamp glows when the switch is opened because of generate a huge voltage on its ends was sufficient to glow. This is because the quick fading of the current through the coil generates a huge self-induced electromotive force on the coil. The coil acts as a source of energy which provides the lamp by an enough voltage to glow. Ministerial Exams: Q.1) What does the amount of the inductance factor of a coil depends on? Q.2) Why the a neon lamp which connected in parallel combination to a coil glow bright for a short period of time in the moment the switch is opened even though the battery is disconnected from the circuit? Q.3) Write the mathematical equation in which the voltage is given in a DC circuit- containing coil, battery, and switch in the following cases: a) When an increasing amount of current flows into the coil? b) When a reduced amount of current flows into the coil? Q.4) What is meant by (self-inductance)? and what does its amount depend on? Q.5) Explain with an activity the generation of the self-induced electromotive force at both ends of the coil? 33 | P a g e (9) MUTUAL INDUCTION Q) What is the phenomenon of mutual inductance? and mention the mathematical equation? It is a phenomenon that generates an induced electromotive force (𝜀234A ) in the secondary coil as a result of a change in the temporal rate of current in the primary coil that is next to or around it. ∆𝑰𝟏 𝝐𝒊𝒏𝒅𝟐 = 𝑴 ∆𝒕 𝑾𝒉𝒆𝒓𝒆: (𝑀)𝑖𝑠 𝑡ℎ𝑒 𝑚𝑢𝑡𝑢𝑎𝑙 𝑖𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 Q) Define the mutual self-inductance coefficient (M), and by what unit is it measured? It is the ratio between the electromotive force induced in the secondary coil to the time rate of current change in the primary coil. 𝜺𝒊𝒏𝒅𝟐 𝑴=− ∆𝑰𝟏 /∆𝒕 Q) The law of the induced electromotive force in the secondary coil (𝜺𝒊𝒏𝒅𝟐 ) was derived in the phenomenon of mutual inductance. Q) What does the mutual self-inductance coefficient (M) between two adjacent coils in the air depends on? 1) Constants of coils (𝐿B, 𝐿A, ) 𝑴 = ^ 𝑳𝟏 × 𝑳𝟐 2) The situation of each coil. 3) he separator between the coils. 34 | P a g e  Q) What does the mutual self-inductance coefficient (M) between two coils; contain a core of closed iron, depends on? Q) What does the mutual self-inductance coefficient (M) between two coils with full magnetic coupling between the coils, (perfect magnetic bonding) depends on? Depends on the Constants of coils (𝐿B, 𝐿A, ). 𝑴 = ^𝑳 𝟏 × 𝑳 𝟐 Q) Explain the phenomenon of mutual inductance? In the adjacent figure, where we have two adjacent wire coils wrapped around a core of wrought iron , the flowing current in the primary coil (1) generates a magnetic field ( B ), and magnetic flux (∅) penetrated the secondary coil (2). if the flowing current in coil (1) changes in time, the magnetic flux (∅) would change accordingly and penetrated the coil (2) in time. According to Faraday’s Law of electromagnetic induction, an induced electromotive force is generated (𝜀234A ) in coil (2), with the number of turns (N2). ∆𝑰𝟏 𝝐𝒊𝒏𝒅𝟐 = 𝑴 ∆𝒕 Q) Where to invest the phenomenon of mutual inductance? Explain. Mutual inductance phenomenon is used in transcranial magnetic stimulation (TMS) device A time-alternating current is applied on the primary coil, which is placed on the patient’s brain, the generated alternating magnetic field penetrates the patient’s brain, which generates an induced electromotive force in it. This, in turn, generates an induced current, which disturbs the electric circuits in the brain, and, using this; mental and psychological diseases are cured. 35 | P a g e  Q) What is (an induced electric field or a nonelectrostatic fields)? It is the field generated as a result of changes in the magnetic flux in unit time, and it is responsible for the movement of electrical charges inside the conductive ring, which are always in tangential directions. Q) What is the difference between stable electric fields and unstable electric fields? Stable electric fields are generated by the static electric charges Unstable electric fields: it is the field generated as a result of changes in the magnetic flux in unit time, and it is responsible for the movement of electrical charges inside the conductive ring, which are always in tangential directions. Ministerial Exams: Q.1) Explain how the phenomenon of mutual inductance is invested in the magnetic stimulation device during the brain. Q.2) Distinguish between: stable electric fields and unstable electric fields. Q.3) Where to invest the phenomenon of mutual inductance? Explain this. Q.4) What does the mutual self-inductance coefficient between two coils wrapped around a sealed wrought iron core (or a perfect magnetic bond between them), depends on? Q.5) What is meant by an unstable electric field? Q.6) What happen? Why? If the appropriate current changed in one of two neighboring coils. 36 | P a g e (10) PRACTICAL APPLICATIONS OF THE ELECTROMAGNETIC INDUCTION PHENOMENON Q) Mention the practical applications of the phenomenon of electromagnetic induction with an explanation of the operation of each application? Credit card When the magnetic credit card slides in wire coil, it induces an electric current then this current is magnified and converted into volt pulses, which contains information. Electric guitar Strings of electric guitar (made of ferromagnetic materials) magnetize when shake by wire coils each containing a magnetic rod, these coils are placed in various places under the strings of the electric guitar, when these strings shake, an alternating current with equal frequency to that of the strings is induced. Then this current is passed to an amplifier, Induction Stove Electromagnetic induction is utilized to manufacture this type of stoves. A wire coil is placed under the upper surface of the stove. An alternating current flow, and it induces an alternating magnetic field outward, when the alternating current passes through the base of the pot (if it was metal). Eddy currents are generated at the base of the pot; hence the base of the pot is heated, then water boils. If the pot was made of glass, no eddy currents generate at its base because the glass is dielectric and doesn't heat the water inside it, if the surface of the induction stove is touched, no heat is felt, Q) Why not use glassware on induction stoves? Because no eddy currents generate at its base because the glass is dielectric and doesn't heat the water inside it, Q) When touching the top surface of the induction stove, the surface does not feel hot. Why? Because the generation of eddy currents, which is the source of heat, occurs at the base of the metal pan and not at the surface of the stove. Ministerial Exams: Q.1) Can an AC current be generated by the strings of the electric guitar. Q.2) Mention some practical applications of the phenomenon of electromagnetic induction, explaining one of them. Q.3) Explain how credit card information is recognized? 37 | P a g e Chapter Questions Q.1) Choose the correct statement for each of the following: 1) Which one of the following figure shows the correct direction of induced current in the conductor ring? 2) In the figure a ring is made of copper placed in the plane of the paper and connected with resistor (R) an exert magnetic field which is perpendicular to the plane and the direction out of the page. In which case induced current in the resistor (R) will be directed from left to right. a) When the magnetic flux that penetrate the ring is increases. b) When the magnetic flux that penetrate the ring is decreases. c) When the magnetic flux that penetrate the ring is constant. d) In all case which is mentioned above 3) When a magnetic rod falls through a wide ring of aluminum placed in horizontally by a holder under the rod, notice figure (59). the direction of induced current in the ring is: a) Always in the direction of clockwise. b) Always in the direction of counterclockwise. c) In the direction of clockwise, then it becomes zero for a moment, then in the direction of counterclockwise. d) In the direction of counterclockwise, then it becomes zero for a moment, then in the direction of clockwise. 4) When a magnetic rod falls through unclosed aluminum ring, which is placed horizontally under the rod magnet, notice figure a) The rod is affected by the repulsive force when approaching to the ring, then it is affected.by attractive force when it is moved away from the ring. b) The rod is affected by the attractive force when approaching to the ring, then it is affected. by repulsive force when it is moved away from the ring. c) The rod is not affected by any force when it is approached to the ring or it’s moved away from it. d) The rod is affected by the repulsive force when it is approached to the ring and affected by the repulsive force moved away from 41 | P a g e 5) In figure a hollow core coil is connected in series to a lamp, resistor, battery and switch. When the switch in the circuit is closed the glow of the lamp is constant. If we enter an iron core into the gap of the coil, the glow of the lamp: a) Increases. b) Decreases. c) Remains constant. d) Increases, then decreases. 6) When a circular coil rotates around a perpendicular axis which is parallel to the coil face within a magnetic field of uniform horizontal flux density ( B ) see figure ، the maximum amount of induced electromotive force (ɛ max) is generated. when triple the number of turns of coil, reduce the dimeter of the coil to half what was it, and double rotational frequency. The maximum amount of induce electro motive force will be: a) 3/2 𝜀.78 b) 1/4 𝜀.78 c) 1/2 𝜀.78 d) 3 𝜀.78 7) The phenomenon of self-induction rerify in a given coil when: a) Pulls a rod magnet away from the face of the coil. b) This coil is placed near another coil in which a variable electric current flows per unit time. c) An electric current flows in this coil of variable amount per unit time. d) This coil rotates in the uniform magnetic field. 42 | P a g e 8) The amount of induced electromotive force between two ends of a conducting rod moving in a uniform magnetic field in static state does not depend on: a) Length of the rod. b) Diameter of the rod. c) the situation of the rod with respect to the magnetic flux. d) Magnetic flux density. 9) When the angular velocity of rotating the core coil of the electric motor decreases as a result of an increase in connected load with its coil, causes a decrease in the amount of : a) Back-electromotive force. b) The voltage between two ends of the core coil. c) The current passing through the engine. d) The lost voltage (IR) between ends of the core coil. 10) It is possible to induce the electric current in a closed conductor ring in one of the following cases except one. In which case the current is not induced: a) A closed conductor ring rotates around its axis that is parallel to its plane and perpendicular to the uniform magnetic flux (B). b) Area vector of the closed conductor ring is parallel to the (B) that changes with the unit time. c) Area vector of the closed conductor ring is perpendicular to the (B) that changes with the unit time. d) The closed conductor ring which area vector parallel to the (B) is pressed from both opposite sides. 11) The unit of magnetic flux density (B) is: a) weber b) weber/s c) weber/m² d) weber. s 12) In the figure (63) when the conductor ring rotates around a perpendicular axis parallel to the it’s face, and the magnetic flux is perpendicular to its axis and reverses twice through each: a) One revolution. b) Quarter revolution. c) Half revolution. d) Two revolutions. 13) Coefficient of self-induction of the coil does not depends on: a) Number of turns of the coil. b) Geometry of the coil. c) The time rate of changing of the current flows in the coil. d) Magnetic permeability of the medium in the core. 43 | P a g e Q.2) Give the reasons of the followings: 1) The neon lamp that is connected in parallel to the coil is brightened strongly for a short time at the moment when the switch is on in spite of that the battery is disconnected from the circuit and does not glow when the switch is off. First: the neon lamp glows when the switch is opened because of generate a huge voltage on its ends was sufficient to glow. This is because the quick fading of the current through the coil generates a huge self-induced electromotive force on the coil. The coil acts as a source of energy which provides the lamp by an enough voltage to glow. Second: the neon lamp doesn’t glow when the switch is closed, because the voltage applied on its ends is not sufficient to glow it. The growth of current from zero to a constant amount is slow because of generate an induced electromotive force in the coil hinders the cause of that force according to Lenz’s law. 2) Water boils inside the metal container which is placed on the induction coil but water does not boil in the glass container which is placed on the same oven. Under the upper surface of the stove a wire coil is placed in which an alternating current flow and this current induces an alternating magnetic field that spreads outward, and with the passage of the alternating magnetic field through the base of the metal pot, eddy currents are generated at the base of the pot and the water placed in it boils. While a pot made of glass does not generate eddy currents at its base (because glass is an insulating material) no heat is generated in it, and the pot is not heated, or the water placed in it. 3) If a change in the electric current flows in one of adjacent coils the induced current is generated in the other coil. According to the phenomenon of mutual induction between two adjacent coils, if the current flowing in the primary coil changes during a unit time, the magnetic flux that penetrates the secondary coil changes during a unit time, and according to Faraday’s law in electromagnetic induction, an induced electromotive force is generated in the secondary coil Q.3) Explain by an experiment how do you know existence of magnetic or electric field in definite area. We throw the charged particle in a perpendicular direction to the field. If the particle moves in a direction parallel to the field lines, then it is an electric field, because the electric force be in a parallel direction to the lines of the electric field. If it moves in a circular path, then it is a magnetic field, because the magnetic force is perpendicular to the velocity vector and perpendicular to the magnetic flux density vector. 44 | P a g e Q.4) When a coil which area of one turn of coil is (A) rotates with angular velocity of (W) inside uniform constant magnetic flux density (B). The magnetic flux that penetrates one turn is given by a cosine function (∅𝑩 = 𝑩𝑨𝒄𝒐𝒔 (𝝎𝒕)) induced electromotive force is supplied as (𝜺𝒊𝒏𝒅 = 𝑵𝑩𝑨𝒔𝒊𝒏 (𝝎𝒕)) Explain it by mathematically. Answer: Q.5) What the meant of the non-electrostatic field? They are the fields that arise by changes in the magnetic field and are responsible for the movement of electrical charges inside the conductor ring, and they are always in tangential directions. Q.6) Define some areas which are eddy currents are used in them and explain each 1) Investing in the brakes of modern trains: Wire coils, each acting as an electromagnet, are placed opposite the rail rails. To stop the train from moving, we close the electrical circuit of the windings, so an electric current flows that generates a strong magnetic field that passes through the tracks, creating eddy currents in them. (Due to the presence of relative motion between the magnetic field and the rods) According to Lenz's law, an excited magnetic field is generated that impedes the movement, and the train stops. 2) Invest in metal detection at checkpoints, especially in airports (pulse induction): - The metal detector contains two coils, one of which is used as a transmitter and the other is a receiver. When the transmitter circuit is closed, an alternating electric current passes through it, generating an alternating magnetic flux. The receiving coil is induced, an induced current pass through it, and the amount of this current is measured when there is air between the two coils. When any metal object passes between the two coils, eddy currents will be generated in the metal body, and these currents will obstruct the magnetic flux generated in the receiver coil, and this causes a decrease in the primary current measured in the future, which indicates the presence of a metal piece in the bags or in the person's clothes. 3) Invest to control the traffic lights on the road. 45 | P a g e Q.7) If a conductor rod (ab) in the figure moves in paper plane horizontally to the left inside uniform magnetic field which directed perpendicular to the page (directed to the viewer), the electric field is generated inside the rod directed to (b). while if the rod is moved to right and inside the same magnetic field, the electric field is reversed to (a). Explain why? When the rod moves to the left perpendicular to the magnetic flux, the magnetic force (FB) affects the positive charges, their direction is towards the end (a) according to the right- hand rule, so the positive charges collect towards the end (a) of the rod and the negative charges at the end (b). Therefore, the direction of the electric field is be from the end (a) towards the end (b). By reversing the direction of movement of the rod (towards the right), the direction of the magnetic force (FB) reverses, so that the positive charges collect at the end and (b) the negative charges at the end (a), so that the field direction is be from the end (b) towards the end (a). Q.8) Indicate the direction of induced current face the ring opposite the wire coil, in the figures Answer: a) When the switch is open, the current is zero, there is no change in the magnetic flux penetrating the coil as (∆∅9 = 0). b) If the switch is closed, there is an increase in the magnetic flux that penetrates the coil, so if we look at the direction of the wire coil from the right side, the direction of the induced current at the moment of the current growth is in a clockwise direction (∆∅9 > 0). c) If the circuit is opened, the magnetic flux that penetrates the coil reactance diminish, so if we look at the direction of the wire coil from the right side, the direction of the induced current at the moment of the current decay is in a counterclockwise direction (∆∅9 < 0). 46 | P a g e Q.9) Consider that the coil and the magnet shown in figure (66) each move at the same relative velocity to the earth. Dose the digital millimeter (or Galvanometer) connected to the coil indicates to current flow in the circuit? Explain that. Answer: No, because there is no induced current flowing in the circuit because there is no relative movement between the magnet and the ring that causes a change in the magnetic flux of the unit of time. Q.10) What is the physical quantity which are measured by followings: a) Weber b) Weber/m² c) Weber/s d) Tesla e) Henry Answer: a) The magnetic flux (∅) is measured in the unit (𝑊𝑒𝑏𝑒𝑟). b) Magnetic field density (𝐵) is measured in unit (𝑊𝑒𝑏𝑒𝑟/𝑚: ). ∆∅ c) The time rate of change in flood ( ) is measured in unit (𝑊𝑒𝑏𝑒𝑟/𝑠)). ∆$ d) Magnetic field density (𝐵) is measured in unit (𝑇𝑒𝑠𝑙𝑎). e) The self-self-inductance coefficient (𝐿) and the mutual self-inductance coefficient (𝑀) are measured in the unit of (𝐻𝑒𝑛𝑟𝑦). 11) How eddy currents work to stop the vibration of metal plate which vibrates perpendicularly to the uniform constant magnetic field? Due to the generation of vortex induced currents in the plate, which generates an induced magnetic field opposite to the direction of the acting magnetic field, and as a result, a magnetic repulsion force is generated that obstructs the direction of movement of the plate within the magnetic field, which acts to suppress its vibration (according to Lenz's law) 12) A copper plate is placed between the poles of uniform electromagnet which has large flux density and perpendicular to the magnetic flux. When the plate is pulled out horizontally from the field by definite velocity this process needs to exert a definite force. And the amount of definite force increases by increasing that speed. What is the correct explanation of the for both condition? As a result of the relative movement between the metal plate and the magnetic flux, eddy currents are generated on the surface of the metal plate according to Faraday's law. In electromagnetic induction, a magnetic force is generated that obstructs the direction of movement of the plate according to Lenz's law. As the amount of that velocity increases, the magnetic force (obstruction) increases. 47 | P a g e Q.13) A copper wire and copper ring are located as in the figures (67, 68). In which figure an induced current form in the ring when the current starts to flow from the wire? Explain. Figure (68) Figure (67) Answer: In the first figure, an induced current does not flow in the ring. Because the magnetic flux density vector is perpendicular to the area vector of the ring, so the angle (𝜃) between the area vector and the magnetic flux, density is equal to (90°), so it is: ∅ = 𝑨𝑩 𝒄𝒐𝒔𝜽 ∅ = 𝑨𝑩 𝒄𝒐𝒔𝟗𝟎 ∅=𝟎 In the second figure, a current is generated because the flux density vector is parallel to the area vector, i.e. the angle equals zero, so it is: ∅ = 𝑨𝑩 𝒄𝒐𝒔𝜽 ∅ = 𝑨𝑩 𝒄𝒐𝒔𝟎 ∅ = 𝑨𝑩 Q.14) You have a wire with constant length, and you want to make simple generator which provides greatest amount of electromotive force. Is it necessary to make coil with one loop or two loops or three loops by the wire, when you rotate the coil which you have get with definite amount angular velocity inside the uniform magnetic field? Explain. Answer: This means that the (𝜺𝒊𝒏𝒅 ) becomes half of what it was. This is when the number of turns is doubled as the length of the wire is fixed. And in the same way for three turns. 48 | P a g e This means that the amount of the (𝜺𝒊𝒏𝒅 ) becomes 1/3 what it was. This is when making the number of turns (3) with the length of the wire fixed. Therefore, we make the wire in the form of a coil with a single circular coil to provide the maximum amount of electromotive force. Q.15) In most coils, laminated soft iron core is used instead of one piece of iron. Explain why? To reduce the effect of eddy currents, the resulting power losses are reduced, and thus the resulting thermal energy is reduced. Ministerial Exams: Q.1) n most coils, the core is made in the form of parallel rods of wrought iron isolated from each other electrically and tightly clamped instead of a core of iron made as one piece, what is the practical benefit of that? Q.2) Why does water boil inside a metal pot placed on the upper surface of an induction stove and not boil water that is inside a glass container placed next to it and on the upper surface of the stove itself? Q.3) Choose the correct answer … Self-inductance coefficient for a coil that does not depend on: a) Number of turns of the coil b) Geometry of the coil c) Time rate of change in the flowing current d) Magnetic permeability of the medium in the core of the coil 49 | P a g e Q.4) Choose the correct answer … The units of measurement for magnetic field density are: a) weber. s b) weber / s c) weber d) weber / m2 Q.5) In the circuit, a hollow coil is connected in series to a light bulb, resistor, battery, and switch, and when the switch was in the circuit closed, the intensity of the lamp's glow was constant. If you inserted a wrought iron leg into the coil cavity, the lamp would glow during the rod entry: a) Increases. b) Decreases. c) Remains constant. d) Increases, then decreases. Q.6) When a conductive ring rotates around a vertical axis parallel to its face and passes through its center and the axis is perpendicular to a regular horizontal magnetic flux. The polarity of the induced electromotive force is a sinusoidal function that changes with time and is reflected twice during each: a) One turn. b) Quarter of a turn. c) Half a turn. d) Two turns. Q.8) Choose the correct answer … the amount of the induced electromotive force at the two ends of a conductor rod moving relative to a magnetic field in a static state that does not depend on: a) Rod length b) Rod diameter c) Magnetic flux density d) Position of the rod relative to the magnetic flux Q.9) Why can't the top surface of the induction stove feel hot when touched by hand? Q.10) Choose the correct answer … when the magnetic rod falls through an un-weighted aluminum ring placed horizontally under the rod. a) The rod is affected by the force of repulsion while approaching the ring and then affected by the force of attraction while moving away from the ring b) It is not affected by any force while approaching the ring or during As it moves away from the ring c) The rod is affected by the force of repulsion as it approaches the ring, as well as by the force of repulsion as it moves away from the ring 50 | P a g e Q.11) Choose the correct answer … when a circle coil rotates around a vertical axis parallel to the coil’s face within a magnetic field, its flux density (B) is uniform (horizontal) generates the greatest amount of induced electromotive force and when increasing the number of coil turns to three times what it was and reducing the diameter of the coil to a third of what Accordingly, and by doubling the rotational frequency of the coil, the maximum amount of the induced electromotive force reactance be: a) 3 𝜺𝒎𝒂𝒙 b) 3/2 𝜺𝒎𝒂𝒙 c) 1/4 𝜺𝒎𝒂𝒙 d) 2/3 𝜺𝒎𝒂𝒙 Q.12) Put sign of (true) or (false) then correct the error … in the below figure a ring of copper material placed in the plane of the paper, that an induced current flows in the resistance in its direction from right to left when the magnetic flux that penetrates the ring increases. Q.13) What the reason of … A neon lamp connected in parallel to a coil, glows bright for a short period of time when the switch is opened despite the battery being disconnected from the circuit, and it does not glow when the switch is closed. 51 | P a g e