Electromagnetism & The Motor Effect Practice Questions

Question 1

6 marks

Figure 1 shows a long straight wire passing vertically through a hole in a horizontal card. There is a current in the wire in the direction shown. Point P is marked on the card.

Figure 1

(a)

Name one piece of equipment that could be placed on the card to show the shape of the magnetic field produced by the current.

/ 1

(b)

Describe the shape of the magnetic field produced around the wire.

/ 2

(c)

When viewed from above the card, the magnetic field lines go anticlockwise around the wire.

State the direction of the field lines, when viewed from above, if the direction of the current in the wire is reversed.

/ 1

(d)

State two changes that would increase the strength of the magnetic field at point P.

/ 2

Question 2

3 marks

A solenoid is a long coil of wire. When there is a current in the solenoid it produces a magnetic field.

(a)

Which of these describes the magnetic field along the centre of a current-carrying solenoid?

A strong and almost uniform
B strong and circular
C weak and almost uniform
D zero everywhere

/ 1

(b)

Explain why the magnetic field inside the solenoid is much stronger than the magnetic field outside it.

/ 2

Question 3

3 marks

A straight wire is placed at right angles to a uniform magnetic field of magnetic flux density $0.15\ \mathrm{N/A\ m}$ .

The length of wire inside the field is $8.0\ \mathrm{cm}$ and the current in the wire is $3.5\ \mathrm{A}$ .

Calculate the size of the force on the wire due to the magnetic field.

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

force = ............................................... N

/ 3

Question 0

4 marks

Figure 2 shows a straight copper wire placed between the poles of two magnets. There is a current in the wire in the direction shown.

Figure 2

(a)

The direction of the force on the wire due to the magnetic field is

A into the page
B out of the page
C to the left
D to the right

/ 1

(b)

The direction of the current in the wire is now reversed. The size of the current is unchanged.

Describe the effect of this on the force on the wire.

/ 1

(c)

State two changes that would increase the size of the force on the wire.

/ 2

Question 0

5 marks

A student makes an electromagnet by winding insulated copper wire around an iron nail and connecting the ends of the wire to a power supply.

The student wants to find out how the number of turns of wire affects the strength of the electromagnet. She will do this by holding the electromagnet just above a pile of steel paperclips and counting how many paperclips are picked up.

(a)

State the independent variable and one control variable for this investigation.

/ 2

(b)

Describe how the student should carry out this investigation to obtain valid results.

/ 3

Question 0

3 marks

A stiff copper wire hangs between the poles of a strong horseshoe magnet. The magnet rests on a bench. When a current is switched on in the wire, the wire moves.

(a)

Explain, in terms of magnetic fields, why the wire experiences a force when there is a current in it.

/ 2

(b)

State the size and direction of the force that acts on the magnet compared with the force on the wire.

/ 1

Question 0

4 marks

A straight wire of length $120\ \mathrm{mm}$ lies at right angles to a uniform magnetic field. When the current in the wire is $4.5\ \mathrm{A}$ , the force on the wire due to the field is $0.36\ \mathrm{N}$ .

(a)

Calculate the magnetic flux density of the field.

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

magnetic flux density = ............................................... $\mathrm{N/A\ m}$

/ 3

(b)

The current in the wire is now halved and the length of wire in the same field is doubled.

State the size of the new force on the wire.

/ 1

Question 0

4 marks

A simple electric motor contains a rectangular coil of wire that is free to rotate on an axle. The coil is placed between the poles of a permanent magnet and a current is passed through the coil.

Explain how the current in the coil causes the coil to rotate.

/ 4

Question 0

6 marks

Figure 3 shows the main parts of a simple d.c. electric motor, seen from above. The coil is free to turn about the axle.

Figure 3

(a)

Explain why the split-ring commutator is needed for the coil to keep rotating in the same direction.

/ 4

(b)

State two changes to the motor that would make the coil rotate faster.

/ 2

Question 0

6 marks

A student places a small magnet on a top pan balance. A stiff horizontal wire is clamped in the magnetic field just above the magnet, at right angles to the field, as shown in Figure 4. The length of wire in the field is $5.0\ \mathrm{cm}$ .

Figure 4

When a current of $4.0\ \mathrm{A}$ is switched on in the wire, the reading on the balance increases by $0.62\ \mathrm{g}$ .

(a)

Explain why the reading on the balance changes when the current is switched on.

/ 2

(b)

Calculate the magnetic flux density of the field at the wire.

(gravitational field strength $g = 10\ \mathrm{N/kg}$ )

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

magnetic flux density = ............................................... $\mathrm{N/A\ m}$

/ 4

Key Concepts of Physics

Motion & Forces

Conservation of Energy

Waves

Light & the Electromagnetic Spectrum

Radioactivity

Astronomy

Energy – Forces Doing Work

Forces & their Effects

Electricity & Circuits

Static Electricity

Magnetism & the Motor Effect

Electromagnetic Induction

Particle Model

Forces & Matter

Question 1

Question 2

Question 3

Question 0

Question 0

Question 0

Question 0

Question 0

Question 0

Question 0