➕ Where the fields add
On one side of the wire the two fields point the same way. They add together and the field is stronger there.
Electromagnetism » The Motor Effect
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.11P, 6.12, 6.13, 6.14
- Why a wire carrying a current in a magnetic field feels a force
- How to use the left-hand rule to find the direction of that force
- How the size and direction of the force change when the current or the field changes
- That a moving charged particle can also feel a force in a magnetic field (Paper 2)
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Unlock This CourseA current in a magnetic field feels a force
In earlier lessons you saw that a current makes its own magnetic field, and that magnets make a field too. What happens when the two fields meet? Place a straight wire between the poles of a magnet, at right angles to the field, and switch the current on. The wire is pushed sideways. This is called the motor effect.
Key terms:
- Motor effect: the force on a current-carrying wire (conductor) placed in a magnetic field.
You can see it by hanging a loose, bendy wire between two magnets facing each other with unlike poles. When the switch is closed, the wire jumps. When the switch is opened, it drops back.
Why does the force happen?
The force comes from two magnetic fields interacting: the field of the magnets and the circular field around the wire.
➖ Where the fields oppose
On the other side the fields point in opposite directions. They partly cancel and the field is weaker there.
The wire is pushed from the side where the field is stronger towards the side where it is weaker.
Picture it
The magnets' field runs from left to right. The current goes into the page, so (by the right-hand grip rule from the lesson on the magnetic field around a current) the wire's field goes clockwise. Above the wire, a clockwise field points to the right, the same way as the magnets' field, so it is stronger. Below the wire, it points left, against the magnets' field, so it is weaker. The wire is pushed downwards, from strong to weak.
The left-hand rule
You do not need to draw the fields every time. The left-hand rule gives the direction of the force when the wire is at right angles to the field. Hold the thumb, first finger and second finger of your left hand at right angles to each other.
| Part of the hand | Points in the direction of |
|---|---|
| Thumb | the force (motion) on the wire |
| First finger | the magnetic field, from north to south |
| Second finger | the current (conventional current, from + to −) |
Learn this
Thumb = Force. First finger = Field. Second finger = Current. Use your left hand here. The right hand is for the grip rule on a wire or a solenoid.
Worked example
A wire carries a current into the page. The magnets give a field from left to right. Find the direction of the force.
Step 1: point the first finger to the right (the field, N to S).
Step 2: point the second finger into the page (the current).
Step 3: your thumb now points down the page.
Answer: the force on the wire is downwards. This matches the stronger-to-weaker picture above.
Changing the force
The force gets bigger or smaller, and can flip direction, when you change the current or the field.
| Change | Effect on the force |
|---|---|
| Bigger current | Bigger force |
| Stronger magnetic field (stronger magnets) | Bigger force |
| Reverse the current | Force reverses (same size) |
| Reverse the field (swap the poles) | Force reverses (same size) |
| Reverse both | Force stays in the same direction |
Reversing both is like turning your left hand half a turn about the thumb: both fingers flip, but the thumb still points the same way. This lesson only asks you to describe these changes. You do not need an equation for the size of the force.
Worked example
In the example above the force was downwards. What happens if the current is reversed so it comes out of the page?
The second finger now points out of the page and the first finger still points right. The thumb points up. The force reverses. If the magnets were also turned round, the field would run right to left and the force would point down again.
Paper 2 only
A moving charged particle (such as an electron or a proton) also feels a force in a magnetic field. This happens as long as its motion is not parallel to the field. A current is a flow of moving charges, so this is the same effect seen in a single particle. If the particle moves along the field lines, there is no force.
Common mistakes
Common mistakes
- Using the right hand. The motor effect needs the left hand.
- Mixing up the fingers. Remember: thumb = force, first finger = field, second finger = current.
- Pointing the second finger the way the electrons move. Use conventional current, from the positive terminal to the negative terminal.
- Pointing the first finger from south to north. Field lines run from north to south outside a magnet.
- Saying that reversing both the current and the field reverses the force. It reverses twice, so it stays the same.
Exam-style question
A straight wire lies at right angles to a magnetic field. There is a current in the wire and it moves upwards.
(a) Explain why the wire experiences a force. [2 marks]
(b) State two ways to increase the size of the force. [2 marks]
(c) The current and the magnetic field are both reversed. State what happens to the direction of the force. [1 mark]
Model answer
(a) The current produces its own magnetic field around the wire (1). This field interacts with the field of the magnets, giving a force on the wire (1).
(b) Increase the current (1). Use a stronger magnet (1).
(c) It stays the same, because reversing each one reverses the force and the two reversals cancel out (1).
Exam tip
For an "explain why" question, say that there are two magnetic fields and that they interact. Saying "the magnet pulls the wire" does not earn the mark.