How to find the current if both positive and negative charges are moving? 
A gas is connected to a copper conductor. In the gas, +6C of charge move to the left every second, and -6C of charge move to the right every second. What is the current in the copper conductor, and what is meant by the current in the gas? 

My attempt at a solution: I've never done any questions involving both positive and negative charges moving, so I'm not sure how to handle this. I feel that -6C moving to the right is 'equivalent' to +6C moving to the left, but I really can't think of any concrete reason  of why this would be the case. For the second part, there are two currents in the gas, so perhaps this is referring to the 'net' current i.e if we were at one point in the gas, how much net charge would we see moving past us each second. But this doesn't take into account the direction the charge is moving in i.e if we had +6C and -6C moving to the left each second, we'd see no net charge moving, so the net current would be 0. But I'm not sure how to extend this analogy to the case in which they are moving in opposite directions. 
 A: *

*If a negative charge moves away from a spot, that spot is now more positively charged - exactly as it would be if a positive charge arrived.

*If a negative charge moves away and a positive arrives, then that spot now effectively has two times a positive net charge. 
So yes, you add the contributions together. 
A: 
I feel that -6C moving to the right is 'equivalent' to +6C moving to
  the left, but I really can't think of any concrete reason of why this
  would be the case.

You're feeling is correct.  But to answer the question, you first choose a reference direction for the electric current; to make it simple, let a positive electric current be to the left.
Then, if 6C/s is flowing to the left, there is an electric current of 6A.
Or, if 6C/s is flowing to the right, there is an electric current of -6A.
Or, if -6C/s is flowing to the left, there is an electric current of -6A.
Or, if -6C/s is flowing to the right there is an electric current of -(-6A) = 6A.
A: By the flemings left hand rule we can find the direction very easily .
If a negative charge moves away from a spot, that spot is now more positively charged - exactly as it would be if a positive charge arrived.
If a negative charge moves away and a positive arrives, then that spot now effectively has two times a positive net charge.
