6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Learn this properly: Magnets and Magnetic FieldsA simple direct current (d.c.) motor consists of a rectangular coil of wire placed between the poles of a permanent magnet, connected to a d.c. supply through a split-ring commutator.
Describe and explain how this d.c. motor works. In your answer you should explain why a force acts on the sides of the coil, why the coil rotates, and the role of the split-ring commutator. (6 marks)
A class investigated the motor effect by measuring the force on a current-carrying wire placed in a magnetic field for different currents, keeping the magnetic flux density and the length of wire in the field constant. Their results are shown below.
| Current / A | Force / N |
|---|---|
| 1.0 | 0.06 |
| 2.0 | 0.12 |
| 3.0 | 0.18 |
| 4.0 | 0.24 |
The length of wire in the field was 0.20 m.
(a) Using a row from the table, calculate the magnetic flux density B. Use F=BIl, rearranged as B=IlF. Show your working and give the unit. (3 marks)
(b) State the relationship between the force and the current shown by the data. (1 mark)
A wire of length 0.15 m carries a current of 5.0 A. It is placed at right angles to a magnetic field of flux density 0.40 T.
(a) Calculate the force on the wire. Use F=BIl. Show your working and give the unit. (2 marks)
(b) State the rule used to find the direction of this force. (1 mark)
A transformer has a primary coil with 2000 turns connected to a 230 V alternating supply. The secondary coil has 100 turns.
(a) Calculate the output (secondary) voltage. Use VsVp=NsNp. Show your working and give the unit. (2 marks)
(b) State whether this is a step-up or a step-down transformer. (1 mark)
A steel paperclip is not a magnet, but it is attracted to a permanent magnet when the magnet is brought near it.
Explain, in terms of induced magnetism, why the paperclip is attracted to the permanent magnet. (2 marks)
Magnetic field lines are used to represent a magnetic field around a bar magnet.
State the direction in which magnetic field lines are drawn (between the poles of a magnet). (1 mark)