Motors and Generators (1 Viewer)

[Dash]

I have a question here that is not outlined in the books.
Well, I was alright with the concept of generators until I came to this link below. Now, I asked sum ppl what the hell was up with the direction of the induced currents, but I didn't find any supporting material in the text. Can someone explain to me why the induced current is travelling in the direction that it is? Shouldn't the current travel in the opposite direction that it is because of the direction the coil is forced to rotate in?

http://www.walter-fendt.de/ph11e/generator_e.htm

 

[...]

woah?

 

[Constip8edSkunk]

u can use the right hand palm law... where the thumb representing the charge movement points in the direction of the wires movement, ie the black arrow. the fingers still represent the field direction and the palm represent the direction of the force on a positive charge ie induced conventional current's direction. reverse for force on electron...

thats just how i do it, there other ways but i found this way easier.

 

[ND]

Remember that induced current goes the opposite direction to the right hand palm rule.

 

[Constip8edSkunk]

well i actually dont use the right palm rule that way, cuz i find that way too complicated, i just use it as if a motor force is acting on a charged particle in the conductor. ie thumb pointing in direction of movement and palm the induced current

 

[Rahul]

lenz's law dash!!!

delete the other 'accidental' thread

 

[ND]

Originally posted by Constip8edSkunk
well i actually dont use the right palm rule that way, cuz i find that way too complicated, i just use it as if a motor force is acting on a charged particle in the conductor. ie thumb pointing in direction of movement and palm the induced current

Actually that was directed at Dash, i thought maybe that was the reason he was confused about the direction of current.

 

[Rahul]

Originally posted by ND
Remember that induced current goes the opposite direction to the right hand palm rule.

hmm...wtf?
why is that?

can you refer me to a text anywhere...

 

[ND]

Originally posted by Rahul
hmm...wtf?
why is that?

can you refer me to a text anywhere...

Umm i don't have any textbooks with me atm.

 

[Rahul]

oh ok...

i never heard that the force would be in the opposite direction

hmm...i gotta do summarise M n G

 

[ND]

Originally posted by Rahul
oh ok...

i never heard that the force would be in the opposite direction

hmm...i gotta do summarise M n G

Check your excel. Remember that the current is set up to oppose the change, and so a current is set up such that it's force opposes it's movement. So when you've got your RHP rule, you ahve to make the force point in the direction opposing the applied movemnet, and so the current is in the opposite direction.

 

[Dash]

Originally posted by ND
Check your excel. Remember that the current is set up to oppose the change, and so a current is set up such that it's force opposes it's movement. So when you've got your RHP rule, you ahve to make the force point in the direction opposing the applied movemnet, and so the current is in the opposite direction.

Ahhhh! Thats the post I was waiting for
So as Rahul said, it is a branch of Lenz's Law...
The current that is induced in the coil is set up to oppose the motion of the coil!!!
Now, why doesn't it say that in the textbook?
I've looked through it, it doesn't say anything about the direction of the induced current in a generator!
Would it be out of the syllabus?

 

[ND]

I don't think it'd be out of the syllabus, it's a direct application of Lenz's law.

 

[Dangar]

Originally posted by ND
Remember that induced current goes the opposite direction to the right hand palm rule.

I've never heard of that either... are you sure?? What page in excel, cos all I can find is on pg. 64 where it says (i quote) "thumb points in direction of induced current"... this is using the RHR.

Originally posted by ND
Remember that the current is set up to oppose the change, and so a current is set up such that it's force opposes it's movement.

I understand that, and that would make sense but wouldn't it mean that generators are really innefficient cos the force generated by the induced current would always be opposing the turning force and so the kinetic energy input would have to be increased etc...

 

[ND]

Originally posted by Dangar
I've never heard of that either... are you sure?? What page in excel, cos all I can find is on pg. 64 where it says (i quote) "thumb points in direction of induced current"... this is using the RHR.

Yep i'm sure. As i said i don't ahve a txtbook with me atm, but i'll have a look through tomorrow. But you just need to think about how it relates to Lenz's law.

I understand that, and that would make sense but wouldn't it mean that generators are really innefficient cos the force generated by the induced current would always be opposing the turning force and so the kinetic energy input would have to be increased etc...

Yes but it wouldn't affect it much, cos the current is so small.

 

[ND]

Excel Physics, Pg 63:

"In these cases [when current is induced], however, your palm points in the opposite direction to the applied force,".