# Direction of Current in a Sliding Bar

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1. May 25, 2015

### 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?

2. May 25, 2015

### davenn

current = electrons, go from negative to positive

Dave

3. May 25, 2015

### 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.

4. May 25, 2015

### patrickbotros

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?

5. May 25, 2015

### nasu

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

6. May 26, 2015

### davenn

you did state that in you opening post .....

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