1. DC Theory - Relationship between Voltage, Current and Resistance

Any basic electrical or electronic circuit consists of three separate but very much related quantities, Voltage, ( V ), Current, ( I ) and Resistance, ( Ω ).

a. Voltage.

Voltage is the potential energy of an electrical supply stored in the form of an electrical charge, and the greater the voltage the greater is its ability to produce an electrical current flowing through a given circuit. As energy has the ability to do work this potential energy can be described as the work required in joules to move the electrical current around a circuit from one point or node to another. The difference in voltage between any two nodes in a circuit is known as the Potential Difference, p.d. or sometimes called Electromotive Force, (EMF) and 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 sub-multiples of the voltage such as microvolts ( μV = 10^-6 V ), millivolts ( mV = 10^-3 V ) or kilovolts ( kV = 10³ V ). Voltage can be either positive or negative.

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.

A simple relationship can be made between a tank of water and a voltage supply. The higher the water tank above the outlet the greater the pressure of the water as more energy is released, the higher the voltage the greater the potential energy as more electrons are released. 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". 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.

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