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Motors and Generators Revision for Trials 1. Two wires of a 2.0 m long cord supplying power to an electrical appliance are 3.0 mm apart and carry a DC current of 8.0 A. Determine the magnitude and the direction of the force between the wires. 2. A coil of wire such as shown is 9.0 cm wide and is 12.0 cm deep (into the page). It carries a current of 2.5 A in a magnetic field of 0.5 T. a) Calculate the force on each wire when the coil is horizontal. b) Calculate the force on each wire when the coil is vertical c) Calculate the maximum torque. 3. Students investigating the construction of a small transformer open up a ‘plugpack’ designed to connect a 4 V mobile phone battery charger to the power mains. They discover that the transformer inside has a thin wire on the coils connected to the power mains and thicker wire on the output to the charger. The core is made up of thin sheets of iron. a) Why would the wire on the low voltage output be thicker than the high voltage input? (1 mark)? b) Why is the core made from thin sheets of iron? (2 marks) 4. With alternating current (AC) the current reverses its direction every 50th of a second. Suppose you applied alternating current to a DC motor as shown here. Predict what you think might happen to the motor. 5. The relative motion between a conductor and a magnetic field can create a voltage which in turn can produce a current in the conductor. Sometimes this induced current is useful while in other devices it is a nuisance and efforts are mode to reduce it. a) Name two devices where the induced current is useful and briefly describe how one of these operates. (3 marks) b) Name two devices where induced currents could be a problem and briefly describe how the induced currents could be reduced in one of these. (3 marks) 6. A rectangular coil in an AC generator has dimensions of 12.0 5.0 cm, and is made up of 54 identical loops. Initially it is lying parallel to a uniform magnetic field of 0.22 T directed horizontally across it, and it can spin about its long axis. y B x Spin axis a) State whether the current will flow towards x or towards y when the coil is turned clockwise in the diagram above. b) Explain how this device could be used to extract energy from the wind. c) Identify two structural changes which could be made to this device so as to improve its performance, and justify one of your statements. d) This device is turned into a motor by connecting the coil contacts to a 18 V battery pack. Calculate the current through the coil and hence the initial force acting on the side xy if the total resistance of the coil is 450 ohms. 7. The number of windings in a transformer’s primary coil is 500 and that of its secondary coil is 1500. The transformer is connected to a 240 V domestic power point. a) Calculate the potential difference across the secondary winding. b) Identify if this is a step up or a step down transformer 8. A pendulum is oscillating to and fro when a magnet is brought near as shown. a) Describe the effect this will have on the motion of the pendulum. b) Predict the direction in which the eddy currents will flow when the disc is in the position shown c) Slits are then cut in the pendulum as shown. Describe the effect this will have on the motion. 1. F = 8.5 x 10-3 N. For a complete circuit the current must flow to and from an appliance. Thus the currents are in opposite directions and the wires repel. 2. a) F = BIl = 0.5 × 2.5 × 0.12 = 0.15 N b) The force is unchanged at 0.15 N, but does not contribute towards the torque. c) max = nAIB = 1.0 × (0.12 ×0.09) × 2.5 × 0.5 = 1.35 × 10–2 Nm OR max = 2Fd = 2 ×0.15 × 0.045 = 1.35 × 10–2 Nm 3. a) Output has a lower voltage but a higher current and hence requires thicker wire b) Thin sheets reduce eddy currents in the core, so reduce losses through heating. Iron increases the magnetic field. 4. The purpose of the commutator is to reverse the direction of flow of the current in the coil each half cycle. This keeps the current flowing in the same direction next to each pole of the magnet. If AC was supplied, this may cancel the effects of the commutator so that the current reverses direction relative to the magnetic poles. This would cause the coil to oscillate but not to rotate. 5. a) Eddy current braking (balances), AC induction motors, induction cooktops, generators. b) Transformers, AC motors, input leads to audio amplifiers. AC motors: Any iron exposed to alternating fields is made of thin laminations tending to prevent circulating currents. The oxide on the iron is a sufficient insulator because the voltage is very small. 6. a) When the upper face of the coil is turned clockwise current will flow in the direction x to y b) The coil could be attached directly to an axle of a wind turbine, so it turns because the blades are forced to rotate as the wind blows on them. Generating electricity always requires energy to be put into the system (conservation of energy) – in this case the wind provides that energy. c) To improve the efficiency of this device the rotor could be forced to rotate in a radial magnetic field created by curved pole-pieces, because maximum emf is induced for more time as the rotor is turned through the field. Another improvement would be to increase the number of loops upwards from 54. d) V = I R 18 = 450 I I = 4.0 10-2 A FB = B I ℓ sin θ FB = 0.22 4.0 10-2 12.0 10-2 sin 90 newtons (per wire). There are 54 identical loops, each having a side x y, so FTOTAL = 5.7 10-2 m 7. a)Vp/Vs = np/ns; Vp = Vs × (np/ns) = 240.0 × (1500.0/500.0) = 720.0 V c) This is a step-up transformer because the voltage is increased. 8.a) As the metal moves through the magnetic field, eddy currents will be induced whose magnetic fields will oppose the change (movement) that induced the current. This will cause the metal sheet to slow down and stop. b For the position shown in which only half the metal sheet is in the magnetic field, the induced current is directed to oppose the movement to the right. The current produces a force to the right thus opposing the motion to the left. Inside the magnetic field this current will be upwards, and it will complete its circuit outside the magnetic field. c The slits reduce the area of metal in which eddy currents can be induced, so that a number of smaller eddy currents are produced. As a result, the induced magnetic field is less and the pendulum will