Current is defined as the net charge that flows through a cross section of conductor per unit time. But is the "net charge" is an algebraic sum of charges or sum of their values? For example, if we have +Q flowing in one way and -Q in the other way, will the current be zero or 2Q?
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It is not zero. It is '2Q'.
In fact this situation actually happens during electrolysis. Positive ions flow in one direction and simultaneously an equal quantity of negative ions flow in the opposite direction. The total current is twice the magnitude of either single current.
The reason is that moving charges have certain properties associated with their movement.
E.g. in the above example:
positive charges moving through the electrolyte produce heating due to collisions;
negative charges moving through the electrolyte produce heating due to collisions.
So you get 'double heating' not zero heating.
At a more advanced level, currents are defined in terms of the magnetic field they produce. Negative charges moving in one direction produced the same magnetic field as positive charges moving in the opposite direction. So if you have equal amounts of positive and negative charges flowing in opposite direction, this doubles the magnetic field - it does not produce a zero magnetic field.
In fact this situation actually happens during electrolysis. Positive ions flow in one direction and simultaneously an equal quantity of negative ions flow in the opposite direction. The total current is twice the magnitude of either single current.
The reason is that moving charges have certain properties associated with their movement.
E.g. in the above example:
positive charges moving through the electrolyte produce heating due to collisions;
negative charges moving through the electrolyte produce heating due to collisions.
So you get 'double heating' not zero heating.
At a more advanced level, currents are defined in terms of the magnetic field they produce. Negative charges moving in one direction produced the same magnetic field as positive charges moving in the opposite direction. So if you have equal amounts of positive and negative charges flowing in opposite direction, this doubles the magnetic field - it does not produce a zero magnetic field.
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spam the question about 50 more times to get more attention.
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Actually, this is exactly what happens in a semiconductor - the current is made up of electrons and holes (+q and -q) flowing in opposite directions - yes they add. ( see diagram at link below)