Current(I) is measured in amperes(A). Amperes is one of the SI base units. It is the rate of flow of charge(Q) - how much charge flows in a certain amount of time. It can be calculated with I=ΔQ/Δt. ΔQ is the charge transferred in coulombs and Δt is the time in seconds.
In a circuit, it can be measured with an Ammeter. Ammeters are always connected in series since, as explained in Kirchhoff's First Law, the current splits at junctions.
Charge(Q) is a derived unit measured in coulombs(C). 1C = 1As⁻¹ meaning that one coulomb of charge passes in 1 second when the current is 1A. Objects with charge exert forces on each other - opposites attract and likes repel. Uncharged particles are neutral.
The smallest unit of charge that can be carried by a particle is called the elementary charge (e). e=1.60 x 10⁻¹⁹C. e is quantised, meaning charge can only come in multiples of e. As a result, the amount of charged particles can be worked out from just the charge by dividing it by the elementary charge. Protons each have a charge of +e and electrons have a charge of -e.
Conventional Current Edit
Conventional current is from + towards -. Electrons would not flow this direction however, as their negative charge would repel from the - end. This means that electron flow in a circuit is opposite to the conventional current.
Kirchhoff's First Law Edit
Says that charge is conserved. As it flows through the circuit, charge is not used up. This means:
∑ currents into a circuit = ∑ currents out of a circuit