F4 Mid Year Exam Study Guide 2024-2025 PDF

Page 1 of 24 1 F4 Mid Year Assessment Study Guide 1.0 General Guideline The main scope of this mid-year examination is from chapter 34 to chapter 48 (except chapter 39, 41 and 42) of the IGCSE Physics textbook. In course of preparation of your examination, studen...

Page 1 of 24 1 F4 Mid Year Assessment Study Guide 1.0 General Guideline The main scope of this mid-year examination is from chapter 34 to chapter 48 (except chapter 39, 41 and 42) of the IGCSE Physics textbook. In course of preparation of your examination, students should use the questions of your classwork and tests as the starting point. Please make sure that you have mastered all those questions first. In addition, several notes are posted in Edmodo. These are also important study material and should also be revised. As always, students must have rulers, protractors and calculators ready for the examination. 2.0 Chapter Guideline 2.01 Chapter 34 – Magnetic fields In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Definitions of soft and hard magnetic material. Soft magnetic materials such as iron relatively easy to magnetize, but their magnetism is only temporary. They are used in the cores of electromagnets and transformers because their magnetic effect can be ‘switched’ on or off or reversed easily. Hard magnetic materials such as steel are difficult to magnetize but do not readily lose their magnetism. They are used for permanent magnets. Hard magnetic materials such as steel are difficult to magnetize but do not readily lose their magnetism. They are used for permanent magnets. (2) Examples of materials that are soft or hard. Examples of non-magnetic materials. Type Soft magnetic Hard magnetic Non-magnetic materials materials materials Examples Iron Steel metals such as brass, copper, zinc, tin, and aluminium, as well as non-metals. (3) Magnetisation and demagnetization of iron and steel. Induced magnetism Materials such as iron and steel are attracted to magnets because they themselves become magnetized when there is a magnet nearby. The magnet induces magnetism in them, as shown in the figure. In each case, the induced pole nearest the magnet is the opposite of the pole at the end of the magnet Chains of small iron nails and steel paper clips can be hung from a magnet Each nail or clip magnetises the one below it and the unlike poles so formed attract. The steel and the iron behave differently when pulled right away from the magnet. Page 2 of 24 If the iron chain is removed by pulling the top nail away from the magnet, the chain collapses, showing that magnetism induced in iron is temporary. When the same is done with the steel chain, it does not collapse; magnetism induced in steel is permanent. (4)Drawing of magnetic field lines and patterns. Magnetic field lines Magnetic field lines show the volume of space around a magnet in which magnetic forces act. Field Line Rules: Magnetic field lines of force show: the shape of the magnetic field. the direction of the field lines – north to south. the strength of the magnetic field – closer together = stronger. Magnetic filed lines never overlap. Field strength decreases with distance from the magnet Between magnets with unlike poles facing, the combined field is almost uniform (even) in strength. However, between like poles, there is a neutral point where the combined field strength is zero. Page 3 of 24 2.02 Chapter 35 – Static electricity In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Explain how an object becomes positively or negatively charged in terms of movement of electrons. An electron is a subatomic particle with a negative electric charge. Certain materials can lose or gain electrons. Atoms, and objects composed of them, are normally electrically neutral. The number of electrons (negative charges) balances the positive atomic charges. If an object gains extra electrons, it becomes negatively charged. If it loses electrons, it becomes positively charged. However, when two materials are rubbed together, electrons may be transferred from one to the other. One material ends up with more electrons than normal and the other with less. So one has a net negative charge, while the other is left with a net positive charge. Rubbing materials together does not make electric charge. It just separates charges that are already there. There are positive and negative charges If two charged objects are close together, + and + or – and – will repel but + and – will attract. Unlike charges attract and that like charges repel Charging a body involves the addition or removal of electrons. (2) S.I. unit of charge Electric charge S.I unit of Electric charge is Coulmbs (C) (3) Charging by induction. Earthing symbol Earthing (4) If Attraction between enough charge uncharged builds and charged up on something, objectsmay be pulled through the air and cause electrons (5) Direction sparks of electric – which field line. To prevent charge building up, objects can be earthed: they can be dangerous. can be connected to the ground by a conducting material so that the unwanted charge flows away. Charging by Induction A conductor can only hold charge if it is insulated from the ground. If it isn’t insulated, it will not hold any charge because electrons transfer between the conductor and the ground.A conductor cannot be charged by rubbing it with a cloth because a cloth is not a good insulator. So it won’t hold charge. Another method of charging is known as induction. In the induction process, a charged object is brought near but not touched to a neutral conducting object. The presence of a charged object near a neutral conductor will force (or induce) electrons within the conductor to move. Induced charges Charges that ‘appear’ on an uncharged object because of a charged object nearby are called induced charges. In the diagram , a metal sphere is being charged by induction. The sphere ends up with an opposite charge to that on the rod, which never actually touches the sphere. Page 4 of 24 Discharging a charged conductor safely A conducting path (e.g a wire ) needs to be provided between the object and the ground. The conducting path allows electrons to transfer between the conductor and the ground.We say the object is earthed. Charging a conductor by induction Example 1 – using positively charged rod An insulated conductor can be charged from a charged body indirectly by induction as well as by direct contact. The figure shows the process of charging an insulated metal sphere by induction using a positively charged rod. Charging by induction gives the conductor the opposite type of charge to the charge on the rod. Also no charge is gained or lost by the rod in this process. Page 5 of 24 Charging a conductor by induction Example 2 - using negatively charged sphere Charging a metal object by induction -Example 2 Start with two objects : ▪ an object A with large negative charge, and ▪ an uncharged metal sphere B on an insulating stand. The method is as follows 1. Object A has a large negative charge. When the metal sphere B is placed near it , electrons in the sphere B are repelled away. The front of the sphere (near A) has an induced positive charge. 2. Now the sphere is touched, either by hand or by a wire connected to earth. This allows electrons to escape from the sphere. 3. The connection is removed. Now the sphere has a positive charge. 4. Finally, the sphere B is taken away from object A. Sphere B has a uniformly distributed positive charge all over it. Charging a conductor by induction Example 3 Bringing a negatively charged polythene strip near to an insulated metal sphere X which is touching a similar sphere Y(Figure 35.5a). Electrons in the spheres are repelled to the far side of Y. Attraction ( 4) If X and Ybetween are separated, withand uncharged thecharged chargedobjects strip still in position, X is left with a positive charge (deficient of electrons) and Y with a negative charge (excess of electrons) (Figure 35.5b). Page 6 of 24 (6) Direction of electric field line. Electric field When an electric charge is placed near to another electric charge it experiences a force. The electric force does not require contact between the two charges so we call it an ‘action-at-a-distance force’ – it acts Chapter 36 – Electric current 2.03space. through The region of In space where addition anskills to the electric chargeinexperiences as stated the checklista at force due of the end to the other charges chapter, is called electric students field. Direction ofshould also electric pay special attention to the following: field The direction of an electric field at a point is the direction of the force on a positive charge at that point (1) S.I. unit of current The lines(2) of Relationship force are shown coming between out ofcurrent charge, a positive and charge time. and going into a negative charge. This is because the lines indicate the direction of the force on a positive charge placed in the field. A positive charge is repelled by another positive charge and attracted by a negative charge. When two oppositely charged objects are placed close together, they attract one another. Two objects with the same charge repel each other. Drawing the electric field lines Exam tip 4 different cases When asked the direction of 1.The field between two oppositely charged parallel plates the electric filed at a point, you 2.a.The field between two opposite point charges must write as: b. two similar point charges The direction of an electric 3.Electric field close to field at a point is the direction a) a negatively charged sphere and of the force on a positive b) negative point charge charge at that point. 4.Electric field close to NOT as: a) a positively charged sphere and Direction is from positive to b) a positive point charge negative. Page 7 of 24 Drawing Electric field lines 1.Electric field between 2. Electric field 3.Electric field close 4. Electric field two parallel plates with between to close to a opposite charges on a. two opposite point a) negatively a) positively them. charges. charged sphere. charged b. two similar point b) A negative point sphere. charges charge b) A positive point charge a. a. a. Electric filed pattern b. b. b. Uniform field-If the electric force felt by a charge is the same everywhere in a The lines of force The lines of region, the field is surrounding a point force Properties uniform Field is non uniform charge or a charged surrounding a Represented by sphere are radial. point charge evenly spaced parallel or a charged lines drawn sphere are perpendicular to the radial. surfaces. Pointing away from Pointing away from The direction of field The direction of positive charge and positive charge and line is towards the field line is away Direction of towards the negative towards the negative sphere and towards the from the sphere the field charge. charge. point charge and away from the point charge. Page 8 of 24 2.03 Chapter 36 – Electric current In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) S.I. unit of current The unit of current is the ampere (A) (2) Relationship between charge, current and time. For a steady current in a circuit , the charge flowing ‘Q’ in a certain time ‘ t ‘ is given by: Charge flowing = current x time (in coulombs) ( in amperes) (in seconds) Q = I x t One coulomb is the charge passing any point in a circuit when a steady current of 1 ampere flows for 1 second. That is,1 C = 1 A s. A charge of 3 C would pass each point in 1 s if the current were 3 A. In 2 s, 3 A × 2 s = 6 A s = 6 C would pass. In general, if a steady current I (amperes) flows for time t (seconds) the charge Q (coulombs) passing any point is given by Q = I × t Symbols and units for some electrical quantities Quantity Symbol Unit Symbol for for unit quantity Current I Ampere A Charge Q Coulombs C Time t Seconds s (4) Difference between the direction of traditional current and direction of flow of electrons. Direction of flow of electrons and direction of conventional current Red=conventional current ▪ Current in a metal is due to flow of electrons. Green= flow of electrons ▪ Current in a liquid is due to flow of ions. In a circuit The direction of electron flow: from – to the + terminal (outside the battery). The direction of current flow : from + to the - terminal Page 9 of 24 (5) Properties of series and parallel circuits. (in terms of current) Series circuit Series = one after the other = there is only one single path for charge to flow The components are in series because the same amount of charge flows through each circuit component every second. The ammeter therefore measures the current in the torch lamp. To measure the current in a circuit ammeter is connected in series. ▪ The current is same all the way round the circuit. ▪ So it does not matter where you connect the ammeter. ▪ So ammeter reads the same (here in the diagram 2A) irrespective of where it is connected (connected in series) Series and parallel circuits Page 10 of 24 Series circuit Parallel circuit In a series circuit the current is the same all the way In a parallel circuit the current splits at the junction round the circuit and is shared between the resistors. Adding resistors in series increases the total resistance in Adding resistors in parallel decreases the total the circuit. resistance of the circuit. In a series circuit, the total resistance is simply the sum In a parallel circuit, the total resistance is given by of the individual resistances, the formula In the example above , 𝟏 𝟏 𝟏 Total resistance = 1 Ω +1 Ω =2 Ω = + 𝑻𝒐𝒕𝒂𝒍 𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆 𝑹𝟏 𝑹𝟐 1 In the example above ,Total resistance = 2 = 0.5 Ω 𝑹𝟏 𝑹𝟐 Total Resistance = 𝑹𝟏 +𝑹𝟐 The combined resistance of 2 resistors in parallel is less than that of either resistor by itself. The potential difference across each resistor can be The potential difference across each resistor can be calculated using V=IR calculated using V=IR In the example above , V= 3 x 1=3V,so each resistor has In the example above , V= 6 x 1=6V,so each resistor a P.D of 3V across it. has a P.D of 6V across it. In a series circuit potential difference is shared between In a parallel circuit potential difference across each the resistors. resistor is the same as the potential difference across the cell. This is because the energy from the cell is shared between the resistors. This is because after picking up energy from the cell charge only passes through one of the resistors (not both). If one of the resistors broke, the circuit would be broken If one of the resistors broke, the current could still and no current would flow. flow through the second resistor, although the current would be smaller because there would now be a greater total resistance in the circuit. Advantages of putting lamps in parallel are: o If one lamp breaks, the other still works. o Each lamp gets maximum PD. o Lamps can be switched on/off independently. (5) Use of an ammeter in circuits (How to connect?) Measuring Potential Difference Measuring Potential Difference Potential difference can be measured using a voltmeter The voltmeter should be connected in parallel ( across the terminals of the cell) with the part of the circuits you want to measure the potential difference of. Page 11 of 24 A voltmeter is connected in parallel with a device to measure its voltage, while an ammeter is connected in series with 2.04aChapter device to 37 – Potential measure its current. difference In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Definition of electromotive force (e.m.f.) e.m.f -Definition ▪ In energy terms, the e.m.f. is defined as the number of joules of electrical energy produced when one coulomb of charge passes through the battery (or cell). Electrical energy produced in joules E ▪ e.m.f of a battery in volts = ; em.f (v) = Charge in coulombs Q ▪ 1 Volt (V) is equivalent to 1 J/C. (2) Definition of potential difference (p.d.) Potential Difference & Energy The potential difference between two points in a circuit is the amount of energy transferred by each unit of charge passing between those two points The unit of voltage, the volt (V), is the same as a joule per coulomb (J/C) 1 V = 1 J/C So, for example: If a bulb has a voltage of 3 V, every coulomb of charge passing through the bulb will lose 3 J of energy If the p.d across a cell is 1 volt, then 1 joule of potential energy is given to each coulomb of charge. In other words, 1 volt = 1 joule per coulomb (1 V = 1 J/C). If the p.d across a cell is 2 volts, then 2 joules of potential energy is given to each coulomb of charge so on. ▪ If 2 J are given up by each coulomb, the p.d. is 2 V. 6𝐽 ▪ If 6 J are transferred when 2 C pass, the p.d. is 2𝐶 = 3 V. (3) S.I. unit of e.m.f. and p.d. S.I Unit of e.m.f and p.d is volts (4) Difference of e.m.f. and p.d. (check the definitions of the terms) ▪ The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts. When a current is being supplied, the voltage is lower because of the energy wastage inside the cell. A cell produces its maximum Potential difference(PD) when not in a circuit and not supplying current. (5) Relationship between p.d., electrical energy and charge. Defining PD E gy j u E PD across two terminals in volts = ; PD (v) = Ch g c u b Q Page 12 of 24 (6) Use of a voltmeter in circuits (How to connect?) Measuring Potential Difference Potential difference can be measured using a voltmeter The voltmeter should be connected in parallel ( across the terminals of the cell) with the part of the circuits you want to measure the potential difference of. (7) Properties of series and parallel circuits (in terms of e.m.f. and p.d.) 2.05 Chapter 38 – Resistance In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) S.I. unit of resistance Ohms Ω (2) Ohm’s law (Definition, equation and calculation) Ohm's Law state that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions, such as temperature, remain constant. A conductor that obeys Ohm's Law is called an ohmic conductor. Copper or constantan wire are examples of ohmic conductors. Page 13 of 24 (3) I-V graphs of ohmic and non-ohmic materials (4) Finding equivalent resistance of series and parallel resistors. Resistors in series Resistors in parallel In a parallel circuit, the total resistance is given by the formula In a series circuit, the total resistance is simply 𝟏 𝟏 𝟏 𝟏 the sum of the individual resistances, = + + 𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆 𝑹𝟏 𝑹𝟐 𝑹𝟑 𝑹𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 = 𝑹𝟏 + 𝑹𝟐 + 𝑹𝟑 𝑹𝟏 𝑹𝟐 𝑹𝟑 𝒑𝒓𝒐𝒅𝒖𝒄𝒕 𝒐𝒇 𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆𝒔 𝑹𝒆𝒇𝒇𝒆 = 𝑹𝟏 +𝑹𝟐 +𝑹𝟑 𝒔𝒖𝒎 𝒐𝒇 𝒓𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆𝒔 The combined resistance of 3 resistors in parallel is less than that of either resistor by itself. Page 14 of 24 (5) Relationship between resistance, length and cross-sectional area of a wire. (6) Definition of resistivity and its related equation. (i) directly proportional to its length l, i.e. R ∝ l, (ii) inversely proportional to its cross-sectional area 1 A, i.e. R ∝ 𝐴 (doubling A halves R). Combining these two statements, we get 𝑙 R∝ 𝐴 𝑙 𝑅=𝜌 𝐴 where 𝜌 is resistivity of the material 𝑹𝟏𝑨𝟏 𝑹𝟐𝑨𝟐 = (useful equation for numericals ) 𝒍𝟏 𝒍𝟐 2.06 Chapter 40 – Electric power In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Equations on page 177 of the textbook (2) Relationship between power, current and p.d. and Ohm’s law. (3) Calculations for circuits with three-heat switch Note: It should be noted that the contents of chapters 36, 37, 38, and 40 will usually be tested together in the form of analyzing a d.c. circuit (One may refer to the classwork for details). Key points Electrical power (in Watts) = current (in amperes) x Potential difference (in volts) P=VI Energy transferred = Power x time = current (amperes I) x potential difference (volts) x time E=IVt Important formulae Symbols and units for some electrical quantities l Q=It R = ρ Quantity Symbol Unit Symbol A for for unit quantity 𝐑𝟏𝐀𝟏 = 𝐑𝟐𝐀𝟐 for a E Current I Ampere A Emf (volts) = 𝒍𝟏 𝒍𝟐 Q wire of same Charge Q Coulombs C material Time t Seconds S E= VQ Potential V Volts V PD (v) = E P = I x V Q difference P= I2 R (PD) P= = E t Electromotive V Volts V 𝑽𝟐 force (EMF) 𝑷= 𝑹 Resistance R Ohms Ω V=IR ; I = V R ; R = V I E = I V t Power P Watts W E= I2 R t Energy E Joules J E= VQ Page 15 of 24 2.07 Chapter 43 – Generators In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Understanding and explanation of the discovery known as electromagnetic induction. (2) Understanding and application of Faraday’s law and Lenz’s law (in IGCSE- styled questions) (3) Application of Fleming’s right hand rule in IGCSE-styled questions. Electromagnetic Induction A current produces a magnetic field. However, the reverse is also possible: a magnetic field can be used to produce a current. Induced e.m.f. and current in a moving wire ▪ When a wire is moved across a magnetic field, as shown above left, a small e.m.f. (voltage) is generated in the wire The effect is called electromagnetic induction. ▪ Scientifically speaking, an e.m.f. is induced in the wire. ▪ If the wire forms part of a complete circuit, the e.m.f. makes a current flow. ▪ A galvanometer detects the small current, and shows deflection. Its pointer moves to the left or right of the zero, depending on the current direction. Definition of Electromagnetic Induction When magnetic field lines are cut by a conductor , an e.m.f is induced in the conductor. The induced e.m.f. (and current) can be increased by: ▪ moving the wire faster ▪ using a stronger magnet ▪ increasing the length of wire in the magnetic field – for example, by looping the wire through the field several times, as shown above right. Page 16 of 24 Induced e.m.f. and current in a coil ▪ If a bar magnet is pushed into a coil, as shown above left, an e.m.f. is induced in the coil. ▪ In this case, it is the magnetic field that is moving rather than the wire, but the result is the same: field lines are being cut. ▪ As the coil is part of a complete circuit, the induced e.m.f. makes a current flow. Increasing the Induced e.m.f. and current in a coil The induced e.m.f. (and current) in the coil can be increased by: ▪ moving the magnet faster ▪ using a stronger magnet ▪ increasing the number of turns on the coil (as this increases the length of wire cutting through the magnetic field). Experiments with the magnet and coil also give the following results. ▪ If the magnet is pulled out of the coil, as shown above right, the direction of the induced e.m.f. (and current) is reversed. ▪ If the S pole of the magnet, rather than the N pole, is pushed into the coil, this also reverses the current direction. ▪ If the magnet is held still, no field lines are cut, so there is no induced e.m.f. or current. Induced current direction: Lenz’s law ▪ The induced current turns the coil into a weak electromagnet whose N pole opposes the approaching N pole of the magnet. ▪ When the magnet is pulled out of the coil, the induced current alters direction and the poles of the coil are reversed. ▪ This time, the coil attracts the magnet as it is pulled away. So, once again, the change is opposed. Page 17 of 24 L z w: An induced current always flows in a direction such that it opposes the change which produced it. Induced current direction: Fleming’s right-hand rule Induced current direction: Fleming’s right-hand rule: If a straight wire (in a complete circuit) is moving at right angles to a magnetic field, the direction of the induced current can be found using Fleming’s right-hand rule, as shown: (4) Construction and operating principle of the A.C. generator (alternator). a.c generator – working The diagram shows a simple a.c. generator. It is providing the current for a small lamp. ▪ When the coil is rotated, it cuts magnetic field lines, so an e.m.f. is generated. This makes a current flow. ▪ As the coil rotates, each side travels upwards, downwards, upwards, downwards... and so on, through the magnetic field. ▪ So the current flows backwards, forwards... and so on. In other words, it is a.c. ▪ The graph shows how the current varies through one cycle (rotation). ▪ It is a maximum when the coil is horizontal and cutting field lines at the fastest rate. ▪ It is zero when the coil is vertical and cutting no field lines. ▪ Slip ring provides continuous connection while the coil is rotating. Page 18 of 24 Increasing the emf (and the current) in the generator The following all increase the maximum e.m.f. (and the current): increasing the number of turns on the coil increasing the area of the coil using a stronger magnet rotating the coil faster. Faster rotation also increases the frequency of the a.c. Mains generators must keep a steady frequency – for example, 50 Hz (cycles per second). Note: Students should refer to the relevant worksheets for details. 2.08 Chapter 44 – Transformers In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: Core of transformer is made of soft (1) Understanding and explanation of how a transformer works. iron and wires are of copper ▪ When alternating current flows in the primary (input) coil, it sets up an alternating magnetic field in the core and, therefore, in the secondary (output) coil. ▪ This changing field induces an alternating voltage in the output coil. Provided all the field lines pass through both coils, and the coils waste no energy because of heating effects. (2) Application of equations in transformers (P. 205 of the textbook) The voltage in each circuit is related to the number of coils on each side of a transformer by the following equation: 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑡𝑢𝑟𝑛𝑠 = 𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑡𝑢𝑟𝑛𝑠 𝑉𝑃 𝑁 𝑉1 𝑁 = 𝑁𝑃 OR =𝑁1 𝑉𝑠 𝑠 𝑉2 2 Power through a transformer If no energy is wasted in a transformer, the power (energy per second) delivered by the output coil will be the same as the power supplied to the input coil. So: Power = V x I Input voltage x input current = output voltage x output current In symbols : V1I1 = V2I2 As voltage x current is the same on both sides of a transformer, it follows that a transformer which increases the voltage will reduce the current in the same proportion, and vice versa. Page 19 of 24 (3) Understanding and application of transmission of electrical power (P.206-207 of the textbook. Paid special attention to “Use of high alternating p.d.s”) High or low voltage? The advantage of transmitting electricity at high voltages, rather than at low voltages. ▪ Transmission cables are good conductors, but they still have significant resistance – especially when they are hundreds of kilometres long. ▪ This means that energy is wasted because of the heating effect of the current. The calculations above demonstrate why less power is lost from a cable if power is transmitted through it at high voltage. ▪ By transmitting electricity at high voltage, the current is reduced, so thinner, lighter, and cheaper cables can be used. ▪ This reduces the heating effect. Power loss due to heating 𝑷 = 𝑰𝟐 𝑹 2.09 Chapter 45 – Electromagnets In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Structure and application of electromagnets Electromagnets consist of a coil of wire wrapped around a magnetically soft core and can be turned on and off. Current in the coil generates a magnetic field This external magnetic field aligns the atom in the soft core in the direction of the magnetic field. As a result the whole soft core become magnetised. When an electric current is passed through the wire, the core becomes magnetised. Electromagnets can be made stronger by Increasing the current Increasing the number of turns An electromagnet attached to a crane is used in scrap yards to lift car bodies or any other objects made of a ferrous material. Page 20 of 24 Distinguish between the design and use of permanent magnets and electromagnets Electromagnets Permanent magnets Soft magnetic material Hard magnetic material Easy to magnetise and demagnetise ( lose its Harder to magnetise and demagnetise (can retain its magnetism easily) magnetism) electromagnets have the ability to be turned on and Permanent magnets are more useful when they do off so they can be used for situations such as not need to be turned off moving scrap metal. Uses: transformers, electric bells, magnetic relays, Uses: D.C motors, A.C generators, galvanometers, reed relays. loud speakers and magnetic door catches. (2) Application of right hand grip rule to a current-carrying straight wire and a solenoid A rule for field direction :The direction of the magnetic field produced by a current is given by the right- hand grip rule shown above right. Imagine gripping the wire with your right hand so that your thumb points in the conventional current direction. Your fingers then point in the same direction as the field lines. other way. being negatively charged, flow the current direction. Electrons, “_” Th c cu The current arrows shown on g h conventional u “ ” Features of magnetic field around a current carrying wire Filed lines are ▪ Are circles centred on the the wire in a plane perpendicular to the wire. ▪ More widely spaced further from the wire. ▪ The spacing of the circles increases as the distance from the wire increases because the magnetic field lines become weaker as the distance from the wire increases. ▪ When the current direction is reversed , the magnetic field direction also reverses. Magnetic fields from coils A current produces a stronger magnetic field if the wire it flows in is wound into a coil. The diagrams show the magnetic field patterns produced by two current-carrying coils. One is just a single turn of wire. The other is a long coil with many turns. A long coil is called a solenoid. Page 21 of 24 A rule for poles - right-hand grip rule. Imagine gripping the coil with your right hand so that your fingers point in the conventional current direction. Your thumb then points towards the N pole of the coil. Features of magnetic field lines of current carrying solenoid The magnetic field produced by a current-carrying coil has these features: the field is similar to that from a bar magnet, and there are magnetic poles at the ends of the coil increasing the current increases the strength of the field increasing the number of turns on the coil increases the strength of the field. (3) Magnetisation and demagnetisation (P.201 of textbook) Methods of magnetisation c. Electrical method a. Stroking method Place the metal in a long coil of wire (solenoid) and pass a 1.Stroke unmagnetized ferromagnetic large DC (direct current) through the coil. material with north pole magnet from one end to the other. 2. Repeat many times 3. The starting end will be north b. Hammering Hammering a magnetic material in a magnetic field causes magnetisation but in the absence of a field it causes demagnetisation. Page 22 of 24 Methods of demagnetisation Hammering in the absence of magnetic field Heating and Use of alternating current (a.c.) in a coil (see fig) 1. Place the magnet in a solenoid. 2. Switch on an alternating current in the coil. 3. Remove the bar from the solenoid, without switching off the current. magnet would be alternately magnetised in different directions. Note: If the current is switched off before the magnet is removed, magnet would remain magnetised in the direction occurring at the moment of switching off. 2.10 Chapter 46 – Electric Motors In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Understanding and explanation of the motor effect The force on a current carrying conductor : ▪ A wire (copper wire) carrying a current in a magnetic field experiences a force. The force arises because the current produces its own magnetic field which acts on the poles of the magnet. ▪ The direction of the force, current or magnetic field is given by Fleming’s left-hand rule. ▪ The direction of the force changes if either the magnetic field or the current is reversed. ▪ The wire moves across the field and it is not attracted to either pole. The force is increased if: the current is increased a stronger magnet is used the length of wire in the field is increased. (2) Application of Fleming’s left hand rule to IGCSE-styled questions. (3) Application of Fleming’s left hand rule to d.c. motors ▪ The field direction is from the N pole of a magnet to the S pole. ▪ The current direction is from the positive (+) terminal of a battery round to the negative (-). This is called the conventional current direction. ▪ Fleming’s left-hand rule only applies if the current and field directions are at right angles. ▪ If they are at some other angle, there is still a force, but its direction is more difficult to predict. ▪ If the current and field are in the same direction, there is no force. Page 23 of 24 If a beam of charged particles (such as electrons) passes through a magnetic field, there is a force on it, just as for a current in a wire: Turning effect on a coil ▪ The coil on the right lies between the poles of a magnet. ▪ The current flows in opposite directions along the two sides of the coil. ▪ So, according to Fleming’s left-hand rule, one side is pushed up and the other side is pushed down. ▪ In other words, there is a turning effect on the coil. ▪ With more turns on the coil, the turning effect is increased. The turning effect can be increased by: Reversing the rotation can be done by: increasing the current reversing the battery using a stronger magnet reversing the poles increasing the number ofturns of coil (increasesshould Note: Students the length ofto refer coil theinrelevant the field)worksheets for details. Electric motors- Principle : If a coil is carrying a current in a magnetic field, as shown above , the forces on it produce a turning effect. Making motors more powerful The motor can be made more powerful by : increasing the current using a stronger magnet increasing the number of turns on the coil increasing the area of the coil. (A longer coil means higher forces because there is a greater length of wire in the magnetic field; a wider coil gives the forces more leverage (turning effect). Page 24 of 24 Fleming’s right-hand and left-hand rules. The two rules apply to different situations: when a current causes motion, the left-hand rule applies when motion causes a current, the right-hand rule applies. Flemming’s Right hand Rule Flemming’s Left hand Rule Right hand rule there is no battery connected in the wire. Left hand rule there is battery connected in the Movement of wire induces current (generator wire effect) Current flowing creates a motion (motor effect) 2.11 Chapter 47 – Electric Meters In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Connection of ammeters to a circuit (2) Connection of voltmeters to a circuit (3) Resistance of an ideal ammeter. (4) Resistance of an ideal voltmeter. 2.11 Chapter 48 – Electrons In addition to the skills as stated in the checklist at the end of the chapter, students should also pay special attention to the following: (1) Deflection of charged particles (positive and negative) under the influence of magnetic and electric fields. (Application of Fleming’s left hand rule in these scenarios) 3.0 Kind reminder This guideline serves to highlight the key points within the scope of this examination. Students should bear in mind that a good study plan and hard work are always keys to success in an examination. Hence, please commence your study early. Should you have any questions, feel free to ask your physics teacher in class or make an appointment if necessary. Let’s hope all of you will pass the examination with flying colours!