Electrical Potential
also known as Voltage
If two negative charges are held in place near each other, they will repel each other. Work must have been done against that repulsion force to bring those charges close together to begin with, and that work was stored as electrical potential energy. We will use the symbol PEelec to represent electrical potential energy. If one of the charges is released, it will accelerate away, gaining kinetic energy as the potential energy decreases.
(This is a situation similar to lifting a weight. As you lift an object, you do work against gravity, and the object gains gravitational potential energy, PEg. Let go, and the object accelerates as it falls, gaining kinetic energy.)
The electrical potential energy per unit charge is called electrical potential. Be careful not to confuse electrical potential energy (measured in Joules) and electrical potential (which is measured in Joules per coulomb, or J/coul). However, the unit "J/coul" is used so often that it has been given a special name, the volt, after Italian physicist Alessandro Volta.
Because electrical potential is measured in volts, it is often called voltage rather than electrical potential.
On an intuitive, gut level, you can think of voltage as a kind of pressure that is pushing electrons apart. The more strongly a battery or generator can cram electrons together, the more desperately they will try to get away from each other, flowing down a wire, through a motor, computer, or whatever, doing work along the way.
Activities & Practice
to do as you read
1. If a battery provides 15 joules of energy to a circuit by delivering 10 coulombs of charge, what is the voltage of the battery?
2. A 9-volt battery delivers nine joules of energy for each coulomb of charge it delivers. One of these batteries can typically deliver about 1800 coulombs of charge before it is "dead". How much energy does it provide over its life?
3. A bolt of lightning typically contains about 35 coulombs of charge, delivered at a voltage of 100 megavolts. What is the total energy delivered?
Examples of Voltage
Batteries deliver charge with a certain amount of energy for each coulomb of charge produced. For most small, disposable batteries, each coulomb they deliver carries 1.5 joules of energy. D, C, A, AA, and AAA batteries are all 1.5V batteries. Car batteries are 12V.
The electricity from the wall outlet of your home is 120V: each coulomb delivered carries 120 joules of energy. (Assuming you live in North or South America; most of the rest of the world uses 220V.)
You experience surprisingly large voltages when you accumulate static charge on your body. For example, when you walk across a rug in the wintertime, you can easily have a potential of thousands of volts. When you have that high a voltage and come near a doorknob, a spark can jump across the gap. (As a rule of thumb, each inch of spark requires about 10,000 volts. The Van de Graaff generator we have used in class usually generates about 80,000 volts.)