# PHY C23: Electromagnetic Induction

Before this, we have seen how a current-carrying conductor in a constant magnetic field will experience a force. Now we turn to the opposite of this effect:

• EM Induction
• Factors affecting the magnitude of induced e.m.f.

What is electromagnetic induction?
The effect where a changing magnetic flux can induce an e.m.f. in a circuit.

This means that if I have a conductor in a magnetic field, & the magnetic flux within that conductor CHANGES somehow, an e.m.f. will be produced which could drive current in the conductor.

A few things to note:

• an e.m.f. will always be induced when there is a change of magnetic flux
• we use the term ‘e.m.f.’ instead of ‘voltage’ because cutting the magnetic field PROVIDES electric potential energy to charges to move
• a current is only induced when the circuit is complete (an incomplete circuit will cause an e.m.f. with no current in the conductor)
•

How does magnetic flux change?
There are a couple of ways to do so:

In these situations, we say that a conductor CUTS the magnetic field lines.
*REMEMBER: there will be no e.m.f. if conductor moves parallel to the field lines.

In practice, it is inefficient to generate e.m.f. with a single wire cutting a magnetic field because the cross-sectional area A is very small.
Instead, multiple turns of wire are used to maximise A:

This is why we use coils in most induction devices.

Before we can explore the characteristics of EM Induction further, it’s time to introduce a new term:

The product of change of magnetic flux (ΔΦ) & the number of turns (N) of a conductor involved in the change of flux.

Basically, this gives the total change of magnetic flux of a circuit.

What are the factors affecting the MAGNITUDE of induced e.m.f.?
Experimentally, scientists have found that:

• increasing cross-sectional area (A) of a single wire perpendicular to the field lines = larger e.m.f
• increasing the number of turns in a coil (N) = larger e.m.f.
• increasing strength of a magnet AKA magnetic flux density (B) = larger e.m.f.
• decreasing time taken (t) in changing magnetic flux = larger e.m.f.

We can put these all together & explore QUANTITATIVELY how much e.m.f. is generated.

# Faraday’s Law of EM Induction

Since Φ = BA, we can also express this as:

This fits in with our experiments: the faster we change the magnetic flux, the more e.m.f. induced!