A current-carrying conductor produces a MAGNETIC FIELD around it.
For a STRAIGHT WIRE:
Field lines form CONCENTRIC CIRCLES around the wire.
The field is in a PLANE PERPENDICULAR to the wire.
DIRECTION: given by the RIGHT-HAND RULE — point thumb in direction of conventional current (+ to −), fingers curl in direction of field lines.
STRENGTH depends on:
SIZE OF CURRENT — larger current → stronger field.
DISTANCE FROM WIRE — further away → weaker field.
This was discovered by Oersted (1820) — a compass placed near a current-carrying wire deflected. First evidence that electric current and magnetism are related.
The Solenoid
A SOLENOID is a coil of wire. When current flows, it produces a uniform magnetic field INSIDE the coil.
FIELD PATTERN:
Outside the solenoid: similar to a bar magnet — lines from N to S.
Inside the solenoid: uniform, parallel field lines along the axis.
The solenoid acts like a BAR MAGNET — has a definite north and south end.
IDENTIFYING POLES:
Right-hand rule for solenoid: curl fingers in direction of conventional current flow around the coil → thumb points towards NORTH pole.
Alternatively: view each end — if current flows ANTICLOCKWISE = NORTH; CLOCKWISE = SOUTH.
STRENGTH of solenoid's field:
Increase CURRENT → stronger field.
Increase NUMBER OF TURNS → stronger field.
Add IRON CORE → much stronger field (core becomes an induced magnet).
Electromagnets
An ELECTROMAGNET is a solenoid with an IRON CORE.
Why iron (not steel)?
IRON is a SOFT magnetic material — easily magnetised and demagnetised.
When current is OFF → iron loses its magnetism quickly → electromagnet switches OFF.
STEEL would retain magnetism even when current is off — less useful as a switch.
ADVANTAGES over permanent magnets:
Can be switched on and off.
Strength can be controlled by changing current.
Polarity can be reversed by reversing current direction.
APPLICATIONS:
ELECTRIC BELL: electromagnet attracts striker → bell rings → circuit broken → striker returns → repeat.
ELECTRIC CRANE (scrapyard): picks up ferromagnetic scrap when on → releases when off.
CIRCUIT BREAKER (relay): electromagnet pulls a switch to break a circuit.
MAGLEV TRAINS: electromagnets in track repel magnets in train → train levitates.
SPEAKERS: varying current → changing force on cone → sound.
⚠️ Common Mistake
An electromagnet uses IRON (soft) — not steel (hard). Iron demagnetises when the current is switched off. Steel would retain magnetism. Increasing CURRENT or NUMBER OF TURNS both increase the strength of an electromagnet's field.
📌 Key Note
Current-carrying wire: circular field lines, stronger with more current, weaker further away. Solenoid: uniform field inside, acts like bar magnet. Electromagnet: solenoid + iron core. Iron (soft): loses magnetism when off. Stronger with more current, more turns, iron core. Applications: cranes, bells, MRI, relays.
🎯 Matching Activity — Electromagnetism Concepts
Match each feature to the correct description. — drag the symbols on the right to match the component names on the left.
Magnetic field around a wire
Drop here
Solenoid field
Drop here
Why iron core is used
Drop here
Increasing field strength
Drop here
Scrapyard crane
Drop here
Electromagnet picks up ferromagnetic scrap when on, releases when current switched off
Concentric circles — stronger near the wire, strengthens with more current
Uniform inside the coil, acts like a bar magnet with N and S poles
Increase current OR increase number of turns OR add iron core
Iron is soft — easily demagnetised when current stops, so electromagnet switches off cleanly
⭐ Higher Tier Only
Describe the motor effect: a current-carrying conductor in a magnetic field experiences a force F = BIl. Apply Fleming's Left-Hand Rule to determine force direction. Explain how the motor effect is used in loudspeakers. Describe electromagnetic induction (generator effect): a conductor moving in a magnetic field induces an EMF — Fleming's Right-Hand Rule gives induced current direction.
🧪 Required Practical
🔬 RP21 (Physics) — Investigate the factors affecting the strength of an electromagnet (number of turns, current). Plot field strength vs current or turns.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. Why is an iron core used in an electromagnet instead of a steel core?
2. What happens to the strength of an electromagnet's field when the current is doubled?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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🤖 Ask Mr Badmus AI
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