1. Concept Overview
Faraday’s first law gives the basic idea behind electromagnetic induction. The law tells me that whenever the magnetic flux linked with a closed conducting loop changes, an emf is induced in the loop. This induced emf may produce a current if the loop is closed.
I find this law very intuitive — electricity appears not because a battery is connected, but because the magnetic environment around the loop is changing. The key is change in flux, not the presence of a magnetic field alone.
1.1. What the law tells me in one line
Change in magnetic flux → induced emf.
If the flux stays constant, no matter how strong the magnetic field is, there is no induced emf.
2. Statement of Faraday’s First Law
Faraday’s first law can be written in simple words as:
Whenever the magnetic flux linked with a circuit changes, an emf is induced in the circuit.
If the circuit happens to be closed, this induced emf makes an induced current flow.
3. Understanding Magnetic Flux in This Context
Magnetic flux \( \Phi_B \) is given by:
\( \Phi_B = B A \cos\theta \)
So flux changes whenever there is a change in:
- magnetic field strength \( B \)
- area \( A \)
- orientation (angle) \( \theta \)
3.1. Which type of change actually matters?
Any of these changes affect \( \Phi_B \). So the conductor doesn’t necessarily need to move — even if the magnet or field around it changes, induction can occur.
4. Ways in Which Flux Can Change
Faraday’s experiments showed that flux can change in simple, practical ways. I keep these cases clearly in my notes because they help in visualising electromagnetic induction.
4.1. 1. Changing the magnetic field
If the magnetic field becomes stronger or weaker near the loop, the flux changes. Example: pushing or pulling a magnet toward or away from a coil.
4.2. 2. Changing the area of the loop
If the loop expands or contracts, or if a sliding rod changes the enclosed area, flux changes even if the field is constant.
4.3. 3. Changing the orientation of the loop
Rotating the loop in a magnetic field alters the angle \( \theta \), and hence the flux. This is the basis of AC generators.
5. Key Observations From Faraday’s Experiments
Faraday carried out simple coil-and-magnet experiments that clearly demonstrated induction. His observations form the backbone of the first law.
5.1. Observation 1: Motion causes induction
When a magnet is moved towards a coil, the galvanometer shows a deflection. When moved away, it shows deflection in the opposite direction. When the magnet is steady, the deflection disappears.
5.2. Observation 2: Relative motion matters
Even if the magnet is fixed but the coil moves toward or away from it, induction still occurs. So what matters is the relative change in magnetic flux, not which object is moving.
5.3. Observation 3: No change → No emf
If the magnetic flux remains constant, the galvanometer shows no deflection. This is true even if the magnetic field is very strong, confirming the importance of flux change.
6. Mathematical Form (Preview of the Second Law)
Faraday’s first law only tells me that an emf is induced when flux changes. It does not tell me how much emf is induced. That part is handled by Faraday’s second law.
6.1. Rate of change idea
Although the first law does not include a formula, it is closely linked to the idea that the magnitude of induced emf depends on how rapidly the flux changes:
\( \varepsilon \propto \dfrac{d\Phi_B}{dt} \)
This proportionality becomes the actual formula in the second law.
7. Simple Real-Life Examples
Faraday’s first law explains many devices that use changing magnetic fields to produce electricity.
7.1. Electric generators
Rotating a coil inside a magnetic field changes the flux continuously. This change induces emf and produces the alternating current used in homes.
7.2. Bicycle dynamo
The wheel rotation moves a magnet around a coil, changing the flux and producing electricity to light up the lamp.
7.3. Induction cookers
A rapidly changing magnetic field from a coil induces currents in a metal vessel, heating it due to resistance.