Magnetism, Induction, and Electromagnetic Devices
Magnetic Fields and Forces
Magnetic effects
Moving charges create magnetic fields, and magnetic fields exert forces on moving charges. The magnetic force on a charge is , so the force is perpendicular to both velocity and field.
A charged particle moves parallel to a magnetic field. What is the magnetic force?
The cross product is zero when velocity is parallel to the magnetic field.
Correct answer: Zero
Why do magnetic forces change direction but not speed in ideal cases?
A perpendicular force changes direction of motion without doing work on the particle in idealized motion.
Correct answer: The force is perpendicular to velocity
Electromagnetic Induction
Faraday’s law
A changing magnetic flux induces an electromotive force. Faraday’s law can be written as . The negative sign reflects Lenz’s law, meaning the induced effect opposes the change producing it.
Why it matters
Induction is the principle behind generators, transformers, induction cooktops, and wireless charging.
Which device directly relies on electromagnetic induction?
A generator converts mechanical motion into electrical energy through changing magnetic flux.
Correct answer: Generator
Motors and generators
Electric motors use current in magnetic fields to create torque, while generators use motion to induce current. These are inverse but closely related energy conversions.
What does Lenz’s law say about the direction of induced effects?
This opposition is essential for energy conservation in induction systems.
Correct answer: They oppose the change in magnetic flux
Device connections
| Device | Main principle |
|---|---|
| Motor | Current in magnetic field produces torque |
| Generator | Changing flux induces emf |
| Transformer | Alternating flux transfers energy between coils |
| Induction cooktop | Changing magnetic field heats conductive cookware |
What is the purpose of a transformer?
Transformers use changing magnetic flux to step AC voltage up or down.
Correct answer: Change AC voltage levels