Question about moving a magnet in and out of a coil regarding EMF

[HarveyD]

Just want to confirm
Moving a coil down while a magnet is inside it - Would create a positive/increasing EMF?
Moving the magnet down with the coil still - Would create a negative EMF/decreasing?

or is it the other way around
or would it not matter :S

[tchung]

Assuming the north pole of the magnet is pointing downwards inside the solenoid, the magnetic flux will be at its maximum downwards initially. However when you move the coil down, the direction of the magnetic flux would still be downwards but with a lower magnitude of flux than initially as it changes. Therefore the change in magnetic flux would be upwards, and according to Lenz's law, the induced current/emf will have a magnetic field such that it opposes the change in flux, creating a downwards magnetic field.

However, moving a magnet down with the coil still is the opposite of the previous situation. The initial magnetic flux will be downwards. As the magnet approaches the centre of the solenoid, the magnitude of the downwards flux increases, therefore there is a downward change in flux. A current/emf will then be induced such that its magnetic field opposes the direction of the change in flux that caused it, i.e (upwards)

In both situations, an emf is induced but in different directions.

[HarveyD]

how can you tell which direction is positive and which is negative
is it just up = positive and down = negative?
or does it not matter

[tchung]

That depends on the orientation of the magnet and the way the solenoid is orientated i.e the direction of the loop which is connected to the circuit / ammeter

[HarveyD]

how would this apply to a horseshoe magnet?
like this?
wire goes through gap ---> increase in magnet field ---> flux opposes this change ---> creating upwards magnetic field because change in flux is down...?

Sorry im not very good at this :S

[b^3]

Looking at my school, no one is good at this, is there a good set of notes somewhere, or a good extract from a textbook that explains it well?

[HarveyD]

did you do the same prac? (involving horseshoe magnet and wire)

[b^3]

Nah it's just no one in either of our classes gets it/understands it and the teachers don't seem to be able to help us get the idea, confusing us more.

[tchung]

how would this apply to a horseshoe magnet?
like this?
wire goes through gap ---> increase in magnet field ---> flux opposes this change ---> creating upwards magnetic field because change in flux is down...?

Sorry im not very good at this :S

I'm not good with horseshoe magnet, but that seems to be right. The direction of induced current can therefore be determined using the RHG rule.

[HarveyD]

ah k thanks

would any of you guys happen to know why an EMF decreases when the coils are further apart? Is it because the magnetic field is less significant?