Lesson focus: Current in a coil creates a controllable magnetic field that can be strengthened by current, turns and a suitable coreA piece of iron put inside a coil to make an electromagnet stronger..
Explain electromagnetA magnet made by passing electricity through a coil of wire; it can be switched on and off. fields and plan a fair investigation of factors affecting their strength.
Build on earlier learning
Magnets and magnetic fieldsPermanent-magnet field conventions now apply to fields produced by electric current.
Practical
Strength of an electromagnet
- Wind wire around an iron nail and connect it to a low-voltage supply.
- Count how many paper clips it picks up.
- Change the number of turns, keeping the current the same.
Record: A table and graph of turns against paper clips lifted.
Safety: Switch off between tests: the wire gets hot.
Strength of an electromagnet
A current in a coil of wire makes a magnetic field. More turns, a bigger current or an iron core make the electromagnet stronger.
Controls
Open the switch and the clips drop: an electromagnet can be turned off.
You will be successful when you can…
- Describe a current magnetic field.
- Predict coil polarity.
- Explain a soft-iron core.
- Investigate field strength validly.
Words that carry the science
- electromagnet
- A magnet whose field is produced by electric current, commonly using a coil and core.
- solenoid
- A cylindrical coil of insulated wire whose field resembles a bar magnet.
- core
- Material placed inside a coil to change and concentrate its magnetic field.
- field strength
- A measure of the magnetic effect at a point.
- coil turns
- The number of loops of wire around the solenoid.
- soft iron
- Iron that magnetises strongly and loses most magnetisation when the field is removed.
Moving charge creates a magnetic field
01Moving charge creates a magnetic field
A current in a straight wire produces circular magnetic-field lines around the wire. In a solenoidA long coil of wire that acts like a bar magnet when current flows through it., fields from many turns combine to make a strong, fairly uniform field inside and a bar-magnet-like pattern outside.
Reversing current reverses the field and swaps the solenoid poles. Switching current off removes the coil field.
02Current, turns and core control the field
Increasing current or the number of turns per unit length usually strengthens the solenoid field. A soft-iron coreA piece of iron put inside a coil to make an electromagnet stronger. becomes magnetised and concentrates the field, greatly increasing the magnetic effect.
Steel may retain magnetisation, so soft ironIron that becomes magnetic easily and stops being magnetic as soon as the current is switched off. is preferable when the field must switch off cleanly, as in relays and scrapyard lifting magnets.
03A fair test needs a measured field proxy
Possible strength measures include maximum mass lifted, paper clips held, force measured at fixed distance or calibrated magnetic flux density. Each has limitations.
Change one factor, keep coil geometry, core, distance and measurement method controlled, repeat readings and limit current duration. Coils can become hot, changing resistance and creating a burn risk.
Design a turns investigation
Test how coil turnsThe number of times the wire is wrapped round to make a coil. affect the maximum mass lifted.
- Use the same soft-iron core and wire material.
- Set the same current for each turn count using an ammeter and adjustable supply.
- Place the same load geometry at the same core face and increase mass consistently.
- Repeat, record the maximum lifted mass and allow the coil to cool between trials.
How to make the reasoning visible: Controlling current is essential because changing wire length can otherwise change resistance and current.
Electromagnet investigation
The current is held at 1.0 A and the same core is used.
| Turns | Lift 1 / g | Lift 2 / g | Mean / g |
|---|---|---|---|
| 20 | 72 | 76 | 74 |
| 40 | 143 | 147 | 145 |
| 60 | 208 | 216 | 212 |
| 80 | 266 | 274 | 270 |
| 100 | 300 | 292 | 296 |
Describe the pattern and one sign it may not stay proportional.
Why is fixed current better than fixed voltage for this turns test?
Check your understanding
Answer from memory, then check against the suggested answer.
What creates a magnetic field around a wire?
How does reversing current affect a solenoid?
Why use soft iron?
Give three ways to strengthen a simple electromagnet.
Why can adding turns at fixed battery voltage fail to strengthen an electromagnet as much as expected?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student adds more turns to an electromagnet connected to the same battery and counts the paper clips it lifts.
Evaluate the experiment. Identify its strengths and weaknesses.
(3)Suggest improvements.
(3)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., identify the confounded variable before suggesting a controlled-current method.