What is electromagnetic induction?
Induction means making something happen without touching it. In electromagnetic induction, a magnet makes a voltage in a wire without touching it.
The rule is simple: when the magnetic field through a coil changes, a voltage is induced across the coil. This voltage is called the induced potential difference (or induced emf).
If the coil is part of a complete circuit, the voltage pushes a current. This is the induced current. If the circuit is open, there is still a voltage, but no current.
The field can change in three ways:
- move a magnet towards or away from a coil;
- move a coil near a magnet (or spin it);
- switch the current in a nearby coil on, off, or make it AC.
A magnet that sits still inside a coil makes nothing. The field is there, but it is not changing.
What makes the induced voltage bigger?
Three things make the induced voltage bigger:
- Faster change: move the magnet faster or spin the coil faster.
- More turns on the coil: each turn adds its own small voltage.
- Stronger magnet (or an iron core inside the coil to make the field stronger).
The direction of the current depends on the direction of motion and on which pole moves. Push the north pole in and the current goes one way; pull it out and the current goes the other way. Turn the magnet round and the current reverses too.
The induced current always makes its own magnetic field that opposes the change that caused it. So you must do work to push the magnet in. That work becomes electrical energy. Energy is never free.
The generator effect: dynamos and alternators
A generator turns movement (kinetic energy) into electrical energy. A coil spins inside a magnetic field (or a magnet spins inside coils).
- Alternator (AC generator): as the coil turns, each side moves up, then down, so the current changes direction every half turn. The output is alternating current (AC). On a graph of voltage against time it is a wave.
- Dynamo (DC generator): a split-ring commutator swaps the connections every half turn, so the current always flows the same way. The output is direct current (DC) that rises and falls but never goes negative.
Spin faster and you get a higher peak voltage and more waves per second (higher frequency).
Microphones use induction too: sound waves move a small coil on a magnet, which makes a tiny changing voltage that copies the sound. A loudspeaker does the reverse (motor effect).
Transformers
A transformer changes the size of an AC voltage. It has a primary coil and a secondary coil wound on one iron core.
- AC in the primary coil makes a magnetic field that keeps changing.
- The soft iron core carries this changing field to the secondary coil.
- The changing field induces an AC voltage in the secondary coil.
Transformers only work with AC. Steady DC makes a steady field, and a steady field induces nothing.
Turns rule: Vp ÷ Vs = Np ÷ Ns.
- Step-up: more turns on the secondary (Ns > Np), so Vs > Vp.
- Step-down: fewer turns on the secondary (Ns < Np), so Vs < Vp.
An ideal transformer wastes no energy, so power in = power out: Vp × Ip = Vs × Is. If the voltage goes up, the current goes down by the same factor.
The power grid: why we step up and step down
A power grid joins power stations to homes across a whole country. Long cables have resistance, and current heats them: power lost = I² × R. Losing heat wastes energy.
To cut this loss, a step-up transformer at the power station raises the voltage to a very high value (for example 400 000 V). For the same power, a higher voltage means a much smaller current, so far less heat is lost. Halve the current and the heat loss drops to one quarter.
Near towns, step-down transformers lower the voltage in stages to a safe value for homes (about 230 V in India and Europe, 120 V in North America).
Try it: a practical
In the 3D: push the magnet in slowly, then fast. Predict first: which swing will be bigger? Then raise the turns from 2 to 12 and test again. In the transformer view, set Np = 10 and Ns = 5 and predict the output before you look.
At school or home (with an adult): wind 50-100 turns of thin insulated copper wire round a toilet-roll tube. Join the two bare ends to a sensitive meter (a galvanometer, or a multimeter on the lowest mV range). Plunge a strong magnet in and out. Watch the reading flip sign. Try fast and slow, and a 20-turn coil versus a 100-turn coil.
Key formulas and definitions
- Induced voltage is bigger for: faster change, more turns, stronger field
- Vp ÷ Vs = Np ÷ Ns
- Vp × Ip = Vs × Is (ideal transformer: power in = power out)
- Power lost in cables = I² × R
- Power P = V × I
Worked examples
1. A magnet rests inside a coil joined to a meter. What does the meter read? Why?
Zero. The field through the coil is not changing, so no voltage is induced.
2. A transformer has 100 turns on the primary and 500 on the secondary. The input is 12 V AC. Find the output voltage. Is it step-up or step-down?
Vs = Vp × Ns ÷ Np = 12 × 500 ÷ 100 = 60 V. Ns > Np, so it is step-up.
3. A phone charger steps 230 V down to 5 V. The primary has 2300 turns. How many turns are on the secondary?
Ns = Np × Vs ÷ Vp = 2300 × 5 ÷ 230 = 50 turns.
4. An ideal transformer has 230 V, 0.5 A in the primary and gives 23 V out. Find the secondary current.
Power in = 230 × 0.5 = 115 W. Is = 115 ÷ 23 = 5 A. The voltage fell 10 times, so the current rose 10 times.
5. A station sends 1 000 000 W. Find the cable current at 10 000 V and at 400 000 V.
I = P ÷ V. At 10 000 V: I = 100 A. At 400 000 V: I = 2.5 A. The current is 40 times smaller.
6. Using the last example, the cables have 10 Ω resistance. Compare the heat lost.
At 100 A: I²R = 100² × 10 = 100 000 W. At 2.5 A: 2.5² × 10 = 62.5 W. Stepping up saves almost all the waste.
Common mistakes
- Thinking a still magnet in a coil makes current. Only a CHANGING field induces a voltage.
- Using a transformer with DC. Steady DC gives a steady field, so the secondary gets nothing.
- Thinking a step-up transformer gives free energy. When voltage goes up, current goes down; power out is never more than power in.
- Flipping the turns ratio: Vs ÷ Vp = Ns ÷ Np, so more secondary turns means MORE output volts, not fewer.