Chapter 7: Review

The chapter problem, solved

The scrapyard crane lifts steel scrap with an electromagnet. The magnet is a coil of 600 turns on an iron core, carrying a current of 4.0 A.

1. Why an electromagnet, and not a permanent magnet. An electromagnet can be switched on and off. The crane switches the current on to pick up the steel scrap, and switches it off to release the load. A permanent magnet would hold on to the scrap and could not let go.

2. Why the magnet attracts steel but not aluminium. Steel is a magnetic material. When the steel comes near the coil, it becomes an induced magnet, and induced magnetism always causes a force of attraction. Aluminium is not a magnetic material, so the magnet does not pull on it. Copper is not magnetic either, so it is left behind with the aluminium.

3. The field of the coil. Inside the solenoid the field is strong and uniform. The field lines are straight, equally spaced and parallel to the axis of the coil. Outside the coil, the field has the same shape as the field of a bar magnet, with a north pole at one end and a south pole at the other.

4. The iron core. The field of the coil magnetises the iron. The magnetised core adds its own field to the field of the coil, so the total field is much stronger. When the current is switched off, the coil's field goes and the iron loses its magnetism, so the steel drops.

5. The transformer. The yard is supplied at 12 000 V and steps this down to 240 V. The primary coil has 4000 turns.

The secondary coil has 80 turns. This is a step-down transformer.

6. The current in the transmission line. The yard uses 24 kW of power.

The current in the 12 000 V line is 2.0 A.

7. The microphone and the loudspeaker. The microphone turns sound into a varying current. Sound waves make the diaphragm and coil move in a magnetic field, so a potential difference is induced. This is the generator effect. The loudspeaker does the opposite. The varying current flows through a coil in a magnetic field, so the coil feels a changing force. This is the motor effect. The force makes the cone vibrate and send out sound.

The judgement that an electromagnet is the right choice for a scrapyard crane is a reasoned answer. It follows from the facts above. Whether the choice is safe and efficient is not the kind of claim a computer can check, so compare your reasoning with the evidence in the working.

Mixed practice — choose the method first

For each question, say which equation or idea you will use before you calculate. Keep your units with your numbers.

1. The scrapyard crane uses an electromagnet instead of a permanent magnet. Explain why.
2. The crane magnet attracts the steel scrap but not the aluminium. Explain why.
3. Describe the magnetic field inside the crane solenoid.
4. Explain how the iron core makes the crane electromagnet stronger.
5. H The yard transformer steps 12 000 V down to 240 V. The primary coil has 4000 turns. Calculate the number of turns on the secondary coil.
6. H The yard uses 24 kW of power from a 12 000 V transmission line. Calculate the current in the line.
7. H Explain how the office microphone turns the yard manager's voice into a varying current.
8. H Explain how the office loudspeaker turns the varying current back into sound.
9. Two magnets are placed with their south poles facing each other. Describe the force between them, and give a reason.
10. The crane magnet is held 15 cm above the scrap. Write this distance in metres.
11. H A wire of length 0.25 m in a motor carries a current of 2.0 A and sits at right angles to a magnetic field of flux density 0.30 T. Calculate the force on the wire.
12. The crane electromagnet runs from a 240 V supply and takes a current of 4.0 A. Calculate the power it uses.
13. A steel block of mass 2.0 kg is lifted 1.5 m onto a pile. Take g = 9.8 N/kg. Calculate the energy transferred to the gravitational store of the block.
14. The crane magnet supports a steel plate of mass 5.0 kg. Take g = 9.8 N/kg. Calculate the weight of the plate.
15. H A transformer has 2000 turns on the primary coil and 500 turns on the secondary coil. The primary potential difference is 240 V. Say which relationship you will use, then calculate the secondary potential difference.
16. H A student says a loudspeaker works by the generator effect. Explain why the student is wrong, and name the correct effect.

What you can now do

• describe the attraction and repulsion between magnetic poles, and tell a permanent magnet from an induced magnet;
• describe a magnetic field, plot it with a compass, and explain the evidence that the Earth's core is magnetic;
• describe how a current creates a magnetic field, and how a solenoid and an iron core make an electromagnet;
• state Fleming's left-hand rule and use $F = BIl$ (Higher tier);
• explain the generator effect and how an alternator and a dynamo work (Higher tier);
• use $\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}$ and $V_s I_s = V_p I_p$, and explain why the national grid uses a high potential difference (Higher tier);
• explain how a microphone and a loudspeaker work, and which effect each one uses (Higher tier).

Your relevance reflection

Return to the prompt from the chapter opener: where in your own week does a magnet hold, move or switch something? Write one more example now that you have finished the chapter. Has the chapter changed what you notice about a doorbell, a speaker or an electric motor?