We use electric circuits to transfer energy from a battery or power supply to
components in the circuit, which then transfer the energy to their surroundings.
For an electric current to flow, two things are needed: a complete circuit for it
to flow around, and something to ‘push’ it around the circuit. The push might be
provided by a cell, battery or power supply. A battery is simply two or more
cells connected end-to-end. In most familiar circuits, metals such as copper or
steel provide the circuit for the current to flow around.
Figure 18.3a shows how a simple circuit can be set up in the laboratory. Once
the switch is closed, there is a continuous metal path for the current to flow
along. Current flows from the positive terminal of the battery (or cell). In the
circuit symbol for a cell, the longer line represents the positive terminal (see
Figure 18.3b). Current flows through the switch and the filament lamp, back to
the negative terminal of the battery. A current that flows in the same direction all
the time is called direct current (d.c.). You will meet alternating current (a.c.)
in Chapter 21 when you learn about transformers. Alternating current is when
current reverses direction. In many countries, mains electricity has a frequency
of 50 Hz so it changes direction 100 times per second.
Figure 18.3b shows the same circuit as represented by a circuit diagram. Each
component has its own standard symbol. If you imagine the switch being pushed
so that it closes, it is clear from the diagram that there is a continuous path for
the current to flow around the circuit.Current & Voltage