All electric circuits have one thing in common they depend on the flow of electric charge. In general, electric charge is carried through a circuit by electrons. This section explores the properties of moving electric charges and what a battery does to the charges.

Key Terms

• electric current
• charge
• conventional current
• primary cell
• secondary cell

Direct current

A current is maintained in a closed circuit by a source of emf. Among such sources are batteries and generators that increase the potential energy of the moving charges. Within a battery, a chemical reaction occurs that transfers electrons from one terminal (+) to another terminal (–). Figure 1 (a) shows the two terminals of a flashlight battery. Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. Figure 1 (b) shows the symbol used to represent a battery in a circuit.

• The maximum potential difference is called the electromotive force (emf) of the battery, for which the symbol ε is used.

The purpose of the battery is, therefore, to produce a potential difference, which can then make charges move. When a continuous conducting path is connected between the terminals of a battery, we have an electric circuit (Figure 2). When such a circuit is formed, charge can move (or flow) through the wires of the circuit, from one terminal of the battery to the other.

Electric Current: Suppose an amount of charge Q flows past a given point in the wire in a time t. The electric current, I, in the wire is simply defined as the amount of charge divided by the time.

The SI unit of current is the ampere, A.
(Coulomb per second, C/s = Ampere, A or 1 A = 1 C/s).

Smaller units of current are often used, such as the milliampere (mA) and microampere (μA).

(1 mA = 10^-3 A and 1μ m = 10^-6A)

• Conventionally, the direction of an electric current is the direction in which positive charges would move. Thus, the current in the external circuit is directed from the positive terminal to the negative terminal of the battery. Such current is known as direct current (DC).