5.2.7Electricity and Magnetism
Investigating the Strength of Electromagnets
5 min read 5 parts Lesson 30 of 30 Unit quiz
Section 1 of 5Method
1
Wind a coil of insulated wire around a soft iron nail, and connect it to a power pack in series with an ammeter and a variable resistor.
2
Set the current to a chosen value and count how many paper clips the electromagnet picks up before they fall.
3
Change only the current, keeping the number of turns fixed, and repeat the count at each current value.
4
Repeat the whole process with a different number of turns, keeping the current fixed, and record each set of results in a table.
Section 2 of 5Variables
Variables
Independent variable
- The factor deliberately changed, such as the current or the number of turns of wire on the coil.
Dependent variable
- The measured quantity, here the number of paper clips picked up, which indicates the strength of the electromagnet.
Control variables
- Factors kept the same, such as the core material, the length of the coil and the type of paper clip used.
Repeatability
- Repeating each reading and calculating a mean reduces the effect of random error in the count.
Section 3 of 5Analysing the Results
Example readings for a coil with a soft iron nail core.
Analysing the Results
Trend
- More current gives a stronger electromagnet, so more paper clips are picked up, although the increase may level off at high currents.
Turns
- Doubling the turns at the same current nearly doubles the number of clips picked up in the example data.
Safety
- Keep the current low and switch off between readings to stop the coil overheating.
Improvement
- Measuring the total mass of clips picked up with a balance gives a more precise reading than a count.
| Number of turns | Current (A) | Paper clips picked up |
|---|---|---|
| 50 | 0.5 | 4 |
| 50 | 1.0 | 8 |
| 50 | 1.5 | 11 |
| 100 | 1.0 | 15 |
Section 4 of 5Worked Example
Question
- A student records 8 paper clips at 1.0 A with 50 turns, and 15 paper clips at 1.0 A with 100 turns.
- Explain what the results show.
- Compare the two readingsthe current is kept at 1.0 A in both.
- The number of turns doubles from 50 to 100.
- The number of clips picked up rises from 8 to 15, which is nearly double.
Answer With the current kept constant, increasing the number of turns increases the strength of the electromagnet.
Section 5 of 5Exam Checklist & Tips
5.2.7 – Key Points for the Exam
- Independent variable The variable that is deliberately changed by the experimenter.
- Dependent variable The variable that is measured in response to the change.
- Control variable A variable kept constant so that it does not affect the results.
- Repeatability The ability to obtain similar results when a measurement is repeated.
- Variable resistor A component whose resistance can be changed to vary the current in a circuit.
Common Mistakes
- Changing two variables at once, so the results cannot show which factor caused the change.
- Counting clips that cling to one another rather than to the core, which inflates the result.
- Plotting the dependent variable on the x-axis; the independent variable belongs on the x-axis.
- Leaving the current switched on for long periods, which heats the coil and can damage it.