Illustration 25.1: Energy and Voltage

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The animation shows a positive test charge in a uniform electric field created by two parallel plates of constant charge.  You can drag the test charge between the plates to any spot and then push "set value and play" to see it move.  You can also read position, voltage, and magnitude of the electric field.  The graph plots the kinetic energy and potential energy as a function of height above the bottom plate.  Restart.

• Does the charge experience a constant force?  Explain. 
• As the charge moves, is the work done on it by the electric force positive or negative?  Explain.
• As you move the charge to a different starting place, how does the total energy change?
• To what positions can you move the charge to decrease the total energy, to increase it, and to keep it the same?

The charge experiences a constant force since the potential energy curve is linear (as with gravity).  Since the kinetic energy increases, the work done by the electrostatic force is positive (and the change in potential energy is negative).  Since work here is F • Δx, the larger the displacement, the larger the work done and the larger the change in kinetic energy.

The potential energy divided by the charge of the particle is the electric potential (measured in Volts).  Triple the charge of the center particle.  In this new configuration the potential energy increases, but the electric potential remains the same (for the charge at the same position).  Why?   What would need to happen to change the electric potential (voltage)?

Illustration authored by Anne J. Cox.
Script authored by Wolfgang Christian and Anne J. Cox.
© 2004 by Prentice-Hall, Inc. A Pearson Company