The motor effect

Figure 7 shows some apparatus used to demonstrate the motor effect.

When the switch is closed the copper rod accelerates.

What direction is the acceleration of the copper rod?

Tick () one box.

The copper rod has a mass of 5.0 g.

The length of the copper rod in the magnetic field is 0.070 m.

The magnetic flux density is 0.30 T.

Calculate the initial acceleration of the rod when the current in the rod is 2.0 A.

Use the Physics Equations Sheet.

Figure 8 shows a simple motor.

There is a magnetic field between the two permanent magnets.

When there is a current in the coil, a magnetic field is produced around the coil.

Explain how these magnetic fields cause the coil to continuously rotate.

In Figure 8 the coil of the motor is in a horizontal position.

Explain why the resultant moment on the coil is zero when the coil is in a vertical position.

A megaphone uses a loudspeaker to amplify sounds that are detected by a microphone. Figure 10 shows a megaphone and microphone.

Complete the sentence.

The microphone is used to convert the pressure variations in sound waves into

The loudspeaker contains a permanent magnet. Which diagram in Figure 11 shows the direction of the magnetic field between the north pole and the south pole of the magnet? The magnets are shown in cross-section. Tick (✓) one box.

Some magnets are permanent magnets and some are induced magnets. What is an induced magnet?

Figure 12 shows the parts of the loudspeaker in the megaphone.

A current in the coil of the loudspeaker causes the coil to move.

What is the name of the effect that causes the coil to move? Tick (✓) one box.

When the current in the coil is 16 mA, the force on the coil is 0.013 N. The length of the wire that makes up the coil is 6.5 m. Calculate the magnetic flux density around the coil in the electromagnet. Use the Physics Equations Sheet.

Megaphones can produce very loud sounds. A person’s hearing can be affected by age and by working in a loud environment. Figure 13 shows how frequency affects the minimum sound level that can be heard by three different people, A, B and C.

Compare how different factors affect the minimum sound level that these people can hear.

The National Grid uses transformers to change potential difference (pd).

Figure 12 shows a transformer.

Identify the parts of the transformer labelled in Figure 12.

There is an alternating input pd of 230 V.

Determine the output pd.

Use the Physics Equations Sheet.

The input pd causes an alternating current.

Explain why there is an alternating current in the output when the transformer is connected to a circuit.

Figure 13 shows a large cable supported by two wooden poles. The cable is

connected to an electricity supply.

There is a force on the cable due to the Earth’s magnetic field when the current is in the direction A to B.

What is the direction of this force?

Tick (✓) one box.

The cable experiences a force of 0.045 N due to the Earth’s magnetic field.

magnetic flux density = 60 T

current = 50 A

Calculate the length of the cable between A and B.

Use the Physics Equations Sheet.

State one assumption you made in your calculation.

A student made a moving-coil loudspeaker. Figure 15 shows a diagram of the loudspeaker.

What is the name of the effect used by the moving-coil loudspeaker to produce sound waves?

Explain how a moving-coil loudspeaker produces a sound wave.

A student investigated how the loudness of sound from the loudspeaker depends on:

• the number of turns on the coil

• the frequency of the supply.

Table 2 shows the results.

Explain why the results cannot be used to make a valid conclusion.

A teacher demonstrated how a magnetic field can cause a copper rod to accelerate. The teacher placed the copper rod on two brass rails in a magnetic field. The copper rod was able to move. Figure 16 shows the equipment used.

The teacher closes the switch and the copper rod accelerates. Explain how Fleming’s left hand rule can be used to predict the direction in which the copper rod will move.

Suggest two changes to the equipment that would increase the force on the copper rod.

The teacher closed the switch and the copper rod accelerated uniformly from rest for 0.15 s. The current in the copper rod was 1.7 A. mass of copper rod = 4.0 g length of copper rod in the magnetic field = 0.050 m magnetic flux density = 0.30 T Calculate the maximum possible velocity of the copper rod when it left the magnetic field.

The circle in Figure 18 represents a straight wire carrying a current. The cross shows that the current is into the plane of the paper.

Figure 18

Complete Figure 18 to show the magnetic field pattern around the wire.

The magnetic flux density 10 cm from the wire is 4 microtesla. Which of the following is the same as 4 microtesla? Tick one box.

Figure 19 shows a simple electric motor.

Figure 19

When there is a current in the coil, the coil rotates continuously. Explain why.