The difference in voltage between any two nodes in a circuit is known as the Potential Difference, p.d. or voltage difference and is the work in joules required to move 1 coulomb of charge from one node to another. Then voltage can be defined as a unit of energy per unit of charge with one volt being defined as one joule of energy required to move one coulomb of charge from one node to the other. Potential difference is measured in volts with the circuit symbol V, or lowercase “v”, although Energy, E lowercase “e” is sometimes used.
A constant voltage source is called a DC Voltage with a voltage that varies periodically with time is called an AC voltage. Voltage is measured in volts, and one volt can be defined as the electrical pressure required to force an electrical current of one ampere through a resistance of one Ohm. Voltages are generally expressed in Volts with prefixes used to denote micro-volts (uV = 10^-6), milli-volts (mV = 10^-3) or kilo-volts (kV = 10^3). Voltage can be either positive or negative.
A voltage can be produced in a number of different ways, static electricity by the separation of charge, by mechanical means as in a generator, by photoelectric means as in a solar cell or by chemical means as in a battery with the voltage produced by these sources of energy being called an electromotive force or e.m.f.
Batteries or power supplies are mostly used to produce a steady D.C. (direct current) voltage source such as 5v, 12v, 24v etc in electronic circuits and systems. While A.C. (alternating current) voltage sources are available for domestic house and industrial power and lighting as well as power transmission. The mains voltage supply in the United Kingdom is currently 230 volts a.c. and 110 volts a.c. in the USA with general electronic circuits operating on a voltage supply of between 1.5V and 24V d.c. The circuit symbol for a constant voltage source usually given as a battery symbol with a positive, + and negative, – sign indicating the direction of the polarity. The circuit symbol for an alternating voltage source is a circle with a sine wave inside.
Voltage is always measured as the difference between any two points in a circuit and the voltage between these two points is generally referred to as the “Voltage drop”. This voltage drop can be measured using a voltmeter connected across it. Any voltage source whether DC or AC likes an open or semi-open circuit condition but hates any short circuit condition as this can destroy it.