Magnetic fields and forces

The study design requires that the student is able to:

  • Apply a field model to magnetic phenomena including shapes and directions of fields produced by bar magnets, and by current-carrying wires, coils and solenoids;
  • Calculate magnitudes, including determining the directions of, and magnetic forces on, current carrying wires, using F = nBIL where the directions of I and B are either perpendicular or parallel to each other;

1.[2 marks](2002 Q9)

Jackie and Jim are studying electromagnetic induction. They have a small permanent magnet and a coil of wire wound around a hollow cylinder as shown below.

Jackie moves the magnet through the coil in the direction shown at constant speed.

Indicate on the diagram the direction of the induced current that flows in the resistor. Explain the physics reason for your choice.

2.[2 marks](2002 Q10)

They next decide to move the magnet, at a constant speed, all the way through the coil and out the other side.Which one of the diagrams (A–D) below best shows how the current through the coil varies with time?

3.[2 marks](2004 Q13)

A cut-away picture of a loudspeaker is shown below. It basically consists of a coil of wire that is attached to a paper cone, and placed in a strong radial magnetic field. In a sound system this coil would be supplied with an alternating current from an audio amplifier. The section view of the unit (also shown below) shows the direction of the magnetic field relative to the coil more clearly.

When the current in the coil is flowing into the page on the left side of the coil (above), which of the statements below gives the direction of motion of the coil (and the attached cone)?

A. It will move up the page.

B. It will move down the page.

C. It will rotate clockwise (viewed from in front of the loudspeaker core).

D. It will rotate anticlockwise (viewed from in front of the loudspeaker core).

4.[3 marks](2004 Q14)

The diameter of the coil is 0.04 m, and consists of 200 turns of wire. The uniform radial magnetic field through the coil is 0.4 T.What is the magnitude of the force on the coil when a current of 0.5 A is flowing?

5.[3 marks](2004 Sample Q1)

Many science books contain experiments where a battery, coil and nail can be used to create an electromagnet, as is shown below.

Briefly describe the process by which the ‘magnetism’ is made when the switch is closed.

6.[2 marks](2004 Sample Q2)

On the figureindicate the following.

i. the north pole of the magnet with a ‘N’

ii. six field lines

7.[3 marks](2004 Sample Q3)

Two separate but similar coils are mounted closely to each other as shown below. The second coil is connected to a battery and switch. The switch is initially open and no current is flowing. An ammeter is connected to the second coil and initially shows no deflection. When the switch is first closed, the ammeter needle is deflected to the left.

Complete the table below by describing how the ammeter responds to the following sequential actions. Write your answer in the two empty entries of the column in the table.

Action / Ammeter
Switch is open / No deflection
Switch is closed for the first time / Meter deflects to LEFT
Switch is left closed for a few seconds
Switch is returned to open position

8.[2 marks](2004 Sample Q6)

A conducting wire is placed in a uniform magnetic field of 0.1 T, as shown below. When a DC voltage is applied between A and B, the wire experiences a constant force as indicated on the diagram.

Choose one of the following options (A–D) that correctly indicates the direction in which the current must flow for the wire to experience the force as indicated.

9.[2 marks](2004 Sample Q7)

Assume that the magnetic field exists only between the poles of the magnet, and is uniform. What current must flow for the wire to experience a force of 0.02 N? (Assume the field associated with the magnet is 2 cm wide.)

10.[2 marks](2004 Sample Q8)

Briefly describe what would happen to the wire if instead of a DC source, the wire was connected to an AC power source of the same voltage and frequency 50 Hz.

11.[2 marks](2004 PilotQ2)

A 10 mm section of a conducting wire is placed in a uniform magnetic field as shown below.

A DC current of I = 1.0 A produces a force of 0.01 N and stretches the wire.

Which one of the following options best describes the direction of the resulting force on the wire?

A. / up / B. / down
C. / towards N / D. / towards S
E. / towards X / F. / towards Y

12.[2 marks](2004 PilotQ3)

Calculate the strength of the magnetic field.

13.[2 marks](2004 PilotQ4)

The wire is connected to an AC power source of the same RMS voltage and frequency 100 Hz instead of a DC source.Briefly explain what happens to the wire.

14.[2 marks](2005 Q1)

A vertical wire carrying a current I is placed opposite the centre of a permanent bar magnet as shown below.

Which of the arrows (A - F) best shows the direction of the magnetic force on the wire at the point P?

15.[3 marks](2005 Q17)

The left side of the figure below shows three sources of magnetic fields.

The right side of the figure shows three possible magnetic field patterns of the shaded planes.

For each of the three sources, draw a line linking the source to the magnetic field pattern it produces in the shaded region.

16.[3 marks](2006 Q1)

The figure below shows a solenoid powered by a battery.

Complete the diagram above by sketching five magnetic field lines created by the solenoid.

Make sure that you clearly show the direction of the field, including both inside and outside the solenoid.

In Questions 17 and 18 use the key, PQRSTU, shown above to indicate direction.

If there is no direction, write none.

17.[2 marks](2006 Q2)

The magnetic field strength in the solenoid is 2.0 × 10-2T (Wb m-2). A U-shaped conducting wire (a, b, c, d),carrying a current of 5.0 A in the direction a d, is placed inside the solenoid as shown below.The highlighted segment, abcd, of size 6.0 cm × 2.0 cm is completely immersed in the magnetic field as shown.

What is the force (magnitude and direction) on the 6.0 cm section of wire, cd?

18.[3 marks](2006 Q3)

What is the force (magnitude and direction) on the 2.0 cm section of wire, bc?

19.[2 marks](2007 Q1)

The figure below shows a bar magnet.

Complete the diagram by sketching five magnetic field lines around the magnet. You must include arrows which show the direction of the magnetic field of the magnet.

20.[2 marks](2007 Q2)

A second bar magnet is placed next to the original as shown below.

Complete the diagram by sketching magnetic field lines to indicate the shape of the magnetic field around the magnets.

21.[3 marks](2007 Q3)

The figurebelow shows a power line at a mining site that carries a DC current of 2000 A running from west to east.

The earth's magnetic field at the mining site is 4.0 × 10-5T, running horizontally from south to north. An engineer is concerned about the electromagnetic force due to the earth's magnetic field on the wire between the two support poles, which are 20 m apart.

Calculate the magnitude and direction (north, south, east, west, up, down) of the force due to the earth's magnetic field on the 20 m section of wire between the two poles.

22.[2 marks](2008 Q1)

23.[2 marks](2008 Q2)

24.[2 marks](2008 Q3)

25.[3 marks](2008 Q4)