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

Faraday's law

Demonstration

Faraday's law of electromagnetic induction is a very important principle. Most of the electrical power in the world is generated by using this principle.

Apparatus and materials

Ferromagnetic cylindrical rod: diameter 10 mm, length 10 cm

Insulated copper wire - SWG 30, 300 g

LED

Demountable transformer, used as a 'step-down' transformer with primary coil connected to mains, with turns ratio to produce 6-10 V across the secondary coil. 

Health & Safety and Technical notes


NB The primary coil must be designed for connection to the mains, e.g. using an IEC connector and mains lead.

Procedure


a Wind the copper wire on the ferromagnetic rod to form a cylinder with the ferromagnetic rod as axis. Leave 0.5 cm of wire on either side of the rod. This is called the test coil. 

b Remove the insulation from the two ends of the copper wire and connect an LED in series. 
 
c Connect the primary coil of the demountable transformer to the AC mains. 
 
d Hold the test coil (ferromagnetic rod with copper wire) in your hand and move the test coil close to the secondary coil, preferably along its axis. (A circular inductor coil will have its magnetic field along its axis.) The LED glows. 
 
e If the primary coil is changed for a smaller one connected to the 20 V DC, the LED glows as the test coil is moved towards or away from the secondary coil. 

Teaching notes


1 When an alternating current flows through the secondary coil, it produces alternating magnetic field along its axis. 

2 When the test coil is positioned correctly, the flux linking with it changes and a voltage is induced. The LED glows as the magnetic field oscillates due to an alternating supply current. 
 
3 If a DC supply is used, the LED glows only when the test coil is moved. 

This experiment was submitted by K.H. Raveesha, Head of Physics at the CMR Institute of Technology in India.