Inducing current, generators and microphones

Welcome to one of the most exciting areas of GCSE Physics! Until now, you have probably focused on how electricity creates magnetism (such as in electromagnets). In this topic, we will turn that on its head and look at electromagnetic induction: how we can use magnetic fields to generate electricity.

This physical principle is how almost all commercial electricity is generated, from giant wind turbines to coal-fired power stations. It is also the technology that makes microphones and loudspeakers work.

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What you'll learn

• How relative motion between a magnet and a conductor induces a potential difference.
• The factors that determine the size and direction of an induced potential difference.
• How alternating current (a.c.) alternators and direct current (d.c.) dynamos generate electricity.
• How electromagnetic induction is used to make microphones and loudspeakers function.

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1. What is Electromagnetic Induction?

If we put a stationary wire inside a magnetic field, absolutely nothing happens. But if we move that wire so that it cuts across the magnetic field lines, a physical phenomenon occurs: a potential difference (voltage) is created across the ends of the wire.

If this conductor is part of a complete circuit, the induced potential difference will cause an electric current to flow through the circuit. This is known as an induced current.

We can produce electromagnetic induction through two types of relative movement:

1. Moving a wire (conductor) through a stationary magnetic field.
2. Moving a magnet into or out of a stationary coil of wire (as shown in the diagram below).

As the magnet moves into the coil, its magnetic field lines cut through the turns of wire in the coil, inducing a potential difference. Because the coil is connected to a complete circuit, the galvanometer needle deflects, indicating that an induced current is flowing.

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2. Factors Affecting the Induced Potential Difference

The size and direction of the induced potential difference (and the resulting current) are not fixed. They depend entirely on how the conductor and the magnetic field interact.

How to Change the Direction

The direction of the induced potential difference and current will reverse if you:

• Reverse the direction of the relative movement (e.g. pulling the magnet out of the coil instead of pushing it in).
• Reverse the polarity of the magnet (e.g. pushing the South pole into the coil instead of the North pole).

How to Increase the Size (Magnitude)

You can increase the size of the induced potential difference by:

1. Moving the wire or magnet faster: This means magnetic field lines are cut at a faster rate.
2. Using a stronger magnet: This increases the density of the magnetic field lines being cut.
3. Adding more turns to the coil of wire: Each turn of wire cuts through the magnetic field lines, adding more potential difference to the total.

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Opposing the Change: Lenz's Principle

An incredibly important rule of electromagnetic induction is that the magnetic field produced by an induced current always opposes the change that created it.

When you push the North pole of a magnet into a coil of wire:

1. A current is induced in the coil.
2. This induced current creates its own magnetic field around the coil.
3. To oppose the incoming North pole, the coil's magnetic field will form a North pole at the end closest to the approaching magnet. Because like poles repel, this works to push your magnet back out.
4. You have to do mechanical work to push the magnet against this repulsive force. Your mechanical work is what is transferred into electrical energy!

If you then pull the North pole out of the coil:

1. The induced current reverses direction.
2. The closest end of the coil becomes a South pole.
3. Because opposite poles attract, the coil tries to pull the magnet back in, opposing your movement of pulling it away.

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3. Large-Scale vs. Small-Scale Generation (Separate Physics Only)

We can demonstrate induction on a small scale in a school laboratory by simply moving a wire up and down between two permanent magnets, or by manually plunging a magnet into a coil of wire.

However, to supply cities with electricity on a large scale, power stations use massive generators.

In a large-scale generator:

• Instead of moving a wire by hand, a energy source (like steam from burning coal, nuclear fission, or falling water in a hydroelectric dam) is used to turn a shaft.
• This shaft rotates a massive, powerful electromagnet (the rotor) inside high-density stationary coils of copper wire (the stator).
• As the electromagnet spins at high speeds, its intense magnetic field lines rapidly sweep through the stationary coils, inducing a massive, continuous alternating potential difference that is fed directly into the National Grid.

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4. Alternators and Dynamos (Separate Physics Only)

Generators are devices that convert kinetic energy into electrical energy using electromagnetic induction. There are two main types of generators that you need to know: alternators (which produce alternating current, or a.c.) and dynamos (which produce direct current, or d.c.).

Both devices consist of a coil of wire rotating inside a uniform magnetic field. As the coil rotates, its sides cut through the magnetic field lines, inducing a potential difference.

The Alternator (Generates a.c.)

As the coil rotates, one side moves up through the magnetic field while the other side moves down. This induces a current in one direction. Half a turn later, the sides swap roles: the side that was moving up is now moving down, which reverses the direction of the induced current.

To keep this alternating current flowing out into the external circuit, the alternator uses:

• Slip rings: Two continuous metal rings connected to the ends of the coil. They rotate with the coil.
• Carbon brushes: Fixed brushes that press lightly against the spinning slip rings, providing a continuous electrical connection to the external circuit without tangling the wires.

Because the connection never swaps, the current in the external circuit changes direction every half-turn. This produces an alternating current (a.c.) output, which looks like a sine wave on an oscilloscope graph.

The Dynamo (Generates d.c.)

A dynamo is very similar to an alternator, but it has one crucial difference in its connection mechanism:

• Split-ring commutator: Instead of two separate slip rings, it uses a single ring split in half down the middle.

The split-ring commutator acts as a automatic rotary switch. Every half-turn, as the coil passes the vertical position where the induced potential difference is zero, the commutator swaps which side of the coil connects to which external brush.

This means that even though the current reverses direction inside the rotating coil every half-turn, the external connection is swapped at the exact same moment. Consequently, the current in the external circuit always flows in the same direction. This produces a direct current (d.c.) output consisting of a series of positive voltage peaks (a "bouncing" wave).

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5. Microphones and Loudspeakers (Separate Physics Only)

Both microphones and loudspeakers convert energy between electrical waves and sound waves. They both contain a coil of wire and a permanent magnet, but they operate in completely opposite ways.

The Microphone (Sound $\to$ Electricity)

A microphone uses electromagnetic induction to convert pressure variations in sound waves into an electrical current.

Sound waves (pressure variations) 
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Diaphragm vibrates 
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Coil moves back and forth relative to permanent magnet
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Magnetic field lines cut
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Alternating potential difference and current induced

1. Pressure waves: Sound waves (which are longitudinal waves of compressions and rarefactions) hit a thin, flexible paper or plastic cone called a diaphragm.
2. Movement: The changing air pressure causes the diaphragm to vibrate back and forth at the same frequency as the sound wave.
3. Induction: A small coil of copper wire is wrapped around the base of the diaphragm and sits over a permanent magnet. As the diaphragm vibrates, the coil moves back and forth through the magnetic field.
4. Output: This relative movement cuts magnetic field lines, inducing an alternating potential difference across the ends of the coil. If connected to a recording circuit, this creates an alternating current whose frequency and amplitude perfectly match the original sound wave.

The Loudspeaker (Electricity $\to$ Sound)

A loudspeaker (or headphone) does the exact opposite. It does not use electromagnetic induction; instead, it uses the motor effect to convert electrical signals back into pressure waves.

1. Input signal: An alternating electrical current (corresponding to a recorded sound) is sent through a coil of wire attached to a speaker cone.
2. Magnetic interaction: The coil is placed inside a magnetic field of a permanent magnet. Because a current-carrying wire in a magnetic field experiences a force (the motor effect), a magnetic force acts on the coil.
3. Vibration: Because the input current is alternating (a.c.), the direction of the magnetic force reverses rapidly. This causes the coil, and therefore the speaker cone (diaphragm), to vibrate back and forth.
4. Sound waves: The vibrating cone pushes the air molecules back and forth, creating compressions and rarefactions that travel through the air as sound waves.

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