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

b. Electric Current.

Electrical Current is the movement of electrical charge and is measured in Amperes, symbol I, for Intensity). It 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. Generally in circuit diagrams the flow of current through the circuit usually has an arrow associated with the symbol, I, or lowercase I to indicate the actual direction of the current flow. However, this arrow usually indicates the direction of conventional current flow and not necessarily the direction of the actual flow.

In electronic circuits, a current source is a circuit element that provides a specified amount of current for example, 1A, 5A 10Amps etc, with the circuit symbol for a constant current source given as a circle with an arrow inside indicating its direction. Current is measured in Amps and an amp or ampere is defined as the number of electrons or charge (Q in Coulombs) passing a certain point in the circuit in one second, (t in Seconds). Current is generally expressed in Amps with prefixes used to denote micro amps (μA = 10^-6A) or milli amps (mA = 10^-3A). Electrical current can be either positive or negative.

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 AC or DC 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. Also, as AC currents (and voltages) are periodic and vary with time the "effective" or "RMS", (Root Mean Squared) value given as I_rms produces the same average power loss equivalent to a DC current I_average . 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. Any current source whether DC or AC likes a short or semi-short circuit condition but hates any open circuit condition as this prevents it from flowing.

Next topic is on Resistance