20.2Magnetic Fields
Magnetic Effect of a Current and Electromagnets — American System Physics Revision Notes & Topical Questions
10 min read 7 parts Lesson 4 of 5 1 quiz
Lesson summary
Describe the magnetic field pattern around a straight wire and a solenoid.A current-carrying wire has circular field lines centred on the wire.Field strength around a straight wire decreases as distance from the wire increases.Reversing the current reverses the direction of the m…
- Looking at the end of a solenoid
- Magnetic effect of a current
- Right-hand grip rule (wire)
- Right-hand grip rule (solenoid)
- Electromagnet
- Electric bell
Section 1 of 7Magnetic Effect of a Current and Electromagnets
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2
Oersted discovered that a compass needle is deflected by a nearby wire carrying a current, showing that moving charges create magnetic fields.
3
The field around a straight wire is a set of concentric circles. The circles are farther apart further from the wire, showing the field is weaker there.
4
Reversing the current reverses the direction of the field lines; increasing the current makes the field stronger (lines closer together).
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In a solenoid the fields of the individual turns add together. Inside it the field is strong and uniform; outside it resembles a bar magnet's field.
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Adding a soft-iron core concentrates the field, as the core itself becomes magnetised by induction, making the electromagnet much stronger.
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Electromagnets are useful because they can be switched on and off and their strength can be adjusted, e.g. scrapyard cranes that lift and drop cars.
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In a relay, a small current in the input circuit magnetises the coil. It attracts an iron armature which pivots and closes the contacts in the output circuit. This lets a low-voltage circuit safely control a high-voltage one (e.g. a car starter motor).
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In an electric bell, current magnetises the electromagnet, which pulls the armature so the hammer hits the gong. This breaks the contact, the magnet switches off, the spring pulls the armature back, the circuit reconnects and the cycle repeats.
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A magnetic circuit breaker has an electromagnet in series with the circuit. If current becomes too large, the magnet becomes strong enough to pull open a switch; it can be reset.
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The right-hand grip rule always uses conventional current (+ to −).
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Magnetic fields from electric currents are used in loudspeakers, MRI scanners and maglev trains.
Section 2 of 7Process & sequence
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Wind insulated wire around a soft-iron nail to make a coil (e.g. 20 turns).
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Connect the coil in series with a switch, ammeter, variable resistor and d.c. supply.
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Set a current, e.g. 0.5 A, and count how many paper clips the electromagnet can lift.
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Increase the current in steps, keeping the number of turns constant, and repeat.
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Repeat with more turns at a fixed current.
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Switch off between readings to prevent overheating and plot number of clips against current.
How a relay switches a large currentwrite these steps in this order; each step is a marking point.
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The input switch is closed, so a small current flows through the relay coil.
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The coil and its soft-iron core become an electromagnet.
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The electromagnet attracts the soft-iron armature, which pivots.
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The armature pushes the output contacts together, completing the high-current circuit.
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When the input current stops, the core loses its magnetism and a spring opens the contacts.
Section 3 of 7Compare
| Change | Effect on field strength |
|---|---|
| Increase the current | Stronger |
| Increase the number of turns | Stronger |
| Add a soft-iron core | Much stronger |
| Reverse the current | Same strength, poles swap |
| Switch off current | Field disappears (soft iron core loses magnetism) |
| Conductor | Field shape | Direction rule |
|---|---|---|
| Straight wire | Concentric circles, spaced farther apart with distance | Right-hand grip: thumb = current |
| Flat circular coil | Circles round each side; nearly straight at centre | Right-hand grip |
| Solenoid | Like a bar magnet outside; uniform, parallel lines inside | Fingers = current, thumb = N pole |
Section 4 of 7Worked examples
S pole
Looking at the end of a solenoid, the current flows clockwise. What pole is at that end, and what happens if the current is reversed?
- Clockwise current seen from an end → that end is a S pole.
- Reversing the current makes it an N pole; the field direction reverses but the strength is unchanged.
30 clips
An electromagnet lifts 12 clips at 0.40 A. Assuming the number lifted is proportional to current (same core and turns), predict the number lifted at 1.0 A.
- Clips ∝ I12 × (1.0/0.40) = 30 clips (in practice fewer, because the core saturates).
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Magnetic effect of a current · Right-hand grip rule (wire) · Solenoid · Right-hand grip rule (solenoid) · Electromagnet · Core · Relay · Armature · Electric bell · Magnetic field strength · Circuit breaker (magnetic)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Words to avoid
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the field goes along the wirethe field lines are concentric circles around the wire.
2
the iron core makes more currentthe soft-iron core becomes magnetised and increases the strength of the magnetic field.
3
use a steel coreuse a soft-iron core so the electromagnet can be switched off.
4
electrons flow in the direction of the thumbthe thumb points in the direction of conventional current.
5
the relay makes the current biggera small current in the relay coil switches on a separate circuit carrying a larger current.
Instant feedbackCheck yourself
Q1. What is the shape of the magnetic field around a long straight current-carrying wire?
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A. Straight lines parallel to the wire
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B. Concentric circles around the wire
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C. Radial lines from the wire
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D. Like a bar magnet
Show answer
B. The field lines form circles centred on the wire.
Q2. Which change would NOT make an electromagnet stronger?
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A. More turns
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B. Larger current
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C. Soft-iron core
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D. Reversing the current
Show answer
D. Reversing the current swaps the poles but does not change strength.
Q3. Why is soft iron used in the core of a relay?
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A. It is a good conductor
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B. It keeps its magnetism
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C. It is easily magnetised and demagnetised
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D. It is cheap
Show answer
C. The relay must release the armature when the current stops.
Q4. What is the SI unit of magnetic field strength (flux density)?
- A. weber
- B. tesla
- C. henry
- D. gauss
Show answer
B. B is measured in tesla (T).
Q5. How does the field of a straight wire change as you move away from it?
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A. It gets stronger
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B. It stays the same
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C. It gets weaker
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D. It reverses
Show answer
C. Field lines get farther apart, so the field weakens with distance.
Q6. What is the main advantage of a relay?
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A. It stores charge
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B. A small current can switch on a large-current circuit
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C. It converts a.c. to d.c.
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D. It increases the p.d.
Show answer
B. The low-current control circuit is kept separate from the high-current circuit.
Marked practiceQuizzes for this lesson
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