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Ever wondered why a compass needle jumps when you bring a wire near it?
When electric current flows, it creates an invisible magnetic field around it. That field can push other magnets or even make a wire move if you place it in another magnetic field.
What is the magnetic effect of current?
The magnetic effect of current is simply the fact that a moving electric charge (like the electrons in a wire) produces a magnetic field. Think of water flowing through a hose – the flow creates pressure that can push things. Similarly, the flow of charge creates a magnetic “pressure” around the conductor.
How to find the direction of the magnetic field
We use the right‑hand thumb rule. Point your thumb in the direction of conventional current (the direction positive charge would move). Then curl your fingers – they show the direction the magnetic field lines wrap around the wire.
Force on a current‑carrying conductor
When a current‑carrying wire sits in an external magnetic field, it feels a force. The formula is F = B I L sin θ where:
- B – magnetic field strength (tesla, T)
- I – current (ampere, A)
- L – length of wire inside the field (metre, m)
- θ – angle between wire and field direction
Notice the sine term – the force is biggest when the wire is perpendicular (θ = 90°) to the field.
Worked example
Find the force on a 2 m long straight wire carrying 5 A, placed perpendicular to a uniform magnetic field of 0.3 T.
Given: I = 5 A, L = 2 m, B = 0.3 T, θ = 90° → sin θ = 1.
Plug into the formula:
F = B I L sin θ = 0.3 × 5 × 2 × 1 = 3 N.
So the wire feels a 3‑newton push, enough to move a small paperclip!
Other common configurations
Different shapes of conductors produce different field patterns.
| Conductor shape | Field pattern | Typical use |
|---|---|---|
| Straight wire | Concentric circles around the wire | Basic experiments, wiring |
| Circular loop | Field lines go through the centre like a bar magnet | Electromagnets, coils |
| Solenoid (many loops) | Uniform field inside, like a bar magnet outside | Relays, loudspeakers, MRI |
Why does this matter for exams?
Questions often ask you to draw field lines, apply the right‑hand rule, or calculate the force on a wire. Knowing the steps and the formula lets you answer quickly.
📝 Likely Exam Questions
- State the right‑hand thumb rule and use it to find the direction of magnetic field around a wire carrying current upwards.
Answer: Thumb up (current direction), fingers curl showing field circles clockwise when viewed from above. - A 0.5 m long wire carries 3 A and lies in a 0.2 T magnetic field at 30° to the field. Calculate the force on the wire.
Answer: F = BIL sinθ = 0.2 × 3 × 0.5 × sin30° = 0.15 N. - Explain why a solenoid behaves like a bar magnet.
Answer: The tightly wound loops make the field inside almost uniform; the ends act as north and south poles, similar to a bar magnet. - Draw the magnetic field lines around a current‑carrying circular loop and label the direction using the right‑hand rule.
Answer: Field lines emerge from one face of the loop and re‑enter the other, forming closed loops; thumb pointing along current gives direction.