## Tuesday, 23 September 2014

### Electro-Magnetic Induction Questions

Question 1
 A circular coil having 20 turns is placed in a uniform magnetic field of flux density 0·2T. The angle between the plane of the coil and the field is 30° and the radius of the coil is 5cm. Calculate the emf induced in the coil when the flux density is reduced steadily to zero in 2·5s.
Question 2
 A piece of copper, 30cm long, is horizontal and is at 90° to a horizontal magnetic field of flux density 1·8×10-5T. The piece of copper is allowed to fall freely (from rest) through this field. Calculate the emf induced in it at the instant when it is 5m below its starting point. Use g = 9·8ms-2.
Question 3
 A rectangular coil, 2cm by 3cm, rotates in a uniform magnetic field of flux density 0.15T. The axis around which the coil rotates is at 90° to the flux lines.The coil has 250 turns and its rotational frequency is 50s-1. a) Calculate the maximum emf induced in the coil. What is the position of the coil (relative to the flux lines) when the induced emf has its maximum value? b) What is the magnitude of the induced emf at a time 5×10-3s after it has passed through its maximum value. c) Calculate the magnitude of the induced emf at a time 2.5×10-3s after it has passed through its maximum value.
Question 4
 An aircraft is flying in a horizontal path. The magnitude of the vertical component of the flux density of the earth’s magnetic field surrounding the aircraft is 1×10-5T. The wingspan of the aircraft is 30m and it is flying at 75ms-1. a) Calculate the emf induced in the wings (it’s a metal aircraft!) b) If a copper wire is used to connect the extremity of the left hand wing to the extremity of the right hand wing, how much current will flow through this wire as a result of the induced emf?
Question 5
 Explain why it is harder to turn a bicycle dynamo when it is connected to a light bulb than when it is not connected to anything.
Question 6
 Some moving coil meters have a coil which is wound around a plastic support. Most moving coil meters have their coil wound around an aluminium support. State and explain the difference in the way the two types of meter behave.
Question 7
 When a simple d.c. electric motor is connected to a battery, the current which flows during the first few seconds varies (approximately) as shown by the graph below. Explain the shape of this graph.
Question 8
 a) Explain the operation of a step-up transformer. b) A transformer has 250 turns on its primary coil and 4000 turns on its secondary coil. It is connected to a 220V supply. Under normal operating conditions the current flowing through the secondary coil is 25mA and the transformer is 90% efficient (assume that themain source of inefficiency is the resistance of the secondary coil). Calculate i) the secondary voltage when on open circuit (that is, when the secondary coil is not connected to anything). ii) the current flowing through the primary coil when the current flowing through the secondary coil is 25mA.
Question 9
 This question is about rms currents and voltages. For the circuit shown above calculate a) the rms value of the current, I b) the maximum value of the current c) the mean power dissipated in R1 d) the rms value of the voltage across R2.