Physics | Electromagnetic induction

Welcome to Electromagnetic Induction!

Hello future physicists! This chapter is one of the most exciting in the entire curriculum because it explains how we generate almost all the electrical energy used in our homes, schools, and cities.

We've already learned that electricity can create magnetism (electromagnetism). Now, we are going to learn the reverse: how magnetism can create electricity. This process is called Electromagnetic Induction (EMI). Don't worry if this seems tricky at first—we will break it down step by step!

Quick Review: Prerequisites

Remember that magnetic fields are areas where magnetic forces act. We represent these fields using magnetic field lines. To understand EMI, you need to remember that these lines point from the North pole (N) to the South pole (S).

I. The Core Idea: Electromagnetic Induction (EMI)

What is Electromagnetic Induction?

Electromagnetic Induction is the process of generating an electric voltage (or electromotive force, EMF) across a conductor (like a wire) when it is exposed to a changing magnetic field.

In simpler terms, if you move a wire near a magnet, you can make current flow!

The Two Ways to Induce a Voltage

For induction to happen, you must have relative movement between the wire and the magnetic field lines.

1. Moving a Conductor: Moving a wire or coil through a stationary magnetic field (e.g., pushing a wire back and forth between two magnets).
2. Moving a Magnetic Field: Moving a magnet near a stationary coil (e.g., pushing a bar magnet into and out of a coiled wire).

The Analogy: Cutting the Field Lines
Imagine the magnetic field lines are invisible strings stretched between the North and South poles. When you move a wire, it must cut across these strings. Every time the wire cuts a field line, a small voltage is induced. If the wire moves parallel to the lines (not cutting them), no voltage is induced.

Factors Affecting the Induced Voltage

The size of the induced voltage (how much electricity we generate) depends on three main factors. If you increase any of these, you increase the voltage:

• Speed of Movement (Rate of Change): The faster the magnet or wire moves, the faster the field lines are cut, resulting in a larger induced voltage.
• Strength of the Magnetic Field: Using a stronger magnet means there are more field lines packed closely together. Cutting more lines results in a larger voltage.
• Number of Turns/Coils: If you use a coil with more turns (more loops of wire), each loop contributes to the voltage, resulting in a significantly larger total induced voltage.

Memory Aid: Think of the factors as the three S's: Speed, Strength (of magnet), and Spirals (number of turns).

Common Mistake to Avoid: A wire sitting still inside a magnetic field will NOT generate electricity. You need relative movement or a changing field.

II. Application 1: The AC Generator (Alternator)

The AC generator, often called an alternator, is the device that turns the mechanical energy of spinning (like water turbines or steam turbines) into electrical energy.

Structure and Function of the AC Generator

The structure of a simple AC generator consists of:

1. A Strong Magnetic Field: Provided by permanent magnets (or electromagnets).
2. A Coil (Armature): A loop of wire placed between the magnetic poles.
3. Slip Rings: Two separate metal rings that rotate with the coil.
4. Brushes: Carbon contacts that rest against the slip rings to conduct the current out to the external circuit.

Step-by-Step: How AC is Generated

1. The coil is rotated (mechanical energy input) within the magnetic field.
2. As the coil spins, its sides cut the magnetic field lines, inducing a voltage.
3. When the coil cuts the lines fastest (moving perpendicular to the field), the voltage is maximum.
4. When the coil moves parallel to the lines (momentarily moving along the field lines), the voltage drops to zero.
5. After half a turn (\(180^\circ\)), the direction in which the coil cuts the field lines reverses.
6. This reversal of movement causes the induced voltage and current to reverse direction.

Because the direction of the voltage (and current) constantly changes (reverses every half cycle), the generator produces Alternating Current (AC).

The Role of Slip Rings

The use of slip rings is crucial for producing AC. Since the rings are separate and each connects continuously to one end of the coil, the external circuit receives the current as it naturally alternates direction during rotation.

III. Application 2: The Transformer

Transformers use the principle of EMI to change the voltage of an AC supply, usually stepping it up for long-distance transmission or stepping it down for safe domestic use.

What is a Transformer?

A transformer is a device used to change a large alternating voltage into a smaller alternating voltage, or vice versa. They work by creating a changing magnetic field in one coil, which then induces a voltage in a second coil.

Key Components

• Primary Coil (\(N_p\)): The input coil connected to the AC power supply.
• Secondary Coil (\(N_s\)): The output coil where the new voltage is induced.
• Soft Iron Core: A loop of magnetic material (laminated soft iron) that guides and concentrates the magnetic field lines efficiently from the primary coil to the secondary coil.

Why Must Transformers Use AC?

This is a critical point!

The primary coil is connected to the AC supply. AC constantly changes direction, meaning the magnetic field it produces constantly changes strength and direction. This changing field is what cuts the secondary coil's wires, inducing the output voltage (EMI).

If you used Direct Current (DC), the primary coil would create a steady, unchanging magnetic field, and no voltage would be induced in the secondary coil. Transformers only work with AC.

Step-Up vs. Step-Down Transformers

The type of transformer is determined by the number of turns on the primary coil (\(N_p\)) compared to the number of turns on the secondary coil (\(N_s\)).

1. Step-Up Transformer

• Purpose: Increases the voltage.
• Turns: The number of turns on the secondary coil is greater than the primary coil. (\(N_s > N_p\)).
• Use: Power stations use step-up transformers to increase voltage for transmission along the National Grid.

2. Step-Down Transformer

• Purpose: Decreases the voltage.
• Turns: The number of turns on the secondary coil is less than the primary coil. (\(N_s < N_p\)).
• Use: Used near homes or within phone chargers to safely reduce the high voltage to a usable, low voltage.

The Transformer Equation (Voltage and Turns)

The ratio of the voltages is directly proportional to the ratio of the number of turns on the coils. This allows us to calculate the output voltage needed.

$$ \frac{\text{Voltage in Primary Coil}}{\text{Voltage in Secondary Coil}} = \frac{\text{Number of Turns in Primary Coil}}{\text{Number of Turns in Secondary Coil}} $$ $$ \frac{V_p}{V_s} = \frac{N_p}{N_s} $$

(Where \(V\) is Voltage and \(N\) is the number of turns.)

The Power and Current Relationship

Transformers are highly efficient, meaning very little energy is wasted (often 99% efficient). For calculations, we usually assume the transformer is 100% efficient, which means the power input equals the power output.

Remember the power formula: \( P = V \times I \).

$$ \text{Power Input} = \text{Power Output} $$ $$ V_p I_p = V_s I_s $$

Crucial Deduction:

• If a transformer steps up the voltage (\(V_s\) increases), the current must step down (\(I_s\) decreases) to keep the power constant.
• If a transformer steps down the voltage (\(V_s\) decreases), the current must step up (\(I_s\) increases).

This relationship is why high voltage is used for transmission: High voltage means low current, and low current reduces energy wasted as heat in the transmission cables.

Conclusion: Bringing it Together

Electromagnetic induction is the fundamental principle behind modern electricity supply. Generators convert movement into AC electricity, and transformers efficiently change the voltage level for distribution and safe use. Mastering these concepts is key to understanding the entire field of electromagnetism!