Magnetic fields due to currents Cambridge International AS & A Level Physics revision
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In plain words
Every electric current makes a magnetic field around itself. Wind the wire into a coil and the fields from each turn add up, until a long coil gives a field just like a bar magnet's. And because a current both makes a field and feels one, two wires side by side push or pull on each other.
4 things to know
- A long straight wire: the field lines are circles round the wire, closer together nearer to it. Right-hand grip rule: point your thumb along the current and your fingers curl the way the field goes.
- A flat circular coil: the lines pass straight through the middle of the coil and loop round outside it on each side.
- A long solenoid: inside, the field is uniform, with parallel, evenly spaced lines. Outside, it looks like the field of a bar magnet. An iron (ferrous) core makes the field much stronger.
- Two parallel wires: each lies in the other's field, so each feels a force. Currents in the same direction attract; currents in opposite directions repel. The two forces are equal and opposite.
Tips and tricks
- "Like currents attract" is the opposite way round from charges, where like charges repel.
- Reversing the current reverses the direction of the field everywhere.
It lands in your notebook with its questions as flashcards.
Magnetic fields due to currents: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What shape are the magnetic field lines round a long straight wire carrying a current?
The grip rule gives their direction.
What is the magnetic field like inside a long solenoid?
The lines are parallel and evenly spaced.
What does an iron core do to the field of a solenoid?
The iron becomes magnetised and adds to the field.
Two parallel wires carry currents in opposite directions. What do they do?
Currents in the same direction attract; opposite currents repel.
The current in a straight wire is reversed. What happens to its magnetic field?
The grip rule now gives the opposite sense.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Magnetic fields due to currents
Cambridge International AS & A Level Physics 9702 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe the magnetic field round a long straight wire that carries a current.[2]
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The field lines are circles centred on the wire, in planes at right angles to it. They are closer together near the wire, where the field is stronger.
State two ways of making the magnetic field inside a solenoid stronger.[2]
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Any two: increase the current, use more turns on the same length, put an iron core inside.
Explain why two parallel wires carrying currents in the same direction attract each other.[3]
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Each wire lies in the magnetic field made by the other, and that field is at right angles to the wire. So each wire feels a force (F = BIL), and Fleming's left-hand rule shows that the force on each is towards the other.
Answers: Magnetic fields due to currents
- 1. The field lines are circles centred on the wire, in planes at right angles to it. They are closer together near the wire, where the field is stronger.
- 2. Any two: increase the current, use more turns on the same length, put an iron core inside.
- 3. Each wire lies in the magnetic field made by the other, and that field is at right angles to the wire. So each wire feels a force (F = BIL), and Fleming's left-hand rule shows that the force on each is towards the other.