Electromagnets.

Step 1 of 6

A current makes a field

In 1820 Hans Christian Ørsted noticed that a compass needle moved every time he switched on a current in a wire beside it. Every electric current makes a magnetic field round it. Round a straight wire the field lines are circles going round the wire, and the field is weak: a few centimetres from a wire carrying one ampere, it is far smaller than the Earth’s field.

To get a strong field, the wire is wound into a coil. Each turn adds its own field, and inside the coil all the shares point the same way. On the board is a coil of copper wire 6 cm long, connected to a power supply, with an iron bolt in it. Right now no current flows, and the clips lie still.

Try it: raise Current and watch the clips jump to the bolt. Then set it back to zero.

Back to the experiment ↑

Step 2 of 6

The field inside a coil

Inside a long coil the field is nearly uniform, and its strength is

$B = \frac{\mu_0 N I}{\ell}$$B =$$\frac{\mu_0 N I}{\ell}$⁠

where $N$⁠ is the number of turns, $I$⁠ the current, $\ell$⁠ the coil’s length in metres, and $\mu_0 = 4\pi \times 10^{-7}\ \mathrm{T \cdot m/A}$$\mu_0 =$$4\pi \times 10^{-7}\ \mathrm{T \cdot m/A}$⁠ the magnetic constant. The field is measured in tesla (T). One tesla is a very strong field; the Earth’s is about 50 µT, so millitesla (mT) are handy.

Now the core is out, and the current and the number of turns are fixed. Work out the field inside the coil.

Step 3 of 6

One product

In the formula, $N$⁠ and $I$⁠ appear only as a product. What sets the field is how much current goes round the coil in all, the ampere-turns: $200$⁠ turns at 1 A and $100$⁠ turns at 2 A give the same field.

Step 4 of 6

An iron core

Soft iron is made of tiny regions, magnetic domains, each of them a small magnet. Usually they point every which way and cancel. In the last Lesson you saw that a magnet lines them up; here the magnet is the coil itself.

Predict what will happen to the field when the bolt goes in, then look.

Step 5 of 6

Saturation

The iron’s boost has a limit. Once nearly all the domains are lined up, the iron saturates, around 1.6 T, and beyond that more current adds only the coil’s own small share. Now the current and the number of turns are at their maximum.

Step 6 of 6

Which end is north

Outside, a coil’s field looks like a bar magnet’s: the lines leave one end and enter the other. Which end is north is given by the right-hand grip rule: curl the fingers of your right hand the way the current runs in the turns, and your thumb points to the north pole. Reverse the current, and the poles swap.

Show the field is at Field: the arrows on the wire show the direction of the current, and the lines leave the end of the bolt.