HSC 2002 Multiple Choice q 10 (1 Viewer)

[Premus]

Hey

I would have thought that the induced emf would be max, when there is greatest change in flux....at point Q ( i thought the answer is A) pls explain?

Also, for the same question, if we were asked to draw graphs of the current against position, flux against position, torque against position ( separately) , what would they look like?

Thank you very much!

 

[chazza]

no its
greatest change in flux that will produce max EMF

 

[Premus]

yeah i thought the answer is A

But it says the answer is B...so i dont understand
Pls explain?
Thanks

 

[chazza]

at p
there is greatest change in flux

at q there is little change in flux

 

[Xayma]

You know the direction of the force.

You know the direction of the magnetic field.

A current will be produced, use your right hand palm rule to determine if there is a current at each of those. There cant be one at Q because the force has no component at right angles to the electric field.

This is more of a qualitative way to determine it, really wont help you if there isnt any points at 0.

 

[chazza]

fine.. ill explain it a bit better
At
P
when the rotor spins a bit, it will experience max Emf coz a change in flux, coz nows more B lines are going into it nows

At B
WHen the rotor spins a bit, it experience min Emf, a there is little change the influx
there B lines going into rotor, are round about the same, Hence little EMF

 

[Premus]

ahhh i see now...

so wherever a current is produced....there will also be an emf.
so for a DC generator, it would also be the same graph yeah?
or would everything be positive?

 

[Xayma]

Absolute value of that graph.

But basically the amount of field lines going left to right is the same.

You are looking for the rate of change of the amount of vertical area it has in order for more field lines to go through (as they are also stacked vertically if that makes sense). The first post of mine is a bit misleading as I was trying to think of a way to explain it.

If you use the rate of change of flux it is better, but mine is a quick alternate way.

Or if you want to get mathsy.

The change in flux is given by the sine curve (as that is how the vertical area is changing and hence the amount of field lines flowing through it) hence the rate of change is given by its derivative a cos curve.

At cos 90° the rate of change is 0.

 

[Premus]

Completely understand it now! Thanks!
but just one last question

for the Flux graph - (sine) - would there be difference for AC or DC (i mean would it be an absolute value graph for DC)?

Thank you

 

[Xayma]

For flux in both cases it is an absolute value sine graph, since you can't have negative flux.

But for AC the change can result in "negative" emf, whereas for DC it can't.

 

[Premus]

are u sure....cause in our catholic trial (CSSA 2004) - (sorry dont have a scanner)

A coil is rotated in an area of constant magnetic flux density. which graph represents the variation of MAGNETIC FLUX as the coil rotates,

question 10 in the multiple choice - i put the answer as D(mod of a sine graph), but the answers say its B( which is a sine graph)??

 

[Xayma]

Well since magnetic flux is magnetic flux density *area and you are given that magnetic flux density is a constant. You can't have negative area in any case.

Magnetic flux will always be non negative.

 

[CrashOveride]

Hey Xayma, or anyone else, can you help out i tried the more mathematical approach here:
http://www.boredofstudies.org/community/showthread.php?t=47506&page=2

Thanks

 

[Steven12]

cant u explain this graph by the fact that maximum torque is produced when armature is parallel to the magnetic field, therefore greatest change in flux? therefore the current will be maximum to begin with... this is just way i look at it.

 

[mojako]

Well since magnetic flux is magnetic flux density *area and you are given that magnetic flux density is a constant. You can't have negative area in any case.

Magnetic flux will always be non negative.

but u can take flux as having some kind of "direction"
and having negative flux enables u to have negative gradient in a flux-time graph which sorta simplifies things because you can then say that the EMF is the gradient of the tangent to the curve.

 

[mojako]

cant u explain this graph by the fact that maximum torque is produced when armature is parallel to the magnetic field, therefore greatest change in flux? therefore the current will be maximum to begin with... this is just way i look at it.

hmm I guess so. this is a good explanation actually.
since torque is greatest, it rotates fastest. since rotates fastest, greatest rate of change in flux.
EDIT: the torque is constant.. as helper said below

 

[Premus]

" but u can take flux as having some kind of "direction"
and having negative flux enables u to have negative gradient in a flux-time graph which sorta simplifies things because you can then say that the EMF is the gradient of the tangent to the curve."

ummm sorry abt this confusion...but what is the right answer? absolute value graph or not?

Thanks

 

[helper]

cant u explain this graph by the fact that maximum torque is produced when armature is parallel to the magnetic field, therefore greatest change in flux? therefore the current will be maximum to begin with... this is just way i look at it.

The torque is constant in a generator. It is produced by an external source

 

[Premus]

" but u can take flux as having some kind of "direction"
and having negative flux enables u to have negative gradient in a flux-time graph which sorta simplifies things because you can then say that the EMF is the gradient of the tangent to the curve."

ummm sorry abt this confusion...but what is the right answer? absolute value graph or not?

Thanks

 

[mojako]

" but u can take flux as having some kind of "direction"
and having negative flux enables u to have negative gradient in a flux-time graph which sorta simplifies things because you can then say that the EMF is the gradient of the tangent to the curve."

ummm sorry abt this confusion...but what is the right answer? absolute value graph or not?

Thanks

umm...
I do not know...
if it says not absolute value in the solution then its not....
if it says absolute value then it is....