Question from 2002 HSC (1 Viewer)

[alexialight]

The answer was End X, I was wondering if anyone could please explain why?

I tried to do the right hand push rule but it didn't seem to work. The direction of the force is in the opposite direction to the motion of the rod, right? (because it's an induced emf), so that would make the force down. The field is left to right, so i ended up with the direction of the positive charges being towards end y. Wouldn't that make end y negative then, because the positive charges are being attracted to it? What have i done wrong?

 

[tempco]

Ok using the right hand push rule, you get the positive charges moving towards End Y... so the accumulation of positive charges at Y means that End X would be more negative than End Y, and therefore it would be negative.

 

[Jezzabelle]

not completely correct.

for starters, positive charges dont 'move' the only particles that do, (or are relatively free too) are electrons. the right hand palm/push rule states the the direction your thumb points gives the directions of CONVENTIONAL current (which by definiton flows from positive to negative P.D), your hands represent the directions of the magnetic field lines and the palm the direction of the force. thus THUMB points to end Y. however ELECTRONS flow in the opposite direction to conventional current (damn old scientists got it wrong in early centuries and now we have to suffer blah!)

As the rod is NOT connected to a circuit, the electrons flow from Y to X and accumalate at X. And as we all know, electrons are negatively charged particles. So therefore RELATIVE to Y, X is more negative. This is why X is the correct answer.

Does that make sense? bringing back memories
cheers! xoxo

 

[tempco]

oh right >_<

thanks for clearing it up!

 

[Jezzabelle]

was that sarcasm i denote?

 

[tempco]

was that sarcasm i denote?

um, nup.

 

[Jezzabelle]

The answer was End X, I was wondering if anyone could please explain why?

I tried to do the right hand push rule but it didn't seem to work. The direction of the force is in the opposite direction to the motion of the rod, right? (because it's an induced emf), so that would make the force down. The field is left to right, so i ended up with the direction of the positive charges being towards end y. Wouldn't that make end y negative then, because the positive charges are being attracted to it? What have i done wrong?

i just re read wat you said... you really concerned me.. umm what is your rank in physics? do you understand it? my msn is beanmasta@hotmail.com if you have any more seemingly obvious questions.. no offence

 

[Jase]

wha???

You just said: thus the thumb points at Y? Dosn't it point at X?

Well the way i see it, you go.. do the right hand rule. Thumb points at X. hence +ve moves from y to x. Since conventional current moves pos to negative, then X must be the negative.

EDIT: Nevermind, my way doesn't make sense. I agree with jezzabelle's one...

 

[BillyMak]

Remember that the rod is moving up, and it is a conductor moving in a magnetic field, so it will experience a force.

Lenz's law states that the force acting on a conductor moving through a magnetic field will be in the opposite direction to it's original motion, and the motion is up so the force is down.
:. Thumb points in Y direction.

 

[Jezzabelle]

Remember that the rod is moving up, and it is a conductor moving in a magnetic field, so it will experience a force.

Lenz's law states that the force acting on a conductor moving through a magnetic field will be in the opposite direction to it's original motion, and the motion is up so the force is down.
:. Thumb points in Y direction.

true true.. man for a one marker this is bloody confusing

 

[BillyMak]

I'd imagine a lot of people got this wrong basically from being tricked and using the push rule in the direction of motion.

 

[Jezzabelle]

I'd imagine a lot of people got this wrong basically from being tricked and using the push rule in the direction of motion.

thats just darn right BOS evil

 

[alexialight]

thankyou so much everyone, i get it now!

 

[JayWalker]

not completely correct.

for starters, positive charges dont 'move' the only particles that do, (or are relatively free too) are electrons. the right hand palm/push rule states the the direction your thumb points gives the directions of CONVENTIONAL current (which by definiton flows from positive to negative P.D), your hands represent the directions of the magnetic field lines and the palm the direction of the force. thus THUMB points to end Y. however ELECTRONS flow in the opposite direction to conventional current (damn old scientists got it wrong in early centuries and now we have to suffer blah!)

As the rod is NOT connected to a circuit, the electrons flow from Y to X and accumalate at X. And as we all know, electrons are negatively charged particles. So therefore RELATIVE to Y, X is more negative. This is why X is the correct answer.

Does that make sense? bringing back memories
cheers! xoxo

-smack-

I'm the Bored of Studies smart ass not you! Get your own role!

 

[kart_racer]

The direction of motion is showing the direction that the force is pushing it. The right hand rule with your palm facing up and your fingers in the direction of the magnetic field lines (south to north) leaves your thumb to point at Y. Therefore X is negative.

Lenz's law doesn't have anything to do with this because the force is pushing the conductor upwards. The right hand palm rule has nothing to do with back-emf (which isnt a force).

 

[Jase]

Yes it does. The emf induced is such that is produces it's own field that opposes the change. This opposing field would in turn give it a force opposing the direction of motion. So by using reverse inspection and the palm rule, you get your palm facing downwards, the magfield still the same, and the current going to Y.

and the direction of the magnetic field lines are North to South..

 

[superbird]

success one hsc states that the correct answer is end X. The board of studies website gives no indication as to what the correct end is.
I originally got end X but since the current is actually induced...im confused

 

[Jase]

The answer is end X. I think jezzabelle explained it best.

You do the whole lenz's law induction thing, and the thumb points to Y. This means 'positive' charge is culminating at point Y.. and negative charge is accumulating at X. Hence X is negative.

 

[d_elmo]

This is one of the easiest questions ive ever seen

right hand palm rule:

fingers point in the direction of the magnetic field-north to south

flip your hand so that your palm is facing up(the direction of the force)

you will notice that your thumb points "out of the screen" (end y to end x)
showing the conventional current in the rod, which we know as from positive
to negative, so that makes end y positive and end x negative.

 

[helper]

elmo, you would have got the mark but your method is totally wrong and if they asked you for the current in the circuit you would have been wrong.