Direction of Current in a Sliding Bar

[patrickbotros]

I'm studying for the Physics SAT Subject Test using The Princeton Review (2011-2012). There's this motional EMF problem about a rod sliding to the right along a u-shaped wire in a uniform magnetic field that's going into the page and the current moving counter clockwise. Problem is: I can't figure out why the current is going counter clockwise. It seems to me that the magnetic force would push electrons in the rod downwards, creating a negative end on the bottom. Therefore, current would go from positive to negative so it would be clockwise, right?

 

[davenn]

Therefore, current would go from positive to negative so it would be clockwise, right?

current = electrons, go from negative to positiveDave

 

[nasu]

If the electrons go down, the conventional current goes up, which means counter-clockwise for the complete circuit.

The current goes from plus to minus in the external circuit. In the source it goes from minus to plus, to close the loop. Here the moving bar is the source.

 

[patrickbotros]

If the electrons go down, the conventional current goes up, which means counter-clockwise for the complete circuit.

The current goes from plus to minus in the external circuit. In the source it goes from minus to plus, to close the loop. Here the moving bar is the source.

Why would the current go from positive to negative? Isn't current the motion of electrons? Why is it in the opposite direction elections go? Is this just a convention or is there a reason?

 

[nasu]

Yes, it is a convention. Look up "conventional current".

 

[davenn]

Why would the current go from positive to negative?

you did state that in you opening post ...

I can't figure out why the current is going counter clockwise. It seems to me that the magnetic force would push electrons in the rod downwards, creating a negative end on the bottom. Therefore, current would go from positive to negative so it would be clockwise, right?

when current flow was discovered, it was long before electrons were discovered and that they moved from neg to pos
As Nasu said do some googling on convention current flow and electron flow Dave