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

Fleming's left hand rule (using the Earth's magnetic field)

Demonstration

The 'catapult field' seen when two magnetic fields interact can reveal the presence of the Earth's magnetic field, making Fleming's rule more memorable to students.

Apparatus and materials

Strip of aluminium foil approx 2 metres long cut to the same width as adhesive tape (25 mm)

Metre strip of adhesive tape, 2

Crocodile clips, 2

Connecting wires of suitable length, 2

Stands and clamps, 2

Power supply (5 to 8 amp DC), or car battery charger, set to 6 volts

Magnetic compass (optional)

Reversing switch (optional) 

Health & Safety and Technical notes


The resistance of the aluminium strip is about an ohm, so it is almost a short-circuit. Make sure the power supply is capable of withstanding such a low-resistance load. Do not use a battery.

Photograph courtesy of Richard Walder

For some more detailed images of the apparatus used in this experiment click here.  

Unroll the aluminium foil and apply adhesive tape to one surface; this makes the foil much more robust and easily cut without tearing. 
 
Ensure power is briefly applied otherwise tape will rupture.

Procedure


a Connect crocodile clips to each end of the tape and suspend with an East-West orientation between the two stands allowing the tape to almost touch the bench. 

b Connect to the power supply and allow tape to settle. 
 
c Briefly switch on and observe a kick in the tape. Reverse polarity and repeat; the kick is in the opposite direction. Rotate the set-up to align North-South and repeat. There will be no movement.

Teaching notes


1 There is a force on the wire only when the current is perpendicular to the magnet's field. The interaction with a current carrying conductor in this experiment demonstrates that the Earth's magnetic field is strong and aligned North to South. 

2 The Earth's magnetic field is used by earth scientists to explain aurorae and by biologists to explain how life on Earth is protected from cell damage. 

This experiment was submitted by Richard Walder from Eastbourne College.

Related experiments


Catapult magnetic field