Induced potential and the generator effect

Figure 13 shows some apparatus used by a teacher in a demonstration.

The teacher moved the wire upwards between the magnets.

The needle on the ammeter deflected to a value of +0.4 mA and then returned to zero.

What effect did this demonstrate?

Explain why a current was detected when the wire in Figure 13 was moved upwards.

The teacher reversed the direction of the magnetic field.

The teacher replaced the wire in its original position.

The teacher moved the wire upwards in the same way as before.

What was the deflection of the needle on the ammeter?

Tick () one box.

Figure 14 shows a sound wave incident on the diaphragm of a moving-coil microphone.

The inside of the microphone includes a small coil of wire and a magnet.

Explain why the sound waves have an effect on the electric circuit.

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.

A dynamo is used to generate an electric current. Figure 22 shows the inside parts of the dynamo connected to a lamp.

The coil is rotated. Explain why a direct current is induced in the coil.

Sketch a graph on Figure 23 to show how the potential difference generated across the lamp varies for two complete revolutions of the dynamo coil.

The lamp is disconnected from the dynamo. Explain why the dynamo becomes much easier to turn.

Figure 10 shows a portable power supply.

The portable power supply has an alternator connected to a transformer. The transformer can be adjusted to have different numbers of turns on the secondary coil. Suggest why.

A lamp is connected to the power supply. The lamp requires an input potential difference of 5.0 V. The alternator generates a potential difference of 1.5 V. The primary coil of the transformer has 150 turns. Calculate the number of turns needed on the secondary coil.

Figure 11 shows the inside parts of the alternator.

The handle of the alternator is turned, causing the coil to rotate. Explain why an alternating current is induced in the coil.

Suggest the purpose of the slip rings.

The alternator from the portable power supply is disconnected from the transformer and lamp. Explain why the handle of the alternator becomes much easier to turn.

Table 3 gives the frequencies in the hearing ranges of five different animals.

Which one of the animals from Table 3 would not be able to hear ultrasound?

Figure 17 shows ultrasound being used to detect a hidden crack in a solid aluminium object. The transmitted and reflected pulses of ultrasound are shown on the screen.

Which of the following is the same as 2 microseconds? Tick (✓) one box.

Ultrasound travels at 6300 m/s in aluminium. Determine the depth of the crack below the top surface of the aluminium. Use information from Figure 17. Give your answer to two significant figures.

Figure 18 shows the parts of a moving-coil microphone.

What is the function of a microphone?

Explain how a moving-coil microphone works.

P-waves and S-waves are two types of seismic wave caused by earthquakes.

Which one of the statements about P-waves and S-waves is correct? Tick one box.

Seismometers on the Earth’s surface record the vibrations caused by seismic waves. Figure 3 shows the vibration recorded by a seismometer for one P-wave.

Calculate the frequency of the P-wave shown in Figure 3.

Write down the equation which links frequency, wavelength and wave speed.

The P-wave shown in Figure 3 is travelling at 7200 m/s. Calculate the wavelength of the P-wave.

Explain why the study of seismic waves provides evidence for the structure of the Earth’s core.

Figure 4 shows a simple seismometer made by a student.

To test that the seismometer works, the student pushes the bar magnet into the coil and then releases the bar magnet.

Why does the movement of the bar magnet induce a potential difference across the coil?

Why is the induced potential difference across the coil alternating?

Figure 5 shows how the potential difference induced across the coil varies after the bar magnet has been released.

Which statement describes the movement of the magnet when the induced potential difference is zero? Tick one box.

The seismometer cannot detect small vibrations. Suggest two changes to the design of the seismometer that would make it more sensitive to small vibrations.