7.2 The Motor Effect

Higher Only

Figure 1 shows a device known as a current balance.

Figure 1

The switch is closed. Describe and explain the subsequent motion of the section of wire labelled A to B.

Higher Only

A small mass of 0.080 kg is used and placed 3.0 cm from the metal support. 

The section of wire labelled A to B is 6.0 cm long and is placed 8.0 cm away from the metal support.

The system is in equilibrium after the switch has been closed. 

Assume the mass of the plastic and the mass of the wire are negligible.

Calculate the magnetic flux density at the location of wire section AB when a current of 0.2 A passes through it. 

Give your answer to Y significant figures.

Take the value of acceleration due to gravity to be 10 m/s² for this question.

Use the Physics Equation Sheet. 

Higher Only

Figure 1 shows a wire passing between a North and South pole, with a flux density of 0.10 T between the poles.

The magnets cover 30 cm of the wire's length.

The wire has a resistance of 0.2 Ω and dissipates 5 J of energy every second. 

The current travels out of the plane of the page.

Figure 1

Calculate the magnitude of the force on the wire and state its direction.

Give your answer to 2 significant figures.   

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Figure 2 shows a diagram of a loudspeaker in a live music venue.

Figure 2

Explain why an alternating current in the coil causes the speaker cone to oscillate.

Higher Only

Figure 2 below shows a straight wire passing between the poles of two magnets.

Figure 2

When a current passes through the wire a force is exerted on the wire.

What is the name given to this effect?

Higher Only

Describe how the direction of the force can be determined using Fleming’s Left-Hand Rule.

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Add an arrow to Figure 2, showing the direction of the force.

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Explain why this force acts on the wire.

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Suggest three changes that would increase the force acting on the wire.

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The apparatus shown in Figure 3 below can be used to measure the force acting on a current-carrying wire placed in a magnetic field.

Figure 3

The balance is initially zeroed so that when the power supply is off, it shows no reading.

When the power supply is turned on, the balance shows a reading.

As the current is increased, the reading increases.

Explain in terms of forces why the reading increases.

Higher Only

For the above apparatus, when the current in the wire was 0.25 A, a force of 1.96 mN was exerted on the wire

The length of wire between the poles of the magnet was 0.042 m

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

Give your answer to 2 significant figures.

(You may need to look up the appropriate formula on the Physics Equations Sheet).

Higher Only

Figure 4 below shows a simple d.c. motor.

Figure 4

When a direct current is supplied to the coil, the coil starts to spin.

Explain why.

Higher Only

Describe three changes that would make the coil spin faster.

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Describe two changes that could reverse the direction of the coil.

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Figure 5 below shows a loudspeaker.

Figure 5

The loudspeaker produces a sound when an alternating current is connected to the coil.

Explain why.