Build an Electromagnet

Wind a coil of wire around an iron nail, switch on the current and pick up paper clips. How strong can you make your electromagnet?

Inside the coil
Field in coil
0.10 T
Amp-turns N × I
40
Clips lifted
6 / 30
Core boost μr
× 100

The maths, live

Field B = μ₀ × μr × N × I ÷ LB = 4π×10⁻⁷ × 100 × 40 × 1.0 ÷ 0.05 = 0.10 T
So B ∝ N × IN × I = 40 × 1.0 = 40 amp-turns
Clips lifted ≈ B ÷ 17 mT101 ÷ 17 ≈ 6 clips (max 30)

Take the nail out. Can the coil lift any clips on its own? Put it back, switch off and watch the clips drop, just like a scrapyard crane.

How it works

An electric current in a wire makes a magnetic field around it. Wind the wire into a coil and the fields from all the turns add up, so the field gets stronger with more turns (N) and more current (I). Switch the current off and the magnetism goes away, which is why scrapyard cranes use electromagnets to drop the cars they lift.

Putting iron inside the coil makes the field much stronger, because the tiny magnets inside the iron line up with the coil's field and add their own. For a long coil the field is B = μ₀ × μr × N × I ÷ L, where L is the coil's length and μr says how much the core boosts the field. For air μr is 1, and a short iron nail boosts it about 100 times.

Try this

  1. Double the number of turns. Does the field double too?
  2. Take out the nail. Can the coil lift any paper clips on its own?
  3. Find two different ways to lift exactly 15 clips.