IGCSE Physics / Magnetism and electromagnetism
The magnetic field around a wire
Edexcel International GCSE Physics (4PH1), chapter 20: Magnetism and electromagnetism, specification point 6.8, 6.10P. Free lesson.
Read the revision notes · IGCSE Physics past papers
A current in a straight wire produces a magnetic field made of circles centred on the wire. The right-hand grip rule gives the field direction, while the circle spacing shows its strength.
What the field around a straight wire looks like
Any current in a conductor produces a magnetic field around it. For a straight wire, the field lines form circles centred on the wire. A plotting compass placed near the wire turns because its needle lines up with the field, not because current flows through the compass. The field lines are closer together near the wire, where the field is stronger. Further from the wire, the circles are more widely spaced, showing a weaker field.
How to find the field direction
Use the right-hand grip rule. Your thumb shows the direction of the current, and your curled fingers show the direction of the magnetic field lines. A dot means current comes out of the page, so the field goes anticlockwise. A cross means current goes into the page, so the field goes clockwise.
How to draw the field
Start at the wire and draw circles centred on it. Add arrows to show which way the field goes. The spacing matters because it shows where the field is stronger or weaker.
What happens when the current changes
Changing the current affects the direction or strength of the field, depending on what you change. Reversing the current reverses the field direction, so every arrow on the drawing flips. It does not change the field's strength. Increasing the current makes the field stronger but does not change its direction. Draw the circles closer together to show this.
Check the direction and avoid drawing slips
If a wire carries current into the page, its symbol is a cross. Point your right thumb into the page: your fingers curl clockwise, so the field lines go clockwise. A compass near a wire swings because the current produces a magnetic field around the wire. The needle lines up with that field; no current reaches the compass. Do not draw the circles evenly spaced or leave off the arrows. Show closer spacing near the wire and arrows for the field direction.