7.2 Magnetic effects of currents and the motor effect — Practise with help

1. It loses most or all of its magnetism. It became an induced magnet only while it was in the field.
2. The unit is the ampere, and the symbol is $I$.

If question 1 went wrong, look again at induced magnets in Section 7.1. If question 2 went wrong, look at current in Section 6.1.

Use a compass, because a compass needle turns when it sits in a magnetic field.

Place the compass near a straight wire. Note where it points with the current off, then switch the current on.

Place a long straight wire on a piece of card and put a compass near it. With the current off, the compass points north. Switch the current on. The needle turns away from north, so the current has made a magnetic field. Reverse the current, and the needle turns the other way. This is a model answer for you to compare with your own wording.

Picture looking along the wire. The field lines form closed loops.

Say that the field lines are circles around the wire, then say what happens to the field strength as the distance grows.

The field lines are circles around the wire. The field is strongest close to the wire. As you move further away, the field gets weaker. A larger current makes the field stronger at every distance.

Think about what happens to the field of each turn of the coil.

Say that the field of each turn adds to the field of the other turns, then describe the field inside the coil.

A solenoid is a coil of many turns. The magnetic field of each turn adds to the field of the other turns, so the fields combine. Inside the solenoid the field is strong and uniform, and outside it the field has the shape of the field of a bar magnet. The magnetic effect is much larger than for one straight wire.

Iron is a magnetic material. Think about what the field of the coil does to the iron.

Say that the iron becomes magnetised by the coil's field and adds to it. Then say what happens to the field when the current stops.

The field of the coil magnetises the iron core. The magnetised core adds its own field to the field of the coil, so the total field is much stronger. When the current is switched off, the coil's field goes and the iron loses its magnetism, so the electromagnet can be switched off.

Compare the field inside the coil with the field around a bar magnet.

Say whether the field inside is strong or weak, and whether it is uniform.

Inside the solenoid the field is strong and uniform. The field lines are straight and equally spaced, and they point along the axis of the coil. Outside the solenoid the field spreads out and has the same shape as the field of a bar magnet.

Hold your left hand with the thumb and the first two fingers at right angles.

Name the field, the current and the force, and match each one to a finger or the thumb.

Hold the left hand so that the thumb, the first finger and the second finger are at right angles to each other. The first finger points in the direction of the magnetic field, from north to south. The second finger points in the direction of the current. The thumb points in the direction of the force on the conductor.

The three quantities you need are $B$, $I$ and $l$. Check that each is in its SI unit before you multiply.

Write $F = BIl$, then put in $B = 0.40$ T, $I = 3.0$ A and $l = 0.20$ m.

You want the current, so rearrange $F = BIl$ before you put the numbers in.

Write $I = \dfrac{F}{Bl}$, then work out $\dfrac{0.60}{0.30 \times 0.50}$.