Electromagnetic Induction, Generators, and Transformers

Electromagnetic Induction, Generators, and Transformers

If moving a current-carrying wire in a magnetic field produces a force (the motor effect), then the reverse is also true: moving a wire through a magnetic field induces a potential difference. This is electromagnetic induction, and it is the principle behind electricity generation.

Electromagnetic Induction

When a conductor (wire) moves through a magnetic field, or when a magnetic field moves past a conductor, a potential difference (p.d.) is induced across the ends of the conductor. If the conductor is part of a complete circuit, an induced current flows.

This also works when the magnetic field through a coil changes — for example, pushing a magnet into or out of a coil.

Key rule: There must be relative motion between the conductor and the magnetic field, or a changing magnetic field. If everything is stationary, no p.d. is induced.

Factors Affecting the Induced P.D.

The size of the induced potential difference increases with:

• Faster movement (moving the magnet or wire more quickly)
• A stronger magnetic field (stronger magnets)
• More turns on the coil
• A larger area of the coil

The direction of the induced p.d. (and current) reverses if:

• The direction of movement is reversed (e.g. pulling the magnet out instead of pushing it in)
• The polarity of the magnet is reversed (e.g. south pole first instead of north)

The Generator Effect

When you push a magnet into a coil connected to a sensitive ammeter:

1. Moving the magnet in produces a current in one direction

2. Holding the magnet still inside the coil produces no current (no change in field)

3. Pulling the magnet out produces a current in the opposite direction

This demonstrates that it is the change in the magnetic field that matters, not the field itself.

The Alternator (a.c. Generator)

An alternator generates alternating current (a.c.). It converts kinetic energy into electrical energy.

How it works:

1. A coil of wire rotates inside a magnetic field (or a magnet rotates inside a coil)

2. As the coil rotates, it cuts through magnetic field lines

3. This induces a changing p.d. across the coil

4. The p.d. reverses direction every half turn, producing a.c.

5. The current is carried out of the generator by slip rings and brushes (sliding contacts)

The output is a sinusoidal (sine wave) a.c. signal. The p.d. is maximum when the coil moves perpendicular to the field lines, and zero when it moves parallel to them.

Increasing the output:

• Rotate the coil faster (increases frequency AND peak voltage)
• Use a stronger magnet
• Use more turns on the coil
• Use a coil with a larger area

The Dynamo (d.c. Generator)

A dynamo is similar to an alternator but uses a split-ring commutator instead of slip rings. The commutator reverses the connection every half turn, so the output is always in the same direction — it produces direct current (d.c.), though the voltage rises and falls.

Transformers

A transformer changes the voltage of an alternating current supply. It consists of:

• A primary coil — connected to the a.c. input
• A secondary coil — connected to the output
• A soft iron core — links the two coils magnetically

How it works:

1. Alternating current in the primary coil produces a changing magnetic field

2. The soft iron core carries this changing field to the secondary coil

3. The changing magnetic field induces an alternating p.d. in the secondary coil

Important: Transformers only work with a.c. — a d.c. supply produces a constant magnetic field, which does not induce a p.d. in the secondary coil (there is no change).

Transformer Equations

The voltage ratio depends on the number of turns:

V_p / V_s = n_p / n_s

Where:

• V_p = primary voltage, V_s = secondary voltage
• n_p = number of turns on primary, n_s = number of turns on secondary

Step-up transformer: More turns on the secondary than the primary. Output voltage is higher than input voltage. (Used in the National Grid to increase voltage for transmission.)

Step-down transformer: Fewer turns on the secondary. Output voltage is lower than input voltage. (Used to reduce voltage for domestic use.)

For a 100% efficient transformer, the power input equals the power output:

V_p x I_p = V_s x I_s

This means if voltage is stepped up, current is stepped down (and vice versa). This is why the National Grid uses high voltage — it reduces the current and therefore reduces energy lost as heat in the cables.

Example: A transformer has 200 turns on the primary and 1,000 turns on the secondary. The input is 50 V.

V_s = V_p x (n_s / n_p) = 50 x (1,000 / 200) = 50 x 5 = 250 V (step-up)

If the input current is 10 A:

I_s = (V_p x I_p) / V_s = (50 x 10) / 250 = 2 A

Exam Tips

• Electromagnetic induction requires a CHANGE in the magnetic field — a stationary magnet in a coil induces nothing
• For generators, increasing the speed increases BOTH the frequency and the peak voltage
• Transformers only work with a.c. — always state this and explain why (the field must be changing)
• In transformer calculations, check whether the question gives turns or asks for them — do not confuse n_p and n_s