magnetic stuff (1 Viewer)

[mojako]

I dont understand these two questions...
Help would be much appreciated.
Thanks

I thought in Untitled-1.jpg the positive terminal should be x.
And in Untitled-2.jpg why isn't the answer B?
The first one is from 2002 STANSW.
The second one is from 2001 HSC.

 

[Brad]

Q14, since in 1 the magnetic field is constantly perp to the wire when its moving.... the other one is only perpendicular at certain points.... It varies througout the movement

 

[ontherun]

this is 4rom success one regarding question 14 hsc:

WITH FRICTION NEGLECTED, ROD Q EXPERIENCES ONLY A FORCE IN A VERTICAL DIRECTION, AND THUS REQUIRES NO FORCE TO MAINTAIN ITS HORIZONTAL CIRCULAR MOTION, WHITLE THE CURRENT IN ROD P PRODUCES A MAGNETIC FIELDOUT OF THE PAGE ONTO ITS LEADING SIDE, AND HENCE REQUIRES CONSTANT FORCE TO BE APPLIED TO KEEP IT ROTATING

 

[shazzam]

I think Brad is right about the second q, and you can use the same principle in the first question: since the field lines are coming out of the table, they are always perpendicular to the plane of movement of the "metal rod", thus the induced emf remains constant (flux change is constant, as rod is revolving at uniform velocity). NB I'm assuming the "rod" is that thing which is in the same plane as the surface of the table...don't think it's the stick thing coming out of that cylindrical object (that may be part of the "pivot")

 

[mojako]

I have Success One but I don't really understand it.

For the first Q (STANSW):
I know it's constant, but I don't know why y is the positive terminal....

For the second Q (HSC):
mm I think I can see what Brad is saying... Thanks.
(the explanation in Success One is a bit different though)

 

[Premus]

so for the HSC q14

If we were to draw graphs...
i would think that the graph for diagram X - would be a straight line - as you guys said the field is always perp.

What would the graph for Diagram Y look like?

Thanks

 

[tempco]

For the first question, don't the electrons move to Y? So X should be the positive end... ?

 

[shazzam]

For the first Q (STANSW):
I know it's constant, but I don't know why y is the positive terminal....

In this particular case there are no obvious terminals we can refer to hence there's really no such thing as a POSITIVE or NEGATIVE terminal, just a direction the induced current is flowing in. If the rod changes direction midway, let's say it performs simple harmonic motion in a semi-circle, then we'd get an oscillating sine curve, ie current flowing in opposite directions at opposite times. However this is not the case, as it is in continual uniform motion.

I think it's important that they only provided ONE answer where the EMF produced is constant, had they provided TWO, one where it's "positive" and one where it's "negative", the answer would have been ambiguous.

 

[mojako]

it says voltage is measured from x to y and the diagram labels which one is x and y.
also in the answer in Success One, they say y is the positive one....
but if everyone thinks x is the positive then at least I'm more confident.

 

[shazzam]

What do you mean Mojako, is this for diagram one or Driagram 2?

 

[mojako]

What do you mean Mojako, is this for diagram one or Driagram 2?

the x y thing is for diagram 1
sorry for the confusion

 

[FcUk]

isnt th first question just a constant emf coz the rod is always at the same angle to the magnetic field and it doesnt rotate or move(others than revovle) from orginal state as opposed to a loop

 

[Jase]

I agree, the postive end is X, but if the answer is B, then Shazzam's right.
Physics most of the time comes down to eliminations and lines of best fits rather than a correct answer.

wait, so why the hell is the answer B??

 

[mojako]

oh.. guys.. really realy sorry I typed the wrong thing in my first post.
I said "And in Untitled-1.jpg the answer should be B."
I meant it's Untitled-2.jpg.
sorry once again.

so my questions are:
in 1.jpg, why isn't X the positive terminal. but this has probably been answered and the solution on the book is wrong.
in 2.jpg, why is the force on diagram Y variable

thanks.

 

[Jase]

LOL! double typo!.. you had why IS the answer B, at first
And now its why ISNT the answer B.. haha that makes more sense!

Well okay on Q, what are the forces acting? Theres centripetal AND gravity. Since g is constant... the force downards is constant. However, centripetal force is also a vector directed towards the centre. So when the rod is at its lowest point, the centripetal force acts up while the gravity force acts down. then at the top, both forces at down and so on.. So the forces on Q are varying sinusoidally.

 

[mojako]

LOL! double typo!.. you had why IS the answer B, at first
And now its why ISNT the answer B.. haha that makes more sense!

Well okay on Q, what are the forces acting? Theres centripetal AND gravity. Since g is constant... the force downards is constant. However, centripetal force is also a vector directed towards the centre. So when the rod is at its lowest point, the centripetal force acts up while the gravity force acts down. then at the top, both forces at down and so on.. So the forces on Q are varying sinusoidally.

re typo:
I wrote that the answer for 2.jpg (I had 1.jpg at the time) should be B [according to my understanding], which means the same thing as why isn't the answer B. so its single typo.. but a very misleading one
the correct answer is C, u can check the examiners' report to confirm

re 2.jpg and ur explanation:
dont they say it's "horizontal" circular paths in the first paragraph?
so if u combine gravity and centripetal including direction then the net force changes but the magnitude stays the same?

in diagram Y, are there any forces whatsoever caused by the presence of the magnetic field and the current?

 

[Jase]

okay you just confused me lol.
Ah okay woops my interpretation was wrong.
that 'horizontal' is a misleading word, it should be both are moving in uniform circular motion.

Yes there is a force exerted by the field. As it moves to a horizontal position, its cutting the field at right angles, and so there's a force resisting it... meaning varying forces.
so brad's first post was essentially right..

 

[mojako]

hmm ok. thanks ^^

I think the circle is in the horizontal plane though.
and the force in diagram Y acts vertically. but if you also count vertical forces it varies with time, as u & Brad said.

 

[CrashOveride]

for the STANSW question , since the rod is always at right angles to the magnetic field, then from the sin@ in the magnetic flux equation (flux=BAsin@) there is always a max magnetic flux? and how i learnt EMF was to take the -ve derivative (so we are looking for negative gradient) of the magnetic flux curve..since magnetic flux is always at a max... derivative is zero.. ??

 

[CrashOveride]

Bump ^^
Thanks if anyone could help