Field of a circular loop

The idea

In a current-carrying circular loop, the field circles around each part of the wire grow so large that they appear as straight lines at the centre, and every section of the wire adds field in the same direction inside.

Bending a wire

A straight wire has circles of field lines round it. Now the same wire is bent into a circular loop, and a current is passed through it.

What do the field lines look like at the centre of the loop?

• Straight lines
• Tiny circles round the centre
• There are no field lines at the centre

From wire to loop

Press Next step to go from one straight wire to a loop.

Adding up inside

Round each bit of the wire there are circles of field lines, and they get larger away from the wire. When we reach the centre of the loop, the arcs of these large circles look straight. The right-hand rule shows that each bit of the wire gives field in the same direction inside the loop.

In a current-carrying circular loop, the field circles around each part of the wire grow so large that they appear as straight lines at the centre, and each section of the wire adds field in the same direction inside the loop.

Notes

In a circular loop the field circles grow so large that they look like straight lines at the centre, and each section of the wire adds field in the same direction inside.

Check yourself

At the centre of a current-carrying circular loop, the magnetic field lines appear as…

Why do the field lines look straight at the centre of the loop?

Inside the loop, the fields due to the different sections of the wire…

The pattern of the magnetic field around a conductor carrying current depends on…

• small circles round the centre. The circles round each part of the wire are largest at the centre, so their arcs look like straight lines.
• no lines, because the field is zero there. Each section of the wire adds field in the same direction inside the loop, so there is a field at the centre.
• straight lines — correct. Yes!
• The circles round each part of the wire have grown so large that their arcs look straight — correct. Yes!
• The current stops flowing at the centre. The current flows round the loop. The lines look straight because the circles round the wire grow larger and larger away from it.
• The fields of different parts of the wire cancel each other. Each section of the wire adds field in the same direction inside the loop, so they do not cancel.
• point in opposite directions. The right-hand rule shows that each section of the wire contributes field in the same direction within the loop.
• all point the same way — correct. Yes!
• have no effect on each other or on the centre. Each section adds to the field inside the loop, and they add in the same direction.
• nothing: it is the same for a straight wire and for a loop. A straight wire gives concentric circles; a loop gives straight lines at its centre. So the pattern depends on the shape.
• the number of compass needles used to look at it. A compass needle only shows the field. The pattern depends on the shape of the conductor.
• the shape of the conductor — correct. Yes!