Electricity and Magnetism
Magnetic effects of electric current
Read a field-line diagram for direction and strength, pick the right-hand thumb rule or Fleming's left-hand rule for the job, and explain what the earth wire and the fuse are doing in a house circuit.
In 1820 Oersted noticed a compass needle twitch as he switched on a current in a nearby wire. Nothing had been moved, and no magnet was involved. That twitch is the whole of this chapter in miniature: a current makes a magnetic field, and a magnetic field pushes on a current. Everything from an electric bell to the motor in a fan comes out of those two sentences.
Two hands, two jobs
Use the right hand to find a field: hold the wire with your thumb along the current, and your curled fingers show which way the field lines wrap round it. Use the left hand to find a force: first finger along the field, second finger along the current, and the thumb points the way the conductor is pushed. Mixing the two up is the commonest mistake in the chapter, and the fix is to ask first whether you are looking for a field or for a push.
Worked example
A field-line diagram of a bar magnet shows the lines close together near the ends and spread out at the sides. What can you read off it?
Where the lines are crowded, the field is stronger.
Closeness of the lines is how the diagram carries magnitude, so the ends of the magnet are where the force on another magnet would be greatest.
Try it together
A copper wire hangs between the poles of a horseshoe magnet, and a current is switched on. The rod jumps.
Work out which rule settles what.
1.You want the direction the rod is pushed. Which rule gives it — the right-hand thumb rule or Fleming's left-hand rule?
Have a go
Have a go on your own. You want to know which way the field lines circle a straight current-carrying wire. Which rule do you reach for?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.