2002 Hsc (2 Viewers)

[helper]

It's a poorly written question.

Nothing wrong with how the question is written.

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.

True if you are talking about the build up of EMF. This current will not be continuos and will stop almost straight away.

End Y is the negative terminal

Wrong

End X is negatively charged

True

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).

The negative terminal is always where there will be a build up of negative charge due to an induced EMF.
It also always the negative terminal of the EMF supply and where conventional current will flow to.

 

[KFunk]

The negative terminal is always where there will be a build up of negative charge due to an induced EMF.
It also always the negative terminal of the EMF supply and where conventional current will flow to.

Initially, end Y is the negative terminal... Why do the electrons move to end X in the first place? ---> because it is the positive terminal. This might change later but while under the effect of the magnetic field the induced 'current' flows as though Y were the negative terminal.

 

[Abtari]

kfunk, i think they mean:

negative in the sense, negative terminal... it would be safe to presume this because it has become conventional for wires, conductors attached to a circuit to go 'this end is negative' meaning it is connected to the negative terminal....

and as for the left hand rule thing, helper, yes it depends on how u are taught. hence u may use the right hand rule, but u must switch the answer u get at the end as we are NOT dealing with a simple motor effect situation...it is an induced current situation...

 

[helper]

End Y is never the negative terminal.

The negatives move to the end X, due to the Force acting on them due to the changing magnetic flux producing a force on a negative charge.

 

[helper]

Abtari, it is a simple motor effect. It is a plain application of Faraday's Law.

Using the Right Hand Palm Rule (I could also use Flemings Left Hand Rule):

1. The movement is upwards, so this is the direction positive particles in the rod will move, so I point my thumb in this direction.
2.The magnetic field is from left to right, so I point my fingers from Left to Right.
3. My palm points intoi the page, so the force on positives is into the page.
4. Negatives go in the opposite direction, so they accumulate at X.
5. So X becomes negative.

 

[Abtari]

Abtari, it is a simple motor effect.

helper, in the diagram given, an emf is being induced. hence a current is also being induced (supposing there is a circuit to join onto the two ends)

how is it a simple motor effect situation? a motor effect situation is when u provide the current to make the conductor move.... (rt hand rule)

this is an induced current situation...quite the opposite of the motor effect situation... (left hand rule/ or right hand rule with the answer reversed)

we are inducing a current here.

 

[serge]

accumulate at X....correct


but that doesnt mean that X is the negative end

I think X is negative as well
using flemings left-hand shoot rule
(ur hand ends up looking like a gun)

helper's right

btw- if BOS made a mistake
we would know about it

 

[Abtari]

yeah i just realised that X is negative...

but its not a simple motor effect as i said earlier

 

[helper]

In a the motor effect, you have electrons moving along a conductor. As the electrons are moving through a magnetic field, their magnetic fields interact, producing a force. This force on the electrons forces them against the side of the conductor and causes it to move.

In an induced current, you have electrons moving through a magnetic field as a conductor is moving and contains electrons. As the electrons are moving through a magnetic field, their magnetic fields interact, producing a force. This force on the electrons forces them towards the end of the conductor. This will produce a potential difference as the electrons are forced to an end.

So in both cases, it is the interaction of moving electrons in a magnetic field producing a force.

So it comes down to what level you want to look at.

Either way you look at it, the answer is the same. When you say you reverse hands, you are also changing what things mean in the hand rule. In the way I do it, the meaning of each thing remains the same.
The only place where my method breaks down is if the magnetic field is changing, instead of movement and in that case, I then think about what do I want the magnetic field being produced to do.

 

[Abtari]

So in both cases, it is the interaction of moving electrons in a magnetic field producing a force.

how is a force produced in the case of induced emf/current??? think of a generator. you don't produce a force. You apply a force, and that induces an emf/current. They are opposite cases. Motor effect is applied in the motor - thats where a force is produced. In the case of the original question, here, the force is being applied...and this allows the induction of an induced emf/current. As I said before, quite opposite cases.

When you say you reverse hands, you are also changing what things mean in the hand rule. In the way I do it, the meaning of each thing remains the same.

for cases involving motor effect, and the production of a force... you use right hand rule to calculate the direction of the force right?

for cases involving the generation/induction of an emf/current (NOT the production of a force)... you use the left hand rule to calculate the direction of the current produced. Alternatively, you can use the right hand rule, taking into account the fact that it is a case of induced current, so the direction would be exactly opposite a normal case i.e. the case when we produce a force.

 

[serge]

for cases involving motor effect, and the production of a force... you use right hand rule to calculate the direction of the force right?

to me right hand and left-hand fleming rule are the same
do both in each hand and you can see they're exactly the same

unless you're talking about a different rule

 

[Haku]

they not the same.

for the generation and emf still just use right. but this has to be done in the direction of lenz's law.

 

[serge]

they not the same.

for the generation and emf still just use right. but this has to be done in the direction of lenz's law.

we're probably talking about two different rules

 

[helper]

Abtari,
You are thinking on a macro scale. A force on the wire. I am talking about on a micro scale.

An induced EMF is a separation of charges. For charges to move a force must act on them.
The force that moves the rod, moves the charges. This results in a current through space but not in the rod.

At this stage don't worry about the method I am talking about. Use the method you are happy with. As long as you are having X as negative and realise, that at no stage Y is negative or a negative terminal.

 

[Haku]

we're probably talking about two different rules

the right hand rule give u direction of movement of current.

by using left hand u find the direction of the electron movement. but its the same but at the same time different rule. to find out electron movement use the right hand rule but point in the direction of the force created by lenz;s law

 

[serge]

by using left hand u find the direction of the electron movement

no the left hand rule i know, gets you conventional current

(not electron movement, thats why i said were
probably talking about something different)

 

[helper]

There are a whole series of versions of the hand rules, so without diagrams or full written explanations, it is hard to compare.

 

[Abtari]

well acc to my method i get X being the negative end, as well...

we probably are thinking of different things

but one point id like to make: its not like oh this hand rule gives the direction of current or what not... even the standard right hand rule which is ubiquitous, can give the direction of any of magnetic field, the force produced, and the current.

nosadness, its not like a certain rule is devised to find the direction of a certain quantity. it's a bit like any other physics relation say v=f.lambda...they give u two, u find the other...

 

[helper]

I just can't get why you keep saying, reverse your answer for the right hand.

Doing it via Lenz's law:
Movement is up, so force is down to oppose motion, Magnetic field is to the right, so induced current is into the page for this section, so positives go to the Y end.

The only way I can think you are talking, is that you are talking about the applied force is in the direction of movement, so you would point your palm in that direction. If that is the case, this is the first time I have heard it explained that way and explains why you reverse the direction or use the other hand.
But as I said, use the method you understand.

 

[Haku]

helper will you be online tomorrow on BOS the whole day like today, or u got to teach ur students?