Describe Oersted's experiment. What does it show, and how is the direction of the magnetic field round a straight wire found?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the discovery. A current in a wire produces a magnetic field around it (Oersted, 1820).
2 · Describe the set-up and observations.
A straight wire held along north-south, just above a compass.
Current on: the needle turns at right angles to the wire.
Current reversed, or wire placed below: the needle turns the other way.
3 · Draw the field lines. [Wire end-on with ⊙; concentric circles with anticlockwise arrows, spaced wider outwards; B along the tangent at P; labels I, r, B]
4 · Give the rule. Right-hand thumb rule: thumb along the current, curled fingers give the field direction.
5 · Real life. A clamp meter measures current by sensing this field round the wire.
6 · Close in one line. Electricity and magnetism are linked: moving charges make magnetic fields.
Scoring tip: Name the rule exactly ('right-hand thumb rule') and show the circles getting farther apart in your diagram.
