The field of a loop
A straight wire has a field in rings. Bend the wire into a circle. What happens to the rings?
- A loop, cut across. Current comes out at one side and goes in at the other.
- Each side has rings round it. Left rings turn anticlockwise. Right rings turn clockwise.
- Between the sides, the lines go up. They add together.
- Look from above. The field points at you. So the current turns anticlockwise.
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Slide the turns and the current. Pick the direction. Read the field.
The rule
The rings from the two sides add up inside the loop. Inside the loop, the field lines point the same way. At the centre, the lines are nearly straight.
Imagine your right hand on the loop. Curl your fingers the way the current goes. Your thumb shows which way the field points through the loop.
This is a second use of your right hand. For a straight wire the thumb is the current. For a loop the fingers are the current.
A turn is one loop of wire. Take a coil of n turns, all one size and close together. Each turn carries the same current. At the centre, the field is n times that of one turn. A bigger current makes a stronger field.
| Current, seen from the front | Field through the loop |
|---|---|
| Clockwise | Away from you |
| Anticlockwise | Towards you |
Outside the loop, in its plane, the lines turn back. They point the other way. The plane of a loop is the flat sheet it lies in.