An introductory experiment showing that electromagnets can conveniently be switched on and off.
Apparatus and materials
For each student group
Copper wire, PVC-covered, 150 cm with bare ends
Nail, large, made of iron
Tintacks or paper clips, supply of
Chemical balance OPTIONAL
Ammeter, 0-1 A OPTIONAL
Health & Safety and Technical notes
The nail should be made of iron which is magnetically soft ('cut' nails are suitable).
The nail may also have gained some magnetism while it has been lying in a cupboard in the Earth's magnetic field. This is easily remedied by heating the nail to 'cherry red heat' and allowing it to cool in the East-West direction. Alternatively use a demagnetising coil, in which an alternating potential difference is connected to a solenoid, and the nail is then slowly withdrawn from the coil to a distance from it.
a Wind a few dozen turns of insulated wire around an iron nail. (Leave enough wire free at either end to make connections to the power supply.)
b Connect the ends of the wire to the low-voltage DC power supply, so that a large current flows round the coil.
c To find out if the nail is a magnet, test it with iron filings. What happens if you turn the current off?
d Offer your electromagnet some larger bits of iron, such as tintacks or paper clips.
e What happens each time you turn the current off?
1 Soft iron is a good temporary magnet. A steel nail will retain a lot of its magnetism once the current in the coil is switched off.
2 Iron filings are chips of soft iron which become temporary magnets when in a magnetic field, and so they line up 'north to south' indicating the direction of the magnetic field.
3 How Science Works extension: This experiment can produce a valid relationship between the number of coils and the strength of the electromagnet without any measurements, only counting.
After a demonstration of the procedure above, students could be asked to design a version of the experiment which would allow them to investigate two factors affecting the strength of the electromagnet: the number of coils and the current flowing in the wire. The number of paper clips held by the electromagnet could indicate the strength of the electromagnet.
This provides an opportunity to discuss the concept of a discrete variable and whether evidence based on discrete variables can lead to a valid conclusion.
The scope of variables here is limited, so this would be suitable as a first investigation that students might plan and carry out themselves, with little or no guidance. Encourage students to find appropriate ways in which to present their results to make them clear and easy to understand.
If students use the mass of iron filings picked up as a measure of the strength, making measurements can prove problematic. One solution is to have a mass of iron filings on a balance pan, use the electromagnet to remove whatever it can and then record the drop in the balance reading.
This experiment was safety-checked in January 2007