Magnetism and Electromagnetic Induction

Magnetism and Electromagnetic Induction

Electricity and magnetism are two sides of the same coin. Every generator producing power for a building and every transformer stepping voltage up or down operates because of one fundamental truth: a changing magnetic field can create voltage in a conductor. Understanding this relationship is essential for any electrician working with AC systems, motors, or power distribution equipment.

Magnetic Fields and Flux

A magnet produces an invisible field of force around itself. We describe this field using imaginary lines called magnetic flux lines. These lines leave the north pole of a magnet, arc through the surrounding space, and return to the south pole. The total number of flux lines in a region is called magnetic flux (symbol: Φ, pronounced "phi"). The denser the flux lines in a given area, the stronger the magnetic field at that point.

When a conductor — such as a loop of wire — sits inside a magnetic field, no voltage is produced as long as the field is steady. Voltage only appears when the flux through the conductor changes. That change can happen by moving the magnet, moving the conductor, or varying the current that creates the field in the first place.

Faraday's Law

Michael Faraday established in the 1830s that the voltage induced in a coil is directly proportional to (1) the rate of change of the magnetic flux and (2) the number of turns in the coil. More turns mean more of the changing flux cuts through wire, adding up the individual small voltages turn by turn. A coil with 200 turns cutting the same changing flux will produce twice the EMF of a 100-turn coil. The induced voltage is called electromotive force, or EMF (symbol: E or ε).

Faraday's Law in plain terms: EMF = N × (rate of change of flux). You do not need to calculate the exact rate of change here, but you must remember the two variables — number of turns and how fast the flux changes.

Lenz's Law

Faraday tells us the magnitude of the induced EMF; Lenz's Law tells us its direction. Lenz's Law states that the induced current always flows in a direction that opposes the change that caused it. Think of it as nature pushing back. When you push a magnet into a coil, the coil responds by generating a magnetic field that repels the incoming magnet. Pull the magnet out, and the coil tries to attract it back. This opposition is why generators require mechanical energy input — you must do work against the opposing magnetic force to keep current flowing.

The direction of galvanometer needle deflection reverses when you reverse the magnet's motion. You will observe this directly in today's demonstration.

Core Material and Its Effect

Passing flux through iron or steel instead of air concentrates the field dramatically. A ferromagnetic core inside a coil guides and intensifies flux, increasing the induced EMF for the same magnet motion or AC current. This is why transformers use laminated steel cores rather than air cores: more flux linkage means more efficient voltage transfer with less energy loss. The laminations (thin steel sheets) reduce energy losses caused by circulating eddy currents inside the core.

Transformers

A transformer uses electromagnetic induction to transfer electrical energy between two coils — primary and secondary — that share a common iron core. AC on the primary coil constantly changes direction, so the flux through the core is always changing, always inducing voltage in the secondary coil.

The voltage ratio between primary and secondary equals the turns ratio:

Vp / Vs = Np / Ns

A step-down transformer has fewer turns on the secondary than the primary, so secondary voltage is lower. A step-up transformer has more secondary turns, producing higher voltage. Ideal transformers also obey the power rule: power in equals power out (Vp × Ip = Vs × Is), meaning that when voltage steps down, current steps up proportionally.

Generators

A generator uses the same induction principle in reverse. Instead of using changing current to create changing flux, a generator rotates a conductor (or a magnet) so that conductors continuously cut through magnetic flux lines, inducing a continuous AC EMF. The output voltage depends on flux strength, the number of conductor turns, and the speed of rotation. Every large generator in a power plant and every portable job-site generator works on this principle.

Safety Notes

During today's demonstration, the instructor will use a 120 V AC source for the transformer portion. Stay behind the safety boundary marked at the bench. Do not reach across energized terminals. The variac (variable autotransformer) allows voltage adjustment — it must be set to zero before any connection is changed. Treat every energized conductor as live until confirmed de-energized and locked out per LOTO (Lockout/Tagout) procedures. Wear safety glasses whenever working near electrical equipment, as arc flash and component failure are always possible, even at low voltages. The galvanometer portion uses no mains voltage and is safe to handle, but ask before touching any connection the instructor has set up.