Electromagnetic Induction (aka EMI)

We've just finished up EMI for the week, so just a short recap and round up of the chapter:

Definitions
I think the more important definitions in this chapter are that of the laws of electromagnetic induction (i.e. Mr Faraday and Mr Lenz are your very very best friends for this chapter). The other two definitions are a little too literal in my opinion but if you happen to be very lucky, the kind where you know the 4D numbers for this week but forgot to buy them then they might just say hi to you during the exams.

So, Mr Faraday likes to say
"the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage"

and Mr Lenz in reply would say
"the direction of the induced current is such as to oppose the change in flux which causes it"

Do note that Mr Lenz is referring to a case whereby the circuit is a closed one (eg a coil) hence there will be current due to the induced emf that Mr Faraday was talking about. However if the circuit is open then there will only be emf induced and what Mr Lenz is doing is predicting what will happen should we close the circuit. More details here.

Concepts
Flux φ vs flux linkage Φ
Magnetic flux φ = BA cos α
Magnetic fluc linkage Φ = Nφ = NBA cos α

As linkage suggests it means that when there are many many coils, everything links up together hence you have to multiply the flux linkage of one coil with the number of coils present.

Induced EMF

1. Wire moving in a field: E= BLv cos α (N = 1)
2. Wire coil moving into or out of a field = same as a wire moving in a field
3. Wire coil moving within a field (laterally) - the currents will cancel out and there's no change in Φ
4. Rotating coil in a field E = NBAω sin (ωt)
5. Spinning Disc in a field E = BAf cos α

"Disturbances within the Force"
Basically things like to maintain status quo. So when anything is disturbed, they tend to resist change and want to return to how things were before. (sound familiar? no? try talking to your parents about them good ol days)

Inducing EMF and resisting change (ie your 5-step guide to answering explanation questions)

So what Mr Faraday and Mr Lenz are essentially saying can be summed up as follows (in the world of EMI):

1. describe the disturbance (who's disturbing what and in what way?)
2. state whether magnetic flux linkage has increased or decreased (be specific!)
3. Quote best friend no 1: Faraday's Law (the increase/decrease ....... leads to an induced emf ....)
4. Quote best friend no 2: Lenz's Law (this current flows in a direction such as to decrease/increase .........)
5. which leads to (insert opposite of point 1)

To note:
Remember your angles and your trigonometry! not every question is super nice and straightforward with right angles between everything :)