ChargeCharge

Electric current is defined as the rate of flow of charge, and it is measured in amperes. It is the amount of current passing a point in the circuit per unit time.

$I = frac{Delta Q}{Delta t}$

Electric charge is measured in coulombs, where one coulomb is the electric charge flowing past a point in one second when there is an electric current of one ampere. Any particle that carries electric charge is a charge carrier.

The elementary charge e equals 1.6× 10⁻¹⁹C. The net charge on a particle or object is quantised (can only take certain values) so is a multiple of e: = ± where n is an integer (e.g. the number of electrons). An electron has the charge –e whereas a proton has the charge e.

Current is the movement of electrons in metals and the movement of ions in electrolytes (liquids that can carry an electric current).

Conventional current flows from the positive terminal to the negative terminal
Electron flow is from the negative terminal to the positive terminal

Ammeters are used to measure the current at any point in a circuit; it is always placed in series and have the lowest possible resistance to reduce the effect they have on the current. The ideal (perfect) ammeter has zero resistance and so no effect on the current that it measures.

Kirchhoff’s Laws:

Kirchhoff’s First Law: for any point in an electrical circuit, the sum of the currents into that point is equal to the sum of the currents out of that point.

$sum I_{in} = sum I_{out}$

This is essentially an extension of the conservation of charge, where charge is the product of the current and the time. Charge can neither be created nor destroyed, so the charge carriers entering a point at a given time must equal the total number of charge carriers leaving that point during that time.

Kirchhoff’s Second Law: the sum of the e.m.f.s is equal to the sum of the IR products in a closed loop (a single possible path for the current).

$sum varepsilon = sum v_{around a closed loop}$

This is essentially an extension of the conservation of energy: the total energy transferred to the charges in a circuit equals the total energy transferred from the charges as they move around the circuit.