Introductory Physics Volume Two

3.2 Electric Current 57
3 Circuits
§ 3.1 Introduction
When you turn on a flashlight, a flow of electrons is passed through
the light bulb. Inside the light bulb is a small filament. As the electrons
pass through the filament they loose energy and in the process the
filament heats up. The filament gets so hot that it glows, this is how
the bulb produces light.
The electrons are supplied from the negative terminal of the battery
and flow to the light bulb through a metal wire. After the electrons
pass through the filament they are carried back to the positive terminal
of the battery by another wire.
A system of electrical devices connected with wires, such as the
flashlight system, is called an electric circuit. Circuits are often represented
in a wiring diagram. The following circuit diagram represents
the flashlight system.
+
–
In this diagram there are three elements and the wires that connect
them. The element on the left is the battery, with the positive terminal
being wider and marked with a + sign. The element on the top is the
switch that turns the flashlight off and on. The element on the right is
the light bulb.
This chapter will be an investigation of electrical circuits and the
common electrical devices from which circuits are built.
§ 3.2 Electric Current
A flow of charge, such as the charge flowing through the light bulb
filament, is called an electric current.