7.2.1Magnetism and Electromagnetism
Magnetic Fields in Wires and Solenoids — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 9
Section 1 of 6Field around a straight wire
Pattern
- Pattern A current in a wire produces a magnetic field in the form of concentric circles around the wire.
- The circles are closer together near the wire, where the field is strongest.
Direction
- Direction The direction depends on the current direction.
- Using the right-hand grip rule, point the thumb along the current and the fingers curl in the direction of the field.
Strength
- Strength The field gets stronger with a larger current and weaker with greater distance from the wire.
Section 2 of 6The solenoid
1Coil the wire
- Coil the wire A solenoid is a long coil of wire. Each turn makes its own circular field.
2Fields add up
- Fields add up Inside the coil the fields from all the turns point the same way and add together.
3Uniform inside
- Uniform inside Inside a long solenoid the field is strong and uniform, shown by parallel, evenly spaced lines.
4Bar magnet shape
- Bar magnet shape Outside, the field looks like that of a bar magnet, with a north pole at one end and a south pole at the other.
Section 3 of 6Increasing the strength of the field
| Change | Effect |
|---|---|
| Larger current | Stronger field |
| More turns on the coil | Stronger field |
| Soft iron core inside the coil | Much stronger field, as the core becomes magnetised |
| Reverse the current | Field direction reverses; poles swap ends |
| Turn the current off | The field disappears when a soft iron core is used |
Section 4 of 6Making and using an electromagnet
1
Wind insulated wire in many tight turns around a soft iron nail or bar.
2
Connect the ends of the wire to a power supply and switch on the current.
3
Test the strength by counting how many paper clips it picks up, and compare after changing the current or number of turns.
4
Switch off the current and the clips drop, showing the magnet can be turned on and off.
5
Uses include scrap-yard cranes, electric bells, relays and loudspeakers.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Solenoid | A long coil of wire that behaves like a bar magnet when carrying a current. |
| Electromagnet | A solenoid with an iron core that is magnetic only while a current flows. |
| Concentric | Circles that share the same centre. |
| Soft iron | Iron that is easily magnetised and demagnetised. |
| Right-hand grip rule | A method to find the direction of the magnetic field around a current-carrying wire. |
Section 6 of 6Exam technique
1
Drawing field circles around a wire as evenly spaced. They get further apart as distance increases.
2
Using steel as the core of an electromagnet. Steel stays magnetised; soft iron does not.
3
Saying a larger current makes the field direction change. It changes only the strength.
4
Forgetting to label the poles at the ends of a solenoid.
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