Voltage
This is the driving force that pushes the current round. This is measured in volts (V)
Resistance
This is anything in the circuit that slows the flow of current down. This is measured in ohms ( W )
Ohm's Law shows that:
These units and symbols for electrical quantities will become very important to know as we begin to explore the relationships between them in circuits. The first, and perhaps most important, relationship between current, voltage, and resistance is called Ohm’s Law, discovered by Georg Simon Ohm and published in his 1827 paper, The Galvanic Circuit Investigated Mathematically . Ohm’s principal discovery was that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given temperature. Ohm expressed his discovery in the form of a simple equation, describing how voltage, current, and resistance interrelate:
Although it is electrons which are the mobile charge carriers which are responsible for electric current in conductors such as wires, it has long been the convention to take the direction of electric current as if it were the positive charges which are moving. Some texts reverse this convention and take electric current direction as the direction the electrons move, an obviously more physically realistic direction, but the vast majority of references use the conventional current direction and that convention will be followed in most of this material. In common applications such as determining the direction of force on a current carrying wire , treating current as positive charge motion or negative charge motion gives identical results. Besides the advantage of agreeing in direction with most texts, the conventional current direction is the direction from high voltage to low voltage, high energy to low energy, and thus has some appeal in its parallel to the flow of water from high pressure to low (see water analogy ).