All materials contain electrons, but in many materials they are all ‘locked’ into the material’s atoms and cannot move about. These materials cannot carry an electric current. and are called electrical insulators. Materials in which there are large numbers of electrons that are free to move around from atom to atom are called conductors.
When there is no current in a conductor, the free electrons move randomly between atoms, with no overall movement When you connect it in an electrical circuit with a power source like a battery, there is a current in the conductor. Now the electrons drift in one direction, while still moving in a random way as well. The drift speed is very slow, often only a few millimetres each second. A current can only flow in a conductor if it is connected in a complete circuit. If the circuit is broken, the current stops.
As a physical quantity, current is the rate at which charge flows past a point on a circuit. As depicted in the diagram below, the current in a circuit can be determined if the quantity of charge Q passing through a cross section of a wire in a time t can be measured. The current is simply the ratio of the quantity of charge and time.
The size of an electric current depends on the number of electrons that are moving and how fast they are moving. But instead of measuring the actual number of electrons we use the total charge carried by the electrons round the circuit each second.
Electric current is measured in amperes, or amps (A).
1 ampere=1 Coulomb/seconds
You use an ammeter to measure current in an electrical circuit. If the current is very small, you might use a milli ammeter, which measures current in milliamps (1 mA = 0.001 A). Even smaller currents are measured with a micro ammeter.
If you want to measure the current in a particular component, such as a lamp or motor, the ammeter must be connected in series with the component. In a series circuit, the current is the same no matter where the ammeter is put. This is not the case with a parallel circuit.