Electric Potential EnergyGravitational Potential Energy

(a reminder)

Electric Potential Energy

(The derivation of this equation is in the textbook).

With both Q = +

and q = + ,

the PE = +

This means work can be done in returning to the reference position of PE = 0.

With a smaller value of r,

the PE increases.

This means work can be done in returning to the original position of larger r.

With both Q = -

and q = - ,

the PE = +

This means work can be done in returning to the reference position of PE = 0.

With a smaller value of r,

the PE increases.

This means work can be done in returning to the original position of larger r.

With Q = +

and q = - ,

the PE = -

This means work must be done from the outside to return to the reference position of PE = 0.

The idea of negative potential energy is important (and sometimes difficult to understand). If a box on a table has zero gravitational potential energy, then the box has negative gravitational potential when it is on the floor. Work must be done on the box from the outside to return it to the table top, the reference position where it has zero potential energy.

What is the electric Potential Energy of a system composed of + 3 C and - 1 C separated by a distance of 0.5 m?

(c) Doug Davis, 2002; all rights reserved

Return to Ch 17, Electrostatics & Electric Energy