2002 Hsc (2 Viewers)

[Haku]

For question 23 b) why is the X end of the rode negative?

 

[Captain Gh3y]

Mistake....

 

[Haku]

well the successone book says that it is and the band 6 exampler say it is also...

and can someone also do part c) of that quetsion, the exampler didn;t explain it.

 

[rama_v]

X is indeed negative and Y is indeed positive

Because the conductor is travelling upwards, so then conventional current should be flowing from X to Y as per the right hand rule.

This means that the electron flow is form Y to X. Thus X becomes negative and Y becomes positive.

I remember I had a huge debate on this forum before it was finally settled. Here it is:

http://community.boredofstudies.org/showthread.php?t=68679&highlight=quick+question+lenzs+law

 

[Haku]

hehe, i remember that too, but forgot where that thread was...

could u please also explain part c) of that question as well?

 

[helper]

Read my answer to
http://community.boredofstudies.org/showthread.php?t=90714

 

[Haku]

X is indeed negative and Y is indeed positive

Because the conductor is travelling upwards, so then conventional current should be flowing from Y to X as per the right hand rule.

This means that the electron flow is form X to Y. Thus X becomes negative and Y becomes positive.

I remember I had a huge debate on this forum before it was finally settled. Here it is:

http://community.boredofstudies.org/showthread.php?t=68679&highlight=quick+question+lenzs+law

there is so much debate going on, so rama_V is the final answer that B is negative cause the conventional current flow from A to B, so electrons flow to A making B negative. is that right?

 

[rama_v]

Umm part (c) I think is just referring to eddy currents. Just state that electromagnetic induction can be used to heat up a conductor if it is placed in a region of changing magnetic field, which will induce eddy currents on its surface. These eddy currents cause the heat which is dissipated throughout the conductor

 

[rama_v]

there is so much debate going on, so rama_V is the final answer that B is negative cause the conventional current flow from A to B, so electrons flow to A making B negative. is that right?

huh? In that question, yeah, the answer was B. Because electrons are moving from A to B, B becomes negative and A becomes positive...

 

[Haku]

*after pondering*

but if the electrons flow from X to Y that means most of the electrons converge at Y, wouldn't that make Y negative?

or is it like a magnet things, where the elctron flowing to Y cause Y is positive and it gets attracted.


Edit: oh sorry, didn;t ask about c), part ii), asking "explain how the emf in the conductor is produced"

 

[rama_v]

*after pondering*

but if the electrons flow from X to Y that means most of the electrons converge at Y, wouldn't that make Y negative?

or is it like a magnet things, where the elctron flowing to Y cause Y is positive and it gets attracted.


Edit: oh sorry, didn;t ask about c), part ii), asking "explain how the emf in the conductor is produced"

Ooops terribly sorry nosadness you are right, I had mixed up the sides in my explanation. Now its all fixed and good

For the explanation of how emf is produced
You have to state that as the rod cuts through the magnetic field, it experiences a change in magnetic flux because the magnetic field lines cut through the rod. You may want to state Faradays Law: "The induced emf in a conductor is proportional to the rate of change of magnetic flux." This sets up a potential difference in the rod, hence an emf is generated in the rod.

 

[helper]

You need to isolate the external circuit and the induced EMF.
The induced EMF is formed by the movement of the rod in the magnetic field. This produces the positive and negative end of the rod.

Once the potential difference is produced, then it will result in a current flowing in the external circuit.

So in this question Y becomes positive by the movement of the rod.
When the wire is connected, then conventional current will flow from the positive end to the negative.

 

[Haku]

Ooops terribly sorry nosadness you are right, I had mixed up the sides in my explanation. Now its all fixed and good

now u are getting me confused...where did u mix up...

please post up a new explaination please. but ur explaination seem to work out for the thread that "helper" provided..

and please answer the second part of the question on "explain how the emf is produced"

 

[rama_v]

Its the same as in the other thread
As helper higlights above, it will all become clear if you attach an external circuit to this rod.

Basically the rod is moving up. So, you say, conventional current flow will be induced to oppose this motion. Using the right hand rule, you want the force on the conductor to be down. S the thumb points in the direction of Y, and the fingers point right, so the direction of the force is down. Hence conventional current flow should be from X to Y. This means the electron flow is from Y to X. Now in a normal circuit, this wouldn't mean much. But the electrons in this rod have no where to go. So when they travel they hit a brick wall when they get to X. So X becomes negative, leaving Y positive.

 

[Haku]

i understand now. i think ill stick with the helpers lenzs law explaination.

helper, can we expect to see ur tips for the hsc tomorrow?

 

[helper]

I will try to get some up.

 

[mattchan]

Its the same as in the other thread
As helper higlights above, it will all become clear if you attach an external circuit to this rod.

Basically the rod is moving up. So, you say, conventional current flow will be induced to oppose this motion. Using the right hand rule, you want the force on the conductor to be down. S the thumb points in the direction of Y, and the fingers point right, so the direction of the force is down. Hence conventional current flow should be from X to Y. This means the electron flow is from Y to X. Now in a normal circuit, this wouldn't mean much. But the electrons in this rod have no where to go. So when they travel they hit a brick wall when they get to X. So X becomes negative, leaving Y positive.

So what happens when there is a circuit attached onto that rod? Will your thumb point towards the direction of the positive current, thus making Y negative, IF there was a circuit attached to this?

 

[Abtari]

my understanding is that as this is an INDUCED CURRENT question...

i.e. concerning electromagnetic induction,

you would NOT use right hand rule, rather the left hand rule or as some people do, use the right hand rule and then just write the opposite of the answer they get

answer should be Y is negative...

according to left hand rule, you have movement of conductor upwards, magnetic field acting from left to right (N to S), so thumb (naturally) points towards Y end...i.e. induced current flows from X to Y...that means conventional current flows from X to Y.... that means....X is positive end...Y is negative

hope that is understandable

 

[KFunk]

It's a poorly written question. Current flows from X to Y, i.e. were there a positively charged particle it would flow from X to Y. Electron flow is from Y to X - the would result in electrons building up at end X giving it a negative charge.

End Y is the negative terminal

End X is negatively charged


Which end is negative is partly a matter of interpretation but I'd be inclined to say end X. It's a shite question. They had some really dodgy stuff in the exemplar answers for that year. I remember seeing a person who used the incorrect hand rule and named X as negative (thinking it to be the negative terminal) and hence they got the answer correct (I believe end X is the answer which was payed marks).

 

[helper]

The use of the left and right hand is based on how you are taught to use them Abtari, there is ways to learn so you only have to use the Right Hand Rule.