CATAGORY: Electricity & Magnetism
QUESTION: Are two wires with currents in the same directions attracted or
repelled?

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Suppose that two parallel wires carry current in the same direction.  
We find the force on wire 2 due to the field generated by wire 1. By the 
right hand rule the magnetic field due to wire 1 comes out of the plane of 
the page above wire 1 in the figure below and into the plane of the page 
below it 
                       ->
. . . . . . . . . . .  B1 out here

-----------------------------------> wire 1 current I1
                       ->
X X X X X X X X X X X  B1 into page here

-----------------------------------> wire 2 current I2
       ->
       L  ------> directed length in this direction

So wire 2 sees a b field into the plane of the page at right angles to the 
plane of the page.  Now the force on wire 2 due to B1 is

     ->       ->  ->
(1)  F   = I2 L x B1

      ->
Where L is the directed length of wire 2 in the direction of the current
in wire 2. By the right hand rule 
->  ->
L x B1 is towards wire 1.  Therefore the wires are attracted.