Electromagnetic induction for Cambridge IGCSE Physics

Reading cue. The arrows leave the north pole and enter the south, so the field runs left to right; the force is perpendicular to both field and wire, so it cannot lie in the plane of the page.

1. aDescribe Describe what happens to the force on the wire in Fig. 5.1 if the current I is reversed so that it flows down the page instead of up.

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   Check answer · 2 marks

   1. the force acts in the opposite direction
   2. its size is unchanged, because neither the current nor the strength of the field has been altered

2. bDescribe The two magnets in Fig. 5.1 are now exchanged, so that a south pole faces the gap from the left and a north pole from the right, and at the same time the current is reversed. Describe the effect of these two changes together on the force on the wire.

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   Check answer · 3 marks

   1. reversing the field alone would reverse the force, and reversing the current alone would reverse it as well
   2. with both reversed the two changes cancel each other
   3. the force therefore acts in the same direction as before and has the same size

3. cExplain The wire in Fig. 5.1 is turned slowly in the plane of the page, away from the vertical position drawn, until it finally lies horizontally along the field arrows. Explain how the size of the force on the wire changes as it is turned, and state the position in which the force is largest.

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   Check answer · 4 marks

   1. the force is largest in the position drawn in Fig. 5.1, with the wire at right angles to the field arrows
   2. as the wire is turned away from that position the force becomes steadily smaller
   3. when the wire lies along the field arrows the force is zero, because no part of the current then crosses the field
   4. throughout the turning the force stays perpendicular to the page, so only its size changes

4. dSuggest Suggest three separate changes to the arrangement in Fig. 5.1, each of which would increase the size of the force on the wire, and explain why only the length of wire lying between the pole faces affects that force.

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   Check answer · 4 marks

   1. increase the current in the wire
   2. use stronger magnets, or bring the pole faces closer together, so that the field across the gap is stronger
   3. increase the length of wire lying in the field, for example by using wider pole faces or by replacing the single wire with several wires side by side carrying the current the same way
   4. outside the gap the field is very weak, so the parts of the wire beyond the pole faces experience almost no force and do not contribute

Reading cue. Take the turns from the labels, not from the loops drawn: five on the left and seven on the right, but the ratio that decides the output is 200 to 5000.

1. aIdentify Identify which coil in Fig. 6.1 has the greater number of turns, and state whether the transformer is a step-up or a step-down transformer.

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   Check answer · 2 marks

   1. the coil wound on the right-hand limb, the secondary, with 5000 turns
   2. it is a step-up transformer, because the secondary has more turns than the primary

2. bExplain The 12 V a.c. supply in Fig. 6.1 is replaced by a 12 V d.c. supply. Explain what a voltmeter connected across the output terminals would read.

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   Check answer · 3 marks

   1. the voltmeter reads zero, apart from a momentary reading as the supply is switched on or off
   2. a steady direct current produces a steady magnetic field in the iron core
   3. there is then no change of magnetic field through the secondary coil, so no e.m.f. is induced in it

3. cDetermine The output terminals in Fig. 6.1 are connected to a lamp, and the current in the primary coil is 0.50 A. Assuming the transformer is 100% efficient, determine the current in the secondary coil and the power delivered to the lamp.

   demanding3 marks

   Check answer · 3 marks

   1. input power = 12 × 0.50 = 6.0 W, and at 100% efficiency the lamp receives 6.0 W
   2. secondary current = primary current × 200/5000 = 0.50 × 0.040
   3. current in the secondary = 0.020 A (20 mA)

4. dExplain The core in Fig. 6.1 is drawn as a solid rectangle of iron. In a real transformer it is built from thin sheets separated by insulation. Explain how this changes what happens in the core, and describe one further reason why a real transformer is not 100% efficient.

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   Check answer · 4 marks

   1. the changing magnetic field induces currents in the iron of the core itself
   2. in a solid core these currents circulate freely and heat the core, so energy from the supply is wasted
   3. insulated sheets break up the paths available to these currents, so they are much smaller and less energy is wasted
   4. one further loss: the copper coils have resistance, so the current in them heats the windings (accept: not all the field from the primary passes through the secondary, or energy is wasted repeatedly magnetising the core)

Reading cue. Read the pole from the end of the magnet nearest the coil, not the labelled far end, and note the arrow sits on the magnet: the magnet moves and the coil stays still.

1. aIdentify Identify the feature of the meter drawn in Fig. 8.1 that lets the direction of the induced current be found, and state what its needle reads before the magnet is moved.

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   Check answer · 3 marks

   1. it is a centre-zero ammeter / zero is at the middle of the scale (1)
   2. the needle can swing either side of zero, so the direction of the current is shown (1)
   3. reads zero before the magnet moves (1)

2. bState For the motion drawn by the arrow in Fig. 8.1, state which magnetic pole is produced at the end of the coil nearest the magnet, and state the effect this has on the magnet as it moves in.

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   Check answer · 2 marks

   1. the near end of the coil becomes a north pole (1)
   2. it repels the approaching north pole of the magnet / opposes the magnet's motion (1)

3. cExplain The magnet in Fig. 8.1 is now held still and the coil is moved to the left towards it, at the same speed. Explain what the ammeter shows.

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   Check answer · 4 marks

   1. the needle deflects in the same direction as before (1)
   2. by the same amount (1)
   3. only the relative movement of magnet and coil matters (1)
   4. the field through the coil changes at the same rate, so the same e.m.f. is induced (1)

4. dExplain The two wires running down from the coil in Fig. 8.1 are disconnected from the ammeter and the magnet is pushed in again at the same speed. Explain why less force is now needed to push the magnet in, and state the source of the energy that was previously measured as a current.

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   Check answer · 4 marks

   1. with the circuit complete an induced current flows in the coil (1)
   2. this current makes the coil into a magnet whose near pole repels the incoming north pole, so a force must be overcome (1)
   3. with the wires disconnected the circuit is broken, so no current flows and there is no opposing force (an e.m.f. is still induced) (1)
   4. the electrical energy came from the work done by the person pushing the magnet / from the magnet's kinetic energy (1)