Zero potential between two plates

[RK1992]

Say there are two charged plates opposite each other, 1m apart; one is +20V and the other is -20V.

There is a point where we say there is zero potential, 50cm from either plate.

I don't understand how we can say that a charge would have zero potential when we define potential energy as an objects potential to do work. Clearly if an object is being acted on by a resultant force, as it is between the two plates, then it can quite clearly do work..

I know the definition of potential is the work done in moving from infinity to the point, but that doesn't fit with an object's potential to do work. What's wrong? :S

 

[rcgldr]

Say there are two charged plates opposite each other, 1m apart; one is +20V and the other is -20V.

Potential would be relatively zero at the surface of the negative plate.

Potential is the work done in moving from infinity to the point

Potential is the work done per unit charge (or in the case of gravity per unit mass). Infinity is used as a reference when a field is generated by a point source, because potential is relative to 1/r (where r is the distance from the point source). For a line with uniform charge per unit length, potential is relative to ln(r), so choosing r = 1 (depending on units) is an option since ln(1) = 0. For an infinite plane with uniform charge per unit area, potential is relative to r, so choosing r = 0, is a common option, such as gravitational potential on the Earth's surface is commonly expresses as g h (and gravitational potential energy as m g h). Between two charged plates, it's convenient to use the surface of the more negatively charge plate as the point of reference where potential is zero.