Magnetism and Electromagnetism
Connect current and magnetic fields, predict induction, and explain how motors, generators, and transformers exchange energy.
Magnetism and Electromagnetism
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Current creates magnetic fields; changing magnetic flux creates voltage
A current-carrying wire produces a magnetic field. Around a straight wire, the field forms circles; around a solenoid, the combined field resembles that of a bar magnet. Reversing current reverses the field.
A moving charge in a magnetic field experiences a force perpendicular to both its motion and the field. For perpendicular motion:
Electromagnetic induction occurs when magnetic flux through a circuit changes. Faster change, stronger fields, and more coil turns generally produce larger induced emf.
Motion alone is not enough
A magnet and coil need relative change in flux. A magnet resting inside a coil produces no continuous induced current.
Lenz’s law is an energy-conservation rule
The induced field opposes the change—not necessarily the field itself. An approaching magnet is resisted; a withdrawing magnet is pulled back.
Separate three electromagnetic devices by their energy direction
Electromagnet: Current → magnetic field.
Motor: Electrical energy → mechanical motion.
Generator: Mechanical motion → electrical energy.
Transformer: Changing current in one coil → changing flux → voltage in another coil.
Why it works
The devices use related coils and fields, so identifying the input and output prevents their functions from being confused.
Five forms you should recognize
Setup: Compasses surround a current-carrying straight wire.
Result: They align tangentially with circular field lines. Reversing current reverses their deflection.
Setup: A coil wraps around a soft-iron nail.
Result: Increasing safe current or adding turns strengthens the field and lets it lift more clips.
Setup: A bar magnet is pushed into a connected coil.
Result: Flux changes and the meter deflects. Stopping the magnet stops the induced current; withdrawing it reverses the deflection.
Problem: Np = 500, Ns = 100, and Vp = 220 V.
Vs = 220(100/500) = 44 VFor fixed delivered power: Raising voltage lowers current.
line heating = I²RLower current therefore sharply reduces energy lost as heat.
Check before you commit
- Saying a stationary magnet produces continuous induction
- Forgetting that reversing current reverses a coil’s poles
- Assuming magnetic force changes a particle’s speed when it acts perpendicularly
- Confusing motor and generator energy conversions
- Using a transformer with steady DC as though flux keeps changing
- Thinking Lenz’s law always means attraction
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Foundations
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Core Practice
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Magnetism and Electromagnetism FAQ
Can a magnetic field do work on a free charged particle?
An ideal magnetic force is perpendicular to velocity, so it changes direction but not kinetic energy.
Why does a transformer not work continuously on steady DC?
After the brief switching transient, current and magnetic flux become constant, so no continuing voltage is induced in the secondary.
Why use soft iron in an electromagnet?
It magnetizes strongly during current flow but does not retain as much magnetism after current stops.
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