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Relationship www.electronics-tutorials.ws/dccircuits/dcp_1.html
Any basic electrical or electronic circuit consists of three seperate but very much related quantities, Voltage (V), Current (I) and Resistance (Ω).

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 abitity to produce an electrical current flowing through a given circuit. The difference in voltage between any two points in a circuit is known as the Potential Difference, p.d. or Electromotive Force, (EMF) and is measured in Volts, symbol (V or energy, E). 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
microvolts (μV = 10-6V), millivolts (mV = 10-3V) or kilovolts (kV = 103V).

Batteries or power supplies are mostly used to produce a steady D.C. (direct current) voltage source 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.

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 diffrence between any two points in a circuit and the voltage between these two points is generally refered to as the "Voltage drop". Any voltage source whether d.c or a.c. likes an open or semi-open circuit condition but hates any short circuit condition as this can destroy it.

Current.
Electrical current is measured in Amperes, symbol (I, Intensity) and is the continuous and uniform flow of electrons (negative particles of an atom) around a circuit that are being "pushed" by the voltage source. In reality, electrons flow from the negative (-ve) terminal to the positive (+ve) terminal of the supply and for ease of circuit understanding conventional current flow assumes that the current flows from the positive to the negative terminal. An ampere is defined as the number of electrons passing a certain point in one second. Current is generally expressed in Amperes with prefixes used to denote microamps (μA = 10-6V) or milliamps (mA = 10-3V).

Current that flows in a single direction is called Direct Current, or D.C. and current that alternates back and forth through the circuit is known as Alternating Current, or A.C.. Whether a.c. or d.c. current only flows through a circuit when a voltage source is connected to it with its "flow" being limited to both the resistance of the circuit and the voltage source pushing it. Current sources are the opposite to voltage sources in that they like short or closed circuit conditions but hate open circuit conditions as no current will flow.

Using the tank of water relationship, current is the equivalent of the flow of water through the pipe with the flow being the same throughout the pipe. The faster the flow of water the greater the current.

Resistance.
The resistance of a circuit is its ability to resist or prevent the flow of current (electron flow) through it and is measured in ohms, greek symbol (Ω, omega) with prefixes used to denote Kilo-ohms (kΩ = 103Ω) and Mega-ohms (MΩ = 106Ω).

The amount of resistance determines whether the circuit is a "good conductor" - low resistance, or a "bad conductor" - high resistance. Low resistance, for example 1Ω or less implies that the circuit is a good conductor made from materials such as copper, aluminium or carbon while a high resistance, 1MΩ or more implies the circuit is a bad conductor made from insulating materials such as glass, porcelene or plastic. A "semiconductor" on the other hand is a material whose resistance is half way between that of a good conductor and a good insulator such as silicon and germanium and is used to make Diodes and Transistors etc.

Resistance in a circuit prevents short circuits (unless its very low) by limiting and controlling the amount of current flowing in a circuit by the voltage supply connected to it and therefore the transfer of power from source to load. Resistance is not affected by frequency and the a.c. impeadance of a pure resistance is equal to its d.c. resistance. Resistance also has the ability to change the characteristics of a circuit by the effect of load resistance or by temperature which changes its resistivity.

For very low values of resistance, for example milli-ohms, (mΩ?s) it is sometimes more easier to use the reciprical of resistance (1/R) rather than resistance (R) itself. The reciprical of resistance is called Conductance, symbol (G) and it is the ability of a conductor or device to conduct electricity with high values of conductance implying a good conductor and low values of conductance implying a bad conductor. The unit of conductance is the Siemen, symbol (S).

Again, using the water relationship, resistance is the diameter or the length of the pipe the water flows through. The smaller the diameter of the pipe the larger the resistance to the flow of water, and therefore the larger the resistance.

Relationship between voltage and current in a circuit of constant resistance.