Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Datalogging electromagnetic induction

Demonstration

Use datalogging to record the e.m.f. induced across the terminals of a coil when a magnet is dropped though it.

Apparatus and materials

1,200 turn coil

Bar magnet (e.g. alnico)

Retort stand

Datalogger

Potential difference/voltage sensor 

Health & Safety and Technical notes


Adding a soft landing area beneath the coil will minimize the risk of the magnet falling on a foot and the risk of damage to the magnet.

You may want to build a guide tube to drop the magnet through, or add a soft landing area underneath. 

1,200 turns gives a good e.m.f. output with an alnico magnet. Fewer turns will also be fine. The magnet needs to be a reasonable fit through the gap in the coils.

Procedure


a Attach the voltage sensor across the coil and set it to record data at least 500 times a second. 

b Put the coil in a retort stand with the central hole vertical. 
 
c Hold the magnet a couple of centimetres above the hole, start the datalogger, and drop the magnet. 
 
d Stop the datalogger.

Teaching notes


The graph produced will look like the one below. 

Graph 
To download a copy of the results used to produce this graph, click here
 
There is a lot of physics in this graph. First the e.m.f. goes negative as the magnet enters the coil, then it goes positive as it leaves. This shows field lines being cut in different directions, or dB/dtbeing negative then positive. Secondly the second e.m.f.peak is shorter in time but higher as the magnet has speeded up. If the software allows it, measure the area under each peak - they should be the same but opposite in sign. 
 
The experiment can be repeated with the magnet reversed - both peaks will reverse in sign. 
 
What would happen if a longer coil were used? Say, much longer than the magnet? 

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.