"Atomic power will make electricity too cheap to meter"
Glenn Seaborg

• It is important to realize that electric potential (potential difference - measured in Volts) is not the same thing as electric potential energy (measured in Joules).  We already know how to calculate the pd dues to a system of charges,

or for a point charge a distance r_A from the charge

 

• What about the Potential Energy, PE ?

Imagine two charges Q and q , with Q at rest.  Now gradually bring q from infinity towards Q to point P.  Then the work done by an external agent in performing this task must be equal to the negative of the work done by the electric field due to Q,

This work is stored in the two charge system as Potential Energy, U_E ,

For systems of more than 2 charges we must consider all possible pairings to determine the total potential energy.

  For U > 0 energy must be provided to create the system
          For U < 0 energy must be provided to break up the system (initially this system is "bound")

  Don't forget  - Change in PE = work needed to accomplish the change.

Do you realize if it weren't for Edison we'd be watching TV by candlelight?
Al Boliska

 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu