For electrical charge to flow through a circuit, two things are needed. The circuit must be a closed loop with no gaps, and it must include a source of potential difference, such as a cell or a battery. An open switch makes a gap, so no charge flows.
An electric current is a flow of electrical charge. In a metal wire the charge is carried by electrons. The size of the current is the rate of flow of charge. A bigger current means more charge passes a point each second.
Charge flow, current and time are linked by the equation charge flow = current × time, or Q = I t. Charge flow Q is measured in coulombs (C), current I in amperes (A), also called amps, and time t in seconds (s). For example, a current of 2 A for 30 s transfers 60 C of charge.
In a single closed loop the current has the same value at any point. An ammeter placed before a lamp reads the same as an ammeter placed after it, because the lamp does not use up the current.