5.2.7Electricity and Magnetism
Investigating the Strength of Electromagnets — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 18 of 18
Part 1 of 5Aim and equipment
| Equipment | Purpose |
|---|---|
| Iron nail (not steel) | Forms the soft iron core of the electromagnet |
| Insulated copper wire | Wound into a coil around the nail |
| Variable d.c. power supply, 0 to 6 V | Provides an adjustable current through the coil |
| Ammeter | Measures the current in the coil |
| Switch | Lets the circuit be turned on and off |
| Paper clips | Picked up to show the strength of the magnet |
| Crocodile clips and leads | Join the coil into the circuit |
| Clamp stand, boss and clamp | Hold the electromagnet in position |
| Top-pan balance | Measures the mass of paper clips picked up |
Part 2 of 5Why these choices?
Iron core, not steel
- Iron core, not steel Iron is easily magnetised and loses its magnetism quickly when the current stops.
- Steel would stay magnetised and spoil the on-off behaviour.
Insulated wire
- Insulated wire The insulation forces current to follow every turn of the coil rather than short-cutting between touching turns.
Paper clips as a measure
- Paper clips as a measure A stronger electromagnet can hold up a larger number of paper clips, so the number picked up is a measure of strength.
Part 3 of 5Method for the effect of current
1
Wind a coil of insulated wire around the iron nail and connect it in series with the power supply, ammeter and switch. Clamp it so that one end of the nail is above the paper clips.
2
Set the power supply to zero and close the switch.
3
Slowly increase the voltage until the electromagnet just holds one paper clip. Record the current.
4
Increase the current until the electromagnet just holds two paper clips, hanging end to end, and record the current again.
5
Continue until six paper clips are held.
6
Repeat the whole experiment at least three times and calculate mean values of current for each number of paper clips.
Part 4 of 5Variables
Electromagnet investigation
Independent variable
- Independent variable Experiment 1: the current in the coil, I. Experiment 2: the number of turns of wire, N.
Dependent variable
- Dependent variable The number of paper clips (or their mass) that the electromagnet can hold.
Control variables
- Control variables In experiment 1: the number of turns, the nail used and the type of paper clip.
- In experiment 2: the current, the nail, and the paper clips.
Second investigation
- Second investigation Keep the current fixed and vary the number of turns on the nail, for example 10, 20, 30, 40 and 50,
- and count the paper clips held each time.
Part 5 of 5Results, safety and conclusion
1Tabulate
- Tabulate Record current (or turns) against number of paper clips, calculate means and, if wanted, mass of clips from the balance.
2Plot
- Plot Plot a graph of strength (paper clips or mass) against current or number of turns, with a line of best fit.
3Conclude
- Conclude A larger current and more turns both make the electromagnet stronger, so it holds more paper clips.
4Safety
- Safety The coil and wire can become hot at high current, so keep the switch open between readings and do not touch the coil.
- Keep paper clips and other metal away from the supply terminals.
Key terms
| Term | Meaning |
|---|---|
| Electromagnet | A magnet created by a current in a coil of wire, usually wound round an iron core. |
| Solenoid | A long coil of wire that forms a magnetic field when a current flows. |
| Core | The iron material inside the coil that strengthens the magnetic field. |
| Independent variable | The variable that you deliberately change in an investigation. |
| Dependent variable | The variable that you measure as a result of the changes. |
| Control variable | A variable kept the same so the test is fair. |
Avoid these mistakes
1
Using a steel nail for the core. Steel keeps its magnetism, so the electromagnet would not switch off properly.
2
Changing the current and the number of turns at the same time. Only one independent variable may change in each experiment.
3
Hanging the paper clips side by side instead of end to end, which does not give a fair measure of strength.
4
Leaving the switch closed between readings, so the wire heats up and the resistance and current change.
5
Doing the experiment only once. Repeat at least three times and use means to reduce the effect of random error.
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