Electromagnetic Induction

**1. Faraday's Discovery — Moving Magnets Make Electricity**

In 1831, Michael Faraday discovered that a changing magnetic field can induce (create) an electromotive force (EMF) and hence a current in a conductor. This is electromagnetic induction — the principle behind every power station in Pakistan, from Tarbela Dam to the wind farms of Jhimpir in Sindh.

Electromagnetic induction occurs when:

• A conductor moves through a magnetic field, OR
• A magnetic field changes around a stationary conductor

The induced EMF drives a current if the circuit is complete.

**2. Faraday's Law and Lenz's Law**

Faraday's Law: The magnitude of the induced EMF is proportional to the **rate of change of magnetic flux linkage**.

In simple terms: move the magnet faster, or use a stronger magnet, and you get a bigger voltage.

Ways to increase induced EMF:

1. Move the magnet/conductor faster
2. Use a stronger magnet
3. Increase the number of turns on the coil
4. Use a soft iron core inside the coil

Lenz's Law: The direction of the induced current is such that it **opposes the change** producing it.

This is a consequence of conservation of energy. If the induced current did NOT oppose the change, you would get energy from nothing — violating physics.

Example: Push a north pole into a coil → the coil becomes a north pole on the entry side (to repel the magnet) → you must do work to push the magnet in → that work is converted to electrical energy.

**3. The AC Generator (Alternator)**

An AC generator converts kinetic energy into electrical energy.

How it works:

1. A coil rotates between two magnets (or magnets rotate around a coil).
2. As the coil cuts through magnetic field lines, an EMF is induced (Faraday's Law).
3. The EMF alternates direction because the coil sides swap between moving up and moving down every half rotation.
4. Slip rings and brushes connect the rotating coil to the external circuit.

Key difference from DC motor: A generator has **slip rings** (continuous rings), not a split-ring commutator. This preserves the alternating nature of the current.

Output: The EMF varies as a **sine wave**. Peak EMF occurs when the coil is horizontal (cutting field lines fastest). Zero EMF when the coil is vertical (moving parallel to field lines, cutting none).

**4. Transformers — Stepping Voltage Up and Down**

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

• A primary coil (input)
• A secondary coil (output)
• A laminated soft iron core (links the magnetic fields)

How it works:

1. AC in the primary coil creates a changing magnetic field.
2. The soft iron core carries this changing field to the secondary coil.
3. The changing field induces an EMF in the secondary coil (electromagnetic induction).

Transformer equation: `Vp/Vs = Np/Ns`

• Vp, Vs = primary and secondary voltages
• Np, Ns = number of turns on primary and secondary coils

Step-up transformer: Ns > Np → higher voltage output (used at power stations).

Step-down transformer: Ns < Np → lower voltage output (used near homes).

For an ideal (100% efficient) transformer: `VpIp = VsIs` (power in = power out)

Pakistan's National Grid: WAPDA generates electricity at ~11,000 V, steps it up to 220,000-500,000 V for transmission across the country (high voltage = low current = less energy wasted as heat in cables), then steps it down to 220 V for homes. Without transformers, Pakistan's electricity system would waste enormous energy in transmission.

**5. Exam Strategy**

• Faraday's Law questions: always state what is changing (flux, speed, number of turns) and how it affects the induced EMF.
• Lenz's Law: the induced effect always OPPOSES the cause. State this explicitly.
• Generator vs motor: generator converts kinetic → electrical (slip rings); motor converts electrical → kinetic (commutator).
• Transformer calculations: write the equation, substitute, solve. Always check if step-up or step-down.
• "Why is the core laminated?" — to reduce eddy currents which waste energy as heat.