UNIT B.4 — ELECTRICITY

Electromagnetism: fields and induction

BRIDGE · EE Foundations Bridge · ~12 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Charges create electric fields — invisible maps of the force a charge would feel at every point in space. Voltage is a summary of that field: the energy per unit charge between two points. Moving charges (currents) additionally create magnetic fields circling the conductor — the effect your clamp meter (Unit 1.4) reads.

Faraday's law is the crown jewel: a CHANGING magnetic field through a loop of wire induces a voltage in it. Spin a magnet near a coil and you've built a generator. Wrap two coils on one iron core, feed one with AC, and the changing field induces voltage in the other — that is a transformer, and the turns ratio sets the voltage ratio.

This closes a loop with your trade knowledge: transformers only work on AC (Unit 1.8) because only alternating current provides the *changing* field Faraday demands. And the entire grid — generation, transmission at high voltage, distribution — is Faraday's law applied at scale.

Go deeper: MIT's Electricity and Magnetism series (8.02x, three parts) — Semester 2 of the Go Deeper pathway.

E-field source

Charges

B-field source

Moving charges

Faraday

Changing B → induced V

Knowledge check

Quiz — 3 questions

1. A transformer transfers energy between windings via:

2. Transformers don't work on steady DC because:

3. A clamp meter reads current by sensing:

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