Electromagnetic induction is one of the key methods to generate power across the country. It's part of everyday life, such as when you charge your phone wirelessly or turn on a bike light by pedalling. It is the process of generating electricity using magnets and movement.
This article will explain what electromagnetic induction is, how it works, and real-world devices, including generators, transformers and dynamos. We'll explore factors that can affect induction, predicting the direction of an induced current and the importance of Lenz's Law. This guide is suitable for GCSE Physics students, including the AQA exam board.
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Electromagnetic Induction
A potential difference (voltage) is created across a conductor when it experiences a change in the magnetic field. This is electromagnetic induction, also known as the generator effect. It is crucial to how electricity is produced, such as in power stations and wind turbines.
Let's say we have a coil of wire connected to a voltmeter. If you push a bar magnet into the coil, the voltmeter needle will come alive and start to flicker. This shows that a voltage has been induced across the wire. The needle will move in the opposite direction when you remove the magnet. If you hold the magnet while it remains in the coil, nothing will happen. This is because induction requires a changing magnetic field, a complicated way of saying that movement is essential.
Voltage is only induced when magnetic field lines are being cut. This happens when a wire moves through a magnetic field, or the field moves across a stationary wire. It doesn't matter which object is moving as long as there is relative motion. If the wire is part of a full loop and it is a complete circuit, the voltage will cause current to flow. This is how generators operate: the coil spins inside a magnetic field, constantly cutting the field lines and producing a continuous supply of electricity.
Another device using this process is called a dynamo. You often find it on bikes near the wheel. It serves as a small generator. When the wheel turns, it spins a magnet inside the coil and induces a current to power the bike light. You only need motion and magnets for it to operate.
1
What will induce a voltage in a coil of wire?
Voltage size and direction
We now need to consider how much voltage is induced and what direction the current flows in. The size of the voltage is affected by the following:
- Speed of movement - The amount of voltage induced will be greater the faster the magnet or coil moves.
- Magnetic field strength - The magnetic field will be denser with a stronger magnet. This means more field lines will be cut through the magnetic field during movement, which increases the voltage.
- Turns in the coil - When a coil of wire has more loops or turns, there will be more magnetic fields cut at once. This will increase the total voltage.
- Coil cross-section - A larger coil will "catch" more field lines as they move, increasing the induced voltage.
- Angle of movement - The greatest voltage is induced when the wire cuts across the magnetic field lines at 90 degrees. Very little is induced when the wire moves along the field lines.
Direction of the current
The induced current will move according to the movement and polarity of the magnetic field. When you reverse the magnet's direction or the wire's motion, the current of electricity will also change.
Physicists use Fleming's Right-Hand rule to predict the direction of the current. It's a simple memory aid:
- Thumb = direction of movement (force)
- First finger = magnetic field (North to South)
- Second finger = induced current (conventional current, from positive to negative)
Make an L-shape with your thumb and first two fingers on your right hand. Make sure the fingers and thumb are 90 degrees to each other. Align them correctly and you'll know which way the current is going. For further reading, Wikipedia has an article on Fleming's Right-Hand rule.
2
What will increase the voltage induced in a wire moving through a magnetic field?
Lenz's Law
Emil Lenz was a physicist in the 19th century. He formulated Lenz's Law in 1834, which explains how nature resists change in magnetic fields. It states:
"The direction of the induced current is always such that it opposes the change that caused it."
Put another way, when a magnetic field induces a current in a wire, the current flows in the direction that creates its own magnetic field. This new magnetic field will try to stop whatever caused the induction in the first place.
For example, let's say you push the north pole of a magnet into a coil. Which way will the induced current flow?
- Lenz's Law states that the coil will oppose the magnet coming in.
- This means the current will create its own north pole at the end closest to the magnet.
- The coil will resist the magnet being pushed in because like poles repel.
Imagine pulling the magnet out of the coil. This time, the coil will create a south pole to try and keep the magnet from leaving. This is because opposites attract. The direction of the induced current will reverse itself.
This "resisting" behaviour is nature's way of conserving energy. If the coil didn't push back, the energy would be generated for free, which breaks the rules of physics.
Here are further examples of Lenz's Law:
- Bicycle dynamos - When you pedal harder on your bike, you are providing more energy against the magnetic resistance. This generates more current.
- Power station generators - Huge coils resist being spun by turbines. This resistance produces electricity.
- Magnetic braking - Trains and rollercoasters sometimes use eddy currents and Lenz's Law to slow the vehicle down safely (an eddy current is a swirling electrical current induced in a conductor by a changing magnetic field).
3
What does Lenz's Law explain in electromagnetic induction?
Real-life applications
Electromagnetic induction is used to power much of the world around it. This induction helps explain how electricity is generated and transferred. It also helps us understand the use of electricity in wireless technologies.
In power stations, electricity is generated by rotating huge coils of wire in magnetic fields. The coil spins inside the magnetic field to induce a current. It is the same for all power stations, including gas turbines, wind turbines and hydroelectric dams. This is how almost all electricity starts its journey to your home.
A transformer is an electrical device that transfers energy from one circuit to another, normally changing the voltage. They rely on electromagnetic induction to change voltage levels. A step-up transformer increases voltage so the electricity can travel long distances via power lines. These are found near a power station. A step-down transformer reduces the voltage to a safe level for use in buildings, such as homes and schools. These transformers are crucial. Without them, the National Grid we depend on wouldn't function.
These are further examples of the use of induction in society:
- Bicycle dynamo - the front light is powered without batteries. The wheel spins a magnet inside a coil as it turns. This generates electricity.
- Induction hob - the pan is heated directly using changing magnetic fields and induced currents in the metal.
- Wireless phone charger - coils and alternating magnetic fields transfer energy without the need for cables.
- Electric guitar - the metal strings vibrate as they are strummed. This is above a coil that disturbs the magnetic fields, inducing signals that become sound.
3
Which of these devices works using electromagnetic induction?
Final thoughts
Electromagnetic induction explains how we generate electricity, the importance of transformers to move this energy efficiently, and how devices like dynamos and induction hobs work. We have learnt that:
- Electromagnetic induction happens when a conductor moves through a magnetic field or when the magnetic field around it changes.
- The size of the induced voltage depends on various factors, including speed, number of coils and the strength of the field.
- Lenz's Law tells us that the induced current will resist the change that caused it.
- Inducting powers the world around us, from homes to charging phones wirelessly.
For further reading, you can learn about transformers from SaveMyExams. You can also tackle GCSE Physics past papers focused on this topic by StudyMind.
If you need support learning about inducting, TeachTutti have qualified GCSE Physics tutors. Every tutor has an enhanced DBS check and you can learn online using the TeachTutti learning platform.
