7.2 Magnetic effects of currents and the motor effect — Learn

Prerequisites: Section 7.1, especially magnetic fields, field lines and induced magnetism; Section 6.1, especially electric current and a complete circuit.

By the end of this section you can: describe how a current creates a magnetic field; describe the field around a straight wire and inside a solenoid; and explain how a solenoid and an iron core make an electromagnet. At Higher tier you can also state Fleming's left-hand rule, calculate the force on a current-carrying conductor, and explain how the force makes an electric motor turn.

1. A current makes a magnetic field

In 1820 a scientist named Hans Christian Oersted placed a wire above a compass needle. When a current flowed in the wire, the needle turned. The current had made a magnetic field.

You can show the effect with a wire and a compass.

Put a long straight wire on a piece of card. Put a compass on the card near the wire.
The compass points north when the current is off.
Switch the current on. The compass needle turns away from north.

The needle turns, so there must be a magnetic field around the wire. Reverse the current, and the needle turns the other way.

Worked example 1. Describe how you would show that a current in a wire creates a magnetic field, and how to show the direction of the field.

A model answer is:

Put a compass near a long straight wire and turn the current on. The needle turns, so the current has made a magnetic field.
Reverse the direction of the current. The needle turns the other way, so the direction of the field depends on the direction of the current.

2. The field around a straight wire

The magnetic field around a long straight wire is a set of circles around the wire. Look along the wire, and the field lines form circles centred on the wire. The field is strongest close to the wire.

The strength of the magnetic field around the wire depends on:

the size of the current. A larger current makes a stronger field;
the distance from the wire. The field is weaker further from the wire.

You can work out the direction of the field with the right-hand grip rule. Point the thumb of your right hand along the wire in the direction of the current. Your curled fingers then point in the direction of the field.

3. The solenoid

A solenoid is a coil of insulated wire, wound into a cylinder. When a current flows, the field of each turn adds to the field of the others. The result is a much stronger field than a single straight wire can make.

Inside the solenoid, the field is strong and uniform. The field lines are straight and equally spaced.
Outside the solenoid, the field has the same shape as the field of a bar magnet. One end of the solenoid acts like a north pole, and the other end acts like a south pole.

4. Electromagnets

An electromagnet is a solenoid with an iron core. Adding an iron core makes the magnetic field much stronger, because the field of the coil magnetises the iron.

An electromagnet has one great advantage over a permanent magnet: you can switch it on and off. When the current stops, the field almost disappears. This is why a crane at a scrapyard uses an electromagnet. It can pick up the steel scrap, and then drop it by switching off the current.

The figure shows a solenoid with an iron core. You can change the current and the number of turns.

Predict first. Do not move the sliders yet. Write down your answers.

If you double the current, what happens to the strength of the field?
If you add more turns to the coil, what happens to the strength of the field?
What happens to the field when the current is switched off?

Test. Set the current to 1.0 A and the turns to 200. Read the field strength. Now set the current to 2.0 A, then set the turns to 400. Finally, switch the current off.

Explain. Why does adding more turns make the field stronger, even when the current stays the same? Write your answer in two or three sentences.

Worked example 2. Explain how a solenoid arrangement can increase the magnetic effect of a current.

A model answer is:

A solenoid is a coil with many turns of wire. The field of each turn adds to the field of the other turns.
The field inside the solenoid is strong and uniform, so the magnetic effect is much larger than for one straight wire carrying the same current.

Worked example 3. A relay has a coil of wire on an iron core, a piece of iron called an armature, and a switch. A small current flows through the coil. Explain how the relay closes a switch in a larger circuit.

A model answer is:

The small current makes the coil into an electromagnet.
The electromagnet attracts the iron armature. Iron is a magnetic material, so the force is one of attraction.
The armature moves and closes the switch. In this way a small current can control a larger current.

Watch out. A stronger field needs more current or more turns, not longer wire alone. Also remember the field is strongest near the wire, and it gets weaker as you move away. Re-check by asking which of the two changes you have made: the current, or the distance from the wire.

5. The motor effect

When a conductor carrying a current is placed in a magnetic field, the magnet and the conductor exert a force on each other. This is called the motor effect.

The force is largest when the conductor is at right angles to the magnetic field. If the conductor is parallel to the field, there is no force on it.

The size of the force depends on:

the size of the current in the conductor;
the strength of the magnetic field, called the magnetic flux density;
the length of the conductor that is inside the magnetic field.

6. Fleming's left-hand rule

Fleming's left-hand rule gives the relative directions of the force, the current and the magnetic field. Hold your left hand so that the thumb, the first finger and the second finger are all 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.

7. The force on a conductor

For a conductor at right angles to a magnetic field, the force is given by:

F = BIl

where $F$ is the force in newtons, $B$ is the magnetic flux density in tesla, $I$ is the current in amperes, and $l$ is the length of the conductor in metres.

The equation is given on the physics equation sheet, so you do not need to memorise it. You do need to choose it and use it correctly.

Worked example 4. A wire of length 0.15 m carries a current of 2.0 A. It sits at right angles to a magnetic field of flux density 0.25 T. Calculate the force on the wire.

\begin{aligned} F &= B I l \\ &= 0.25 \times 2.0 \times 0.15 \\ &= 0.075\,\text{N} \end{aligned}

Worked example 5. A wire of length 0.50 m carries a current of 4.0 A. The force on it is 0.20 N. Calculate the magnetic flux density.

\begin{aligned} F &= B I l \\ B &= \frac{F}{I l} \\ &= \frac{0.20}{4.0 \times 0.50} \\ &= 0.10\,\text{T} \end{aligned}

8. Electric motors

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.

One side of the coil carries a current in one direction, and the other side carries the current in the opposite direction. The two sides sit in the same field, so the forces on them act in opposite directions. The two forces form a turning effect, and the coil rotates.

In a simple motor, the current in the coil is reversed every half turn. The commutator does this. Without the reversal, the forces would stop the coil and then push it back the other way.

Progress check

Return to the chapter problem. The crane magnet is a solenoid with an iron core. Describe the field inside the coil, and explain why the iron core makes the magnet stronger. Then explain why the steel scrap falls when the current is switched off.

You should now be able to

describe how to show that a current creates a magnetic field;
describe the field around a straight wire, and state what the field strength depends on;
describe the field inside and outside a solenoid;
explain how a solenoid and an iron core make an electromagnet, and why an electromagnet can be switched off;
interpret a simple diagram of an electromagnetic device, such as a relay;
state Fleming's left-hand rule and describe the motor effect (Higher tier);
use $F = BIl$ to calculate a force, a current or a magnetic flux density (Higher tier);
explain how the force on a coil makes an electric motor turn (Higher tier).