Generator Current Direction Help needed please!!! (1 Viewer)

[NichNich]

Hi, I have this horrible question in Jacaranda Physics which I do not understand even after reading the answers over and over again... and I'm getting it very confused with Question 23 (b) of the 2002 HSC paper- which was a question here in an earlier thread...

http://img76.exs.cx/img76/6260/physicsquestion.jpg

Can anyone please help me???! I'm sooo confused

Many thanks in advance!

 

[Jase]

Here's a similar if not exact same thread:

http://www.boredofstudies.org/community/showthread.php?t=47630

It's a confusing question definately.

 

[LSP]

Is the answer from jacaranda: terminal A is negative and B is positive?

 

[Brad]

Yeah B is positive.. use the right hand push rule....

 

[LSP]

yeah, use the right hand rule and you will find out the answer.

Check carefully the slip rings which wire is connected to which. as soon as you find out which way current is flowing, then you will find out the answer!

Same as the HSC question.

 

[NichNich]

I did use the right hand push rule, and it's telling me that current flows from L to K to N to M, right?

If this happened in a motor, current flowing from L to K to N to M, it will make terminal A positive, and terminal B negative.....

Why is this different in a generator?

Thanks for replies!

 

[Brad]

remeber that its a generator so Current is flowing Out of it.....

 

[ashtor]

Yeah since its a generator it can be thought of as a battery, where current flows from negative to positive in the internal circuit making B positive. In a battery the terminal that supplies current to the external circuit is the positive one. In a motor everything's reversed because the coils in a motor are the external circuit (fed from a power supply).

 

[jumb]

If it's AC, wouldnt the polarity change?

 

[asdf_is]

if u do right hand palm rule and lead it back to the source, it shows that B is where the postive current is coming from, isnt that the way?

 

[tennille]

The right hand push rule is only applicable to motors. For genrators, you can use the left hand push rule (same thing, just with the left hand).

 

[helper]

Right Hand Rule can be used for both if you understand it.

The Generator produces an EMF. The EMF is exerting a force on the positive nucleus to B, however they don't move. The electrons receive a force pushing and moving them to A. This makes A negative and B positive.

If you connect this to an external circuit, the current will flow from B to A in the external circuit from positive to negative.

So inside the generator the current is flowing towards the positive as they are having their potential increased.

 

[tennille]

Yes, they can be used for both. But i find it much easier to apply the left hand palm rule for generators.

 

[Brad]

your thinking of the right hand PALM rule.... which is different to the right hand PUSH rule....

 

[helper]

That is the problem there is so many different version of the vector product rule and people give them different names. It is hard to tell which one someone is talking about.

Technically there is only one that is correct and very few people use it.

 

[bboyelement]

The EMF is exerting a force on the positive nucleus to B, however they don't move. The electrons receive a force pushing and moving them to A. This makes A negative and B positive.


So inside the generator the current is flowing towards the positive as they are having their potential increased.

hmm... i dont get the bit where you said the electrons receive a force pushing and moving them to A. Each time i revise physics i always get stuck on this bit ... confusing the blood hell out of me ... please help

 

[helper]

Do you understand how to work out the force acting on a moving charge in a magnetic field?

Assuming you do, when you work out the direction of the force, the direction on a positive charge and a negative charge will be opposite. The same thing happens in a conductor when it is moved in a magnetic field. This forces the positive and negative charges to opposite ends of the conductor.

http://www.uq.edu.au/_School_Science_Lessons/30.0.GIF

If there is nowhere for the charges to move then one end becomes positive and the other negative.

If there is a wire connected to make a circuit, then the current will flow from the positive end of the rod to the negative end. The rod acts as the source of EMF, like a battery.

 

[bboyelement]

Do you understand how to work out the force acting on a moving charge in a magnetic field?

Assuming you do, when you work out the direction of the force, the direction on a positive charge and a negative charge will be opposite. The same thing happens in a conductor when it is moved in a magnetic field. This forces the positive and negative charges to opposite ends of the conductor.

http://www.uq.edu.au/_School_Science_Lessons/30.0.GIF

If there is nowhere for the charges to move then one end becomes positive and the other negative.

If there is a wire connected to make a circuit, then the current will flow from the positive end of the rod to the negative end. The rod acts as the source of EMF, like a battery.

ahha ... got it so the direction where the positive charge is forced to is the postive end and the negative charge is forced in the opposite direction to the negative end... right?
thanks