Current, Potential difference and Resistance

In an electric circuit, there are three things to measure; Current, Potential difference and Resistance.

Current (I)
An electric current in a wire is a drift of electrons, but according to the convention adopted in 1800, current is regarded as the flow of positive electricity.
Current can also be defined as charge flowing in a circuit per unit time. Current (I) = Charge (Q)/time (t).
Current is measured in Amperes.
An Ampere is the Current which, if flowing in two straight parallel and infinitely long wires, placed one meter apart in a vacuum with a negligible cross sectional area, will produce on each of the wires a Force of 0.0000002 N per meter length of the wire.

Potential difference

Potential difference between 2 points x and y, in a circuit, is the work done when moving a unit of charge from x to y.

The SI unit is the Volt. A volt is the potential difference between 2 points of a circuit carrying a constant current of one ampere when the power dissipated between these points is one Watt.

The circuit consists of resistors R1 and R2 each having a potential difference, in volts, V1 and V2 depending on the value of their resistances. The arrow shows the path of convention current.

Resistance

This is the opposition to the flow of Current through a conductor. The SI unit of Resistance is the Ohm. An Ohm is the resistance in which current of one ampere flowing for one second generates one joule of thermal energy.