The motor effect

The battery inside an electric toothbrush is recharged when the toothbrush is placed on its charger.

Figure 9 shows part of the toothbrush and the charger.

Explain how the current is produced in the secondary coil of the transformer in Figure 9.

The potential difference across the primary coil is 230 V.

The potential difference across the secondary coil is 1.5 V.

The primary coil has 1840 turns of wire.

Calculate the number of turns of wire in the secondary coil.

number of turns =_........

In Figure 9, a diode is connected in series with the rechargeable battery.

Explain why a diode is needed in the toothbrush in this part of the circuit.

Figure 10 shows a simplified diagram of the split ring commutator in a d.c. electric motor.

Explain the function of the commutator in a d.c. electric motor.

Figure 13 represents the Earth.

Figure 13 shows two magnetic compass needles placed near to the Earth’s surface, at points Q and T.

Each magnetic compass needle can rotate about its central dot.

A compass needle is placed at point P and another at point R, near to the Earth’s surface.

On Figure 13, draw an arrow at point P and an arrow at point R to show the direction of the compass needle at each point.

Explain why the arrows point in the directions you have drawn in part (i).

You may draw on Figure 13 to help your answer.

Figure 14 shows a wire placed between the poles of a U-shaped magnet.

The wire is connected to a resistor and a battery.

The wire carries a current in the direction shown.

The wire is perpendicular to the magnetic field of the magnet.

Draw an arrow on Figure 14 to show the direction of the force, F, acting on the wire.

Label this arrow ‘F’.

State one practical way of reversing the direction of force F.

In Figure 14

• current in the wire = 3.2 A

• length of wire in the magnetic field = 0.042 m

• magnitude of the force on the wire = 0.078 N

Calculate the magnitude of the magnetic flux density between the two poles of the magnet.

magnetic flux density =_........

Figure 8 shows two magnets with their N poles facing each other.

Figure 8

On Figure 8, draw the shape and direction of the magnetic field between the two magnets.

Figure 9 shows a toy that has a plastic cylinder, a plastic base and two similar magnets. Each of the two magnets is in the shape of a ring.

The upper magnet seems to float in the air above the lower magnet.

Describe the forces acting on the upper magnet.

Use the idea of magnetic fields in your answer.

Figure 10 shows a current‑carrying wire between the poles of a magnet.

The magnet and the wire each experience a force when there is a current in the wire.

1 State the direction of the force on the wire.

2 State the direction of the force on the magnet.

The force on the wire is 0.15 N.

The current in the wire is 2.7 A.

The magnet produces a field with a magnetic flux density of 0.50 T.

Calculate the length of the wire in the magnetic field.

Use an equation selected from the list of equations given at the end of the question paper.

length of the wire in the magnetic field =_........

Figure 11 shows the shape of the magnetic field near a bar magnet.

Figure 11

Draw arrows on the field lines in Figure 11 to show the direction of the magnetic field.

Place a letter X on Figure 11 at a place where the magnetic field is strongest.

Describe two differences between the magnetic field shown in Figure 11 and a uniform magnetic field.

State how a uniform magnetic field may be obtained in a school laboratory.

Figure 12 shows the directions of some plotting compass needles placed at different points near the Earth’s surface.

Figure 12

Sketch, on Figure 12, the Earth’s magnetic field outside and inside the Earth.

State which part of the Earth generates its magnetic field.

A wire is placed at right angles to the Earth’s magnetic field.

The wire is 0.600 m long and carries a current of 93.1 mA.

The force on the wire is 1.11 × 10⁻⁵N.

Calculate the magnetic flux density of the Earth’s magnetic field.

Use the equation

F = B × I × l

magnetic flux density =_........

A student investigates moments of forces.

Figure 14 shows the apparatus used.

The pivot is under the centre of the rod.

A magnet is fixed to one end of the rod.

A piece of modelling clay is fixed to the other end of the rod.

The system is in equilibrium.

State the relationship between the moment of the weight of the magnet and the moment of the weight of the piece of modelling clay about the pivot.

The student fixes a coil to the bench under the magnet as shown in Figure 15.

The coil of wire is connected to a d.c. power supply so that there is a current in the coil.

To bring the system back into equilibrium, the student hangs a 0.050 N weight on the rod, 8.4 cm away from the pivot, as shown in Figure 15.

Calculate the size of the force between the magnet and the coil.

Describe how the student could develop the investigation to determine if the size of the force between the magnet and the coil is directly proportional to the size of the current in the coil.

The student reverses the direction of the current in the coil. Describe how the student can bring the system back into equilibrium without making any changes to the magnet.

Figure 11 shows the magnetic field of a magnet.

At which point is the magnetic field strongest?

Figure 12 shows a wire carrying a current.

Draw, on the card in Figure 12, the magnetic field that is produced by the current.

Figure 13 shows two metal rods carrying a current.

A metal roller touches both rods and completes the circuit. The roller is in the magnetic field produced by a magnet.

The magnetic flux density of the magnetic field at the roller is 1.2 T.

The current in the roller is 2.5 A.

The length of the roller carrying the current is 0.060 m.

Calculate the force on the roller.

Use the equation

Describe how Fleming’s left‑hand rule can be used to determine the direction of the force acting on the roller.

You may draw a diagram to help your answer.

Draw an arrow on Figure 13 to show the direction of the force acting on the roller.

A student uses a plotting compass to investigate the magnetic field around a wire.

Figure 10 shows the wire going straight through a card.

Figure 10 shows the compass needle when there is no current in the wire.

Which of these shows a possible direction of the compass needle when there is a current in the wire going from P to Q?

Describe how the student could develop the investigation to find the shape of the magnetic field produced by the current.

Figure 11 shows a copper wire between two magnetic poles.

The current in the wire is in the direction shown by the arrow.

The wire experiences a force due to the magnetic field.

The direction of the force due to the magnetic field is

The interaction between the magnetic fields produced by the magnet and the current in the wire produces forces on the magnet and the wire.

Compare these two forces.

Figure 12 shows a different wire inside a uniform magnetic field.

The magnetic flux density of the magnetic field is 0.72 N/A m. The length of the wire inside the field is 30 mm. The size of the force due to the magnetic field on the wire is 0.045 N.

Calculate the size of the current in the wire.

Use an equation selected from the list of equations at the end of this paper.