Physics Induced Emf Help! (1 Viewer)

[lookoutastroboy]

Hey guys,

Just doing some physics problems but having some difficulty with interpreting emf vs time graphs. Especially when they give you a situation like a coil moving in a field or something and the conditions change.

There was 1 question in the 2010 Independent Physics Trial paper that I hope people can explain. (for some reason - pics of answers won't save - just the question).

1. A metal ring has its plane perpendicular to a magnetic field as shown (see pic)

Over a period of time, the strength of the magnetic field is reduced at a uniform rate from 0.5T to zero. Then its direction is reversed and the strength increased, at the same uniform rate, back up to 0.5T.

Which graph shows a possible emf induced in the ring during this time?

So the answer is a graph is just a straight horizontal line on the emf axis (above time axis). Was wondering why it is a constant EMF given the conditions...

p.s. orientation of pic is screwed; hope yall can read it

Cheers,
L.A.

 

[failwhale]

isn't it because it is always experiencing a change in flux, meaning a constant induced emf over time? idk ahha

 

[imd94]

yeah pretty sure that's it; emf = (change in flux)/(change in time) so since the flux is changing from 0.5 to 0 and back from 0 to 0.5 both over the same time period, the change in flux and the change in time are both constant, giving a constant induced emf.

 

[lookoutastroboy]

thanks for the responses guys - just one thing imd - say in am emf vs time graph - the line passes through to the negative emf (y-axis) - what does this actually indicate - i know its direction - but its hard to visualise this - for these type of Qs - do you have to use the EMF = -n(change in flux)/(change in time) formula every time?

 

[imd94]

hmm yeah these emf graphs are my weak point too. so i'm probably not the person to ask. But i'll give it a crack:
basically if it goes into the negative, that means the emf has changed direction as you said. This is because the flux is now going through the coil in the opposite way. It's helpful to hold a ring or something up and rotate it to see that the flux can go through a coil in different ways. and yeah if you want to use the formula, that works too just remember that flux has a direction.

 

[Mixalli]

hmm yeah these emf graphs are my weak point too. so i'm probably not the person to ask. But i'll give it a crack:
basically if it goes into the negative, that means the emf has changed direction as you said. This is because the flux is now going through the coil in the opposite way. It's helpful to hold a ring or something up and rotate it to see that the flux can go through a coil in different ways. and yeah if you want to use the formula, that works too just remember that flux has a direction.

flux is a scalar and hence does not have direction, only magnitude. From my understanding, its because area can be a vector quantity that the graph can therefore be negative (and hence the flux is passing through a 'negative' area or the area opposite to the initial). But i think its one of the many things in physics which are beyond the syllabus and we just have to remember without really understanding it :/

 

[imd94]

its because area can be a vector quantity that the graph can therefore be negative (and hence the flux is passing through a 'negative' area or the area opposite to the initial).

ah, i stand corrected. i mean that the flux can travel different directions through the coil, i.e. from side A to B or side B to A which constitutes two different directions and thus one is negative. but yeah we had a terrible teacher so all of this is coming from HSC online hahah

 

[someth1ng]

Yeah, change of flux is constant so the emf produced is constant.