Potential Energy
The concept of voltage is tied into the concept of potential energy. We therefore
look briefly at energy.
In mechanics, potential energy is the energy that a body possesses
because of its position. For example, a bag of sand hoisted by a rope over a
pulley has the potential to do work when it is released. The amount of work
that went into giving it this potential energy is equal to the product of force
times the distance through which the bag was lifted (i.e., work equals force
times distance).
In a similar fashion, work is required to move positive and negative
charges apart. This gives them potential energy. To understand why, consider
again the cloud of Figure 2–7. Assume the cloud is initially uncharged. Now
assume a charge of Q electrons is moved from the top of the cloud to the
bottom. The positive charge left at the top of the cloud exerts a force on the
electrons that tries to pull them back as they are being moved away. Since
the electrons are being moved against this force, work (force times distance)
is required. Since the separated charges experience a force to return to the
top of the cloud, they have the potential to do work if released, i.e., they possess
potential energy