Potential Difference
If electrons have greater potential energy at one place than at another, then a potential difference is said to exist between the two places. Potential difference is the difference in potential energy per unit of charge, between two points.
In the metric system, potential energy is measured in joules. The charge is measured in coulombs. If the difference in potential energy between two places is 1 joule for every coulomb of charge, then there is a potential difference of 1 joule per coulomb. This amount of potential difference is called 1 volt (abbreviated V).
1 volt = 1 joule of potential energy/coulomb of charge
Suppose you place a small electric heater in a small amount of water say, 1 gram, which occupies a volume of 1 cubic centimeter. Suppose further that the resistance of the heater allows a current of 1 A to flow when the heater is connected to a 1 V cell.
If you let the current flow for exactly 1 second, I coulomb of charge will flow through the heater. As it flows, the electrons will lose energy to the water through the heater. The temperature of the water will rise by slightly less than 0.25°C.
In general, if a conductive path exists between any two points, the charge will flow between the points as long as the potential difference exists. But the current reduces the charge at one point and increases the charge at the other. The effect is to reduce the potential difference between the two places.
If the potential difference decreases to zero, the current stops. Therefore, to maintain a current between two points, you must maintain the potential difference as the charge flows.
A battery or generator acts as a pump, moving electrons through itself to maintain a potential difference between two points. The electrical utility runs wires to your plant from its generators. The utility maintains a certain potential difference between these to keep the equipment running in the plant.
Potential differences can vary from zero to several million volts. The potential difference between the terminals of a dry cell is about 1.5 V. The potential difference between the terminals of an automobile storage battery is about 12 V. The potential differences commonly applied to the terminals of industrial electric motors are 120, 200, 240, 430, 2400, and 6K.
Measuring Potential Difference
The meter used for measuring potential difference is called a voltmeter because it measures the potential difference in volts. A digital voltmeter displays the value as a numeral. A D’Arsonval voltmeter displays the value as the position of a pointer on a scale.
A D’Arsonval voltmeter is connected internally much like a D’Arsonval ammeter. But instead of shunt to carry most of the current around the meter movement, it has a series resistor connected to the movement.
The series resistor has a very high resistance. Therefore, the amount of current flowing through the meter movement is very low. The value of the resistor determines the sensitivity and the range of the volt-meter.
The higher the resistance, the lower the current that flows through the meter movement for a given potential difference. Decreasing the current lowers, the sensitivity of the meter and increases the potential difference required to produce a full-scale deflection of the meter movement.