Simple Magnetism & Magnetic Fields (Cambridge O Level Physics)

A student magnetises a steel rod by using a permanent magnet.

Describe a method that the student could use.

Explain how the student could test that the steel rod has been magnetised.

Magnets can be made from soft iron or from steel.

State one difference between the magnetic properties of soft iron and steel.

Fig. 8.1 shows a sheet of paper. A bar magnet is underneath the paper. A student sprinkles iron filings onto the paper.

On Fig. 8.1

• Label the position of each pole of the magnet. Use the label N for the north pole and S for the south pole.
• Draw magnetic field lines above the magnet and draw magnetic field lines below the magnet.
• Add arrows to show the direction of the magnetic field.

A student places a soft iron rod inside a coil of insulated wire. The coil is connected to a battery, as shown in Fig. 8.2.

Bar magnets produce a magnetic field. 

State where, in relation to the bar magnet, the magnetic field is strongest and explain how scientists know this.

You may wish to use a diagram in your answer.

A student is making a compass from a steel sewing needle, a cork and a bowl of water. Fig. 1.1 shows this apparatus.

A compass consists of a freely rotating magnet.

Suggest how the student might make a compass from this equipment.

When the student brings a magnet close to the needle-cork compass, the needle spins to face the north pole of the magnet. When the student moves the magnet away, the needle always returns to the same position. 

Suggest why this is the case. 

A wooden toy train has permanent magnets that hold the carriages together. Fig. 1.1 shows the arrangement. 

The centre carriage (B) is removed and turned 180°. 

Describe the effect this will have on the train.

Figure 1.2 shows the one of the carriages with a magnet attached to each end.

Fig. 1.2

Sketch the magnetic field lines inside the carriage.

A student is given three unknown metal strips and a bar magnet. 

The student is also given the following information. 

1 strip is magnetic and magnetised

1 strip is magnetic but not magnetised

1 strip is not magnetic

Describe a method the student could use to identify each strip.

Describe the difference between a magnetised and an unmagnetised magnetic material.

The student places a bar magnet on a piece of paper, as shown in Fig. 9.1.

Show the pattern of magnetic field lines around the bar magnet.

Draw two lines above the magnet and two lines below the magnet. Start and finish each line at a pole. Include one arrow to show the direction of the magnetic field.

The student rubs a plastic rod with a dry cloth. The plastic rod becomes positively charged.

Explain why the friction between the plastic and the cloth causes the plastic to become positively charged.

The student investigates the forces between two pairs of objects.

Fig. 9.2 and Fig. 9.3 show the pairs of objects.

State whether there is a force of attraction, a force of repulsion, or no force between the pairs of objects. Draw a ring around one phrase for each pair of objects.

1. two positively charged spheres

Fig. 9.1 shows the magnetic field pattern around a bar magnet.

(i)

On Fig. 9.1, write the letters N and S to indicate the north and south poles of the magnet.

[1]

 

(ii)

Fig. 9.2 shows a soft-iron bar placed close to a permanent magnet.

 

Three balls P, Q and R are electrically charged. The balls are suspended by threads of insulating material. Fig. 9.3 shows the arrangement.

Ball P is negatively charged.

A long straight wire passes through a piece of card. There is a current in the wire, as shown in Fig. 11.1.

Fig. 11.2 shows the view of the card from above. The current is into the page.

The current in the wire produces a magnetic field around the wire. One magnetic field line is drawn.

 
On Fig. 11.2, draw two more magnetic field lines around the wire. Show the direction of the magnetic field by drawing an arrow on each field line.

Fig. 11.3 shows the circuit for an electric bell.

Explain how the circuit causes the hammer to hit the bell repeatedly when the switch is closed.

 
Use your ideas about circuits and electromagnets.

Apart from an electric door bell, suggest another use for electromagnets.

A student is using plotting compasses to investigate the magnetic field lines of bar magnets. 

The student places two bar magnets in the arrangement shown in Fig. 1.1.

Fig. 1.1

Draw the magnetic field lines that the student will discover between the magnets.

The student changed the orientation of the bottom magnet by rotating it 180°. Fig. 1.2 shows the predictive sketch drawn by the student before they added the plotting compasses. 

State two differences between the sketch in Fig 1.2 and the pattern the student will actually see when they add the plotting compasses.

The student attempts to look at the field lines by adding iron filings around the magnet.

Whilst collecting the iron filings, they spill some on the magnet. 

Fig. 1.3 shows a side view of the magnet and the filings. 

Explain why the iron filings are in this position. 

Scientists can explain magnetic materials and non-magnetic materials using domain theory. 

All atoms contain electrons, and electrons act as tiny magnets with a north and a south pole, as shown in Fig. 1.1.

Metals are collections of many millions of atoms. For non-magnetic materials the electrons are randomly aligned as shown in Fig. 1.2.

For magnetic materials, the electrons are arranged in groups called domains. Within each domain, the electrons are aligned so that they all point in the same direction, and each domain acts as a magnet itself. Fig. 1.3 shows this arrangement. 

Use domain theory to explain

Using arrows in a rectangle to represent electrons in a metal as in part (a), draw a sketch of a magnetised material.