As free electrons move through a circuit, the particles which do not move get in the way. This opposition to the flow of electric current is called resistance. Electrical resistance converts electrical energy to other forms such as heat and light. Electrical resistance is measured in ohms (Ω) using an ohmmeter.
Symbol
Potential difference V
(Voltage) Current
I Resistance R Units Volts
Amperes (amps) Ohms
Unit symbol V
A Ω
Measuring instrument Voltmeter Ammeter Ohmmeter
31.4 Designing and building simple electrical circuits
Electrical circuits can be represented using circuit diagrams. Circuit diagrams show all the components of a circuit and how they are connected to each other. Circuit diagrams are useful for designing and building electric circuits.
Cells and batteries A battery is formed by connecting cells together. A typical AA ‘battery’ is actually one cell with a voltage of 1.5 V. A typical 9 V battery really is a battery because it has a number of cells inside it. The symbols used for cells and batteries when designing circuits are shown here in Fig. 8 along with some other commonly used symbols.
Fig. 7 A 9 volt battery ammeter cell
battery (2-cell)
lamp switch (open)
switch (closed)
voltmeter resistor resistor (variable) Fig. 8 Symbols used to represent electrical components in electrical circuit diagrams.
The circuit shown in the photograph in Fig. 9 was designed to enable a bulb to be turned on and off. The circuit diagram shows a cell, a switch and a bulb. The different parts are connected by wires to form a circuit. When the switch is pressed the circuit is complete, current flows and the bulb lights up. When the switch is not held closed the circuit is broken, no current flows and the bulb does not light.
Fig. 9 A photograph and a diagram of a simple circuit. In the photograph, the switch is being held closed to complete an electrical circuit.
