Practical Activity for 14-16
To demonstrate the motor effect in a wire carrying an electric current.
Apparatus and Materials
- Shallow dish, 80 mm diameter and 20 mm high (Petri dish)
- Sheet of conducting metal, i.e. lead or copper approx 30 mm by approx 100 mm placed in base of dish with one end folded at right angles to clear top of dish and form electrode terminal
- Neodymium magnet (10 mm diameter) glued to top face of metal sheet and placed centrally
- Copper sulfate solution, saturated
- Fuse wire, 5 amp. Use rheostat to limit current
- Copper rod, 16 SWG or thicker, approx 100 mm long
- Stand and clamp
- Crocodile clips, 2
- Power supply (0-12 V DC), 5 amps
- Reversing switch (optional)
- Ammeter (optional) to keep current to around 5 amps
Health & Safety and Technical Notes
The apparatus can produce some gas if current is too high.
Fuse wire may rupture if the current is too high, so the demonstration should be done behind a screen. Why not use a rheostat and ammeter to adjust the current? (Copper sulfate is toxic.)
The copper sulphate should not be left in the dish after the experiment is complete since it tends to attack the iron of the magnet.
Neodymium magnets are available from...
Insulate side of magnet with glue or ring of adhesive tape.
Recent developments in magnet technology have made this possible using brine or copper sulfate solution as the liquid conductor, instead of mercury (used by Faraday).
If you use copper sulfate solution, the device will deposit copper from solution so can only be operated for short periods. Brine does not prevent the problem.
Copper wire can be salvaged from offcuts of electrical earthing cable or use a nail / toy car axle etc.
- Connect the lead strip electrode, using a crocodile clip, to the reversing switch.
- Wrap or solder fuse wire to one end of the copper rod. Suspend the copper rod from clamp with 5 amp fuse wire so that the rod just clears the lead strip and can rotate freely around the magnet without fouling the magnet or strip.
- Connect the free end of fuse wire, using a crocodile clip, to the reversing switch.
- Pour sufficient copper sulfate into dish to just cover the magnet.
- Starting at zero, increase the voltage until the copper wire starts to rotate around the magnet (it may need a helping hand to start). Reduce the voltage once rotation is established in order to keep the current to around 5 amps.
- Reversing the polarity will reverse the direction of rotation.
- The copper sulfate solution forms an electrical conductor since it has free ions and completes the electrical circuit.
- An interesting but unwanted side effect is the reduction of copper sulfate by electrolysis to copper metal. This can be seen forming like iron filings at the end of copper rod as it rotates.
- This was the first motor ever produced repeatedly by Michael Faraday in his laboratory at the Royal Institution, London.
This experiment was submitted by Richard Walder from Eastbourne College.