Electromagnetic Effects Notes (Physics 0625) PDF

Physics 0625 Electromagnetic Effects The magnetic effect of a current The effect in a wire A magnetic field is created when an electric current (charge) flows through a wire. A d.c. creates a constant magnetic field, while an a.c. creates an alternating magnetic field. The magnet...

Physics 0625 Electromagnetic Effects The magnetic effect of a current The effect in a wire A magnetic field is created when an electric current (charge) flows through a wire. A d.c. creates a constant magnetic field, while an a.c. creates an alternating magnetic field. The magnetic field around a current-carrying wire is circular, perpendicular to the wire ( Figure 1 ), and gets weaker with distance from the wire. The arrows show the direction of the magnetic field ( B ). Extended If you place a compass at any point around the wire, the north point of the needle will point in the direction of the magnetic field. Thus the direction of a magnetic field line at a point is the direction of the force on the north pole of a magnet at that point. The effect in a solenoid When the wire is arranged as a coil (known as a solenoid ), the resulting magnetic field is the same as that of a bar magnet ( Figure 2 ). Extended In Figure 2 , notice that the general direction of the current flow (left to right) indicates the direction of the magnetic field ( B ) in the solenoid. Places where the field lines are closest together indicate where the field is strongest – in this case, inside the solenoid. To increase the strength of the magnetic field you can:  Increase the current through the solenoid. Increase the number of turns of wire in the solenoid. Supplying a coil with a.c. has the effect of changing the direction of the current and therefore the direction of the magnetic field every half cycle. Applications of electromagnets Electromagnets are very useful. They can be designed to be extremely strong and can be turned on and off with a switch. They are commonly used in cranes to sort scrap metal, door locks, starter motors, relays and school bells. You need to be able to interpret diagrams of devices using electromagnets and explain how they work. The important points to remember are: A flow of current results in the generation of a magnetic field around a coil. The magnetic field will attract a magnetic material, for example an iron bar, and close the circuit. The closed circuit will now perform an action, such as ring a bell, turn on a different circuit or lock a door. Figure 3 is a relay, used to switch between separate circuits. When the electromagnet is supplied with current, a magnetic field is generated. The field attracts the iron bar, which is pivoted and allows the conducting contacts at A to touch. The second circuit containing the bulb is now closed and current flows. Q Which rule should you use to find out the direction of the magnetic field around a straight wire? #1 Left-hand rule #2 Corkscrew rule #3 Right-hand rule #4 Wave equation Force on a current-carrying conductor Force, magnetic field and current You already know that a current-carrying wire has a magnetic field around it. By placing the wire within another magnetic field, it is possible for the wire to experience a force and therefore move. To increase the size of the force: Increase the current in the wire. Increase the number of individual wires. Increase the strength of the magnetic field. Increase the length of the wire within the magnetic field. Changing the direction of the magnetic field or current changes the direction of the force. To find out the direction of the force, use Fleming’s left-hand rule (Figure 1). Notice that the force (F), current (I) and field (B) are perpendicular to each other. If the field and current are parallel (going in the same direction), no force will be acting. The apparatus shown in Figure 2 can be used to determine the force on a current- carrying conductor in a magnetic field. Extended By applying Fleming’s left-hand rule, you can deduce the direction of the force on the wire in Figure 2: Field: left to right Current: into the page Force: down If either the current or the field are reversed, the force will be upwards. Forces on charged particles Any moving charged particle within a magnetic field will experience a force. Conventional current flows from positive to negative, and so a positive charge will move in that direction. A negative charge moves in the opposite direction to current flow. To use Fleming's left- hand rule with a negative charge, you must first label current flow in the opposite direction to the negative charge’s motion. Figure 3 shows the path taken by charged particles in an experiment to determine the direction of forces on the particles. Although you can see the path of the particles in Figure 3, you could use Fleming's left-hand rule to deduce the direction of the force on the positive and negative particles. Remember the notations for 'into the page' and 'out of page' can be used for magnetic fields too. Positive particle: Field: into the page. Current: left to right. Force: up. Negative particle: Field: into the page. Current: right to left (because it is a negative particle). Force: down. Q A current-carrying wire is perpendicular to a magnetic field and moves up. If the current is reversed, in which direction will the wire move? #1 No movement #2 Down #3 Left #4 Right The d.c. motor Electric motors are incredibly reliable. They only have one moving part and can spin very slowly or very quickly without the use of gears. They can be controlled precisely and are used in thousands of applications including lifts, washing machines, trains, blenders, robots, drones and even cars. We know that a current-carrying wire in a magnetic field experiences a force. This principle combined with clever design gives us the d.c. motor. Watch this animation to see a d.c. motor in motion. a current-carrying wire in a magnetic field experiencing a turning effect. In order to make the motor spin more quickly, you can: Increase the strength of the magnets and thus the magnetic field. Increase the number of turns of wire in the coil. Increase the current to the coil from the power supply. Explaining the d.c. motor Extended You can use the left-hand rule to explain how the motor achieves rotation. The left-hand side of the coil is perpendicular to the magnetic field and therefore experiences a force downwards. (Field: left to right. Current: into the page. Force: down.)  The right arm of the coil is carrying current in the opposite direction and is also perpendicular to the magnetic field, and so experiences a force upwards. A turning force causes the coil to spin about the pivot. Brushes allow constant contact whilst the coil spins freely. The split ring commutator ensures that current always flows the same way around the loop. In this case, the turning force always acts in a anticlockwise direction. Electromagnetic Induction Induction As you know from magnetism, 'induce' means to generate or bring about something. This subtopic describes how electromotive force e.m.f is induced. This is the basic principle behind all electricity generation. A wire near a changing magnetic field will experience an induced e.m.f. This is known as Faraday's Law. Try this simulation to observe Faraday's Law. Move the magnet in and out of the coil at different speeds and observe the brightness of the bulb and the magnitude and sign of the voltage. Try the different coils and reversing the magnet. Note the effects. https://phet.colorado.edu/sims/html/faradays-law/latest/faradays-law_en.html An e.m.f. will also be induced if the magnet remains stationary and the coil of wire is moved. There are therefore two ways to induce an e.m.f. Moving a magnet so that its field lines are cut by a wire. Moving a wire within a magnetic field. This practical activity demonstrates electromagnetic induction. Having done the simulation and the practical activity, you should understand that: Increasing the force or speed at which you move the wire or magnet increases the size of the induced e.m.f. Increasing the strength of the magnet increases the size of the induced e.m.f. Increasing the number of turns in the wire increases the size of the induced e.m.f. Note: An e.m.f. is not induced when the wire moves parallel to the magnetic field. Extended Fleming’s right-hand rule When an e.m.f. is induced in a wire, use Fleming’s right-hand rule to work out the direction of force (motion), magnetic field and induced current ( Figure 2 ). The direction of M otion of the wire (relative to the field) is represented by the thu M b. The direction of the magnetic F ield is represented by the F irst finger. The direction of the C urrent is represented by the se C ond finger. Solution: Follow the guidance about using the right-hand rule to see if you agree with the following answers. (a) Field: up. Motion: right. Current: out of page. (b) Field: right. Motion: up. Current: into page. Lenz's Law: Opposing change Lenz’s Law states that the direction of the induced e.m.f. opposes the change that creates it. Watch the video, which shows a magnet falling slowly through a copper tube. The moving magnet creates a changing magnetic field, as in the earlier simulation. An e.m.f. is therefore induced in the copper tube. The direction of the induced e.m.f. is such that it opposes the change that created it (i.e., the falling magnet), and thus provides a force which slows the magnet’s fall. https://www.youtube.com/watch?v=_Nj69iJLo-w (2) FeelFlux Original – YouTube Question: Which of the following would not induce an e.m.f.? a.c. generator a.c. and d.c. Current is the rate of flow of charges (electrons) around a circuit ( see subtopic 4.2 ). We can generate two different types of current: a.c. – alternating current: electrons continuously change direction. d.c. – direct current: electrons flow in one direction only. A voltage (e.m.f.) source is needed to make both a.c. and d.c. For a.c., the source must cause electrons to move back and forth; for d.c., electrons must flow in one direction. Figure 1 shows how the voltage of a.c. and d.c. sources compare. Notice that for an a.c. supply, the voltage must reduce to zero and then become negative, causing the electron to move in the opposite direction. Extended An a.c. generator You have learnt that a moving wire in a magnetic field will induce an e.m.f. Figure 2 shows the arrangement for an a.c. generator, which allows a rectangular coil of wire to continuously rotate within a magnetic field, and so an e.m.f. is induced. You can relate the position of the generator coil to the peaks and zeros of the voltage output shown in the voltage–time graph in Figure 3. A The top and bottom sides of the coil are moving parallel to the magnetic field, and so no e.m.f. is induced. B The long sides of the rectangular coil move exactly perpendicular (90°) to the magnetic field, and so maximum e.m.f. is induced. C Again, the top and bottom sides of the coil are moving parallel to the magnetic field, and so no e.m.f. is induced. D Again, the long sides of the coil move perpendicular (90°) to the field but in the opposite direction. Therefore, the induced e.m.f. is again maximum, but in the opposite direction. You can determine the direction of the induced current in the coil using Fleming's right- hand rule. Generating d.c. (rectification) To generate d.c., you can either simply use an electrical cell, which makes use of a chemical reaction, or you can rectify an a.c. supply to make it d.c. If you connect an a.c. source to a diode, only one half of the supply will pass through the diode – the negative half of the current cycle is blocked by the diode ( Figure 4 ). Notice that although the rectified waveform (on the right) increases and decreases, it never becomes negative. The rectified output is therefore d.c. – the electrons can move in only one direction, even though the e.m.f. provided by the power supply varies. Transformer What is a transformer? A transformer is a device that is able to increase or decrease the size of an alternating e.m.f. ( Figure 1 ). There are two types of transformer: a step-up transformer increases voltage a step-down transformer decreases voltage. A transformer consists of: A primary coil through which a.c. is supplied; this is the origin of all electrical energy used in a transformer. A soft iron core , which is designed to allow the transition of magnetic flux to a secondary coil. The secondary coil is the output of the transformer and will have more or less coils than the primary depending on whether it is a step-up or a step-down transformer. How do transformers work? Extended Transformers only work with a.c. because they require a changing magnetic field. The primary coil is supplied with an alternating current and behaves like a bar magnet that is constantly switching its poles due to the constantly changing current (a bit like a rotating bar magnet).  A constantly changing magnetic field is transferred to the secondary coil via the soft iron core. Because we now have a conductor (secondary coil) in the presence of a changing magnetic field, an e.m.f. is induced across the secondary coil. The e.m.f induced across the secondary coil is also constantly changing and therefore provides an alternating current. The size of the induced e.m.f. depends on how many turns there are in the secondary coil compared to the primary coil. A step-up transformer has more turns on the secondary coil than on the primary coil. A step-down transformer has fewer turns on the secondary coil than on the primary coil. For example, if the secondary coil has twice as many turns as the primary coil, then the output voltage will be twice as big. This concept can be applied using the following equation: Worked example 1 A step-down transformer has 200 turns of wire on the primary coil and 50 turns of wire on the secondary coil. If an e.m.f. of 320 V is supplied to the transformer, what is the output e.m.f.? Transformers can be designed to have hundreds of times more turns on the secondary coil in order to produce huge output voltages. For example, a Tesla coil is a device used to demonstrate the arcing of electricity when very large voltages are present. A Tesla coil acts as a large step-up transformer. Extended Conservation of energy in a transformer You may be wondering why we cannot make a lot of voltage by simply wrapping more coils of wire around an iron core, and thereby lowering energy costs for every household. However, conservation of energy still applies (if the voltage increases, the current decreases), and so the power in is equal to the power out (assuming no losses): Note that this is only true when we assume the transformer is 100% efficient. Worked example 2 A step-up transformer has an output power of 1000 W. If the current in the primary coil is 25 A, what is the input e.m.f. in the primary coil? Efficiency in transformers In physics, we usually assume that the efficiency of a transformer is 100%, so the electrical power on the primary coil is equal to the electrical power induced on the secondary coil. However, you may be asked to calculate the efficiency of a transformer. In which case, use one or more of the following equations, depending on the information you are supplied with: The efficiency of a transformer can be increased by: Using low resistance coils to reduce the power wasted due to the heating effect of the current. Using a laminated core which consists of layers of iron separated by layers of insulation; this reduces heating in the iron core and prevents currents being induced in the core itself (referred to as eddy currents). Why do we need transformers? Electricity companies are keen to eliminate any waste. If electrical transmission cables heat up, energy is not making it to the consumers’ homes and therefore cannot be sold for a profit. An inefficient transformer wastes power and money! Transformers are designed to increase the voltage at which electricity is transmitted. The advantages of such high-voltage transmission are that the current in the cables is lower and less energy is wasted as heat. In addition, the cables used to carry the current have a large cross-sectional area, and this lowers the resistance and energy wasted. Figure 2 shows a system of step-up transformers being used to increase the voltage (reducing the current) of an alternating electrical supply as it leaves a power station. Thick cables held above the ground by pylons carry the supply to your neighborhood. A series of step-down transformers lowers the voltage to a safe level and increases the current to be used in your home. Exam questions: Q1. Why an AC voltage is connected with the primary coil of a transformer? Ans: An AC voltage keeps changing the magnetic field, due to which an emf is induced in the secondary coil. Q2. Why a soft iron core is used in a transformer? Ans: Soft Iron can be easily magnetised, the core increases the flux linkage bwtween the two coils. Q3. Why a laminated core is used in a transformer? Ans: To avoid the Eddy currents and hence to reduce the heat loss. Q4. Why a split ring commutatator is used in a DC motor? Ans: To reverse the direction of current every half rotation and keep the coil rotating through 3600 in one direction. Q5. What is meant by the direction of an electric field? Ans: The direction of an electric field is the direction of force on a positive charge placed at that point. Q6. What is meant by the direction of magnetic field? Ans: The direction of a magnetic field is the direction of force on a north pole of a magnet placed at that point.