This is why the circuit must be complete: the battery must be connected to the components in a circuit to allow the charges to drop through them and convert their energy.
When there is a point of high electrical energy and a point of low electrical energy in a complete circuit, we say that there is a difference in electrical potential energy between those points. This is known as potential difference.
Potential difference provides the energy that pushes electrons around a circuit. It is needed to make a current flow.
Measuring Potential Difference
Potential difference is also called voltage. The SI unit for voltage is volts. The symbol for voltage is the letter V.
A voltmeter is used to measure the potential difference across a component. In other words, it measures the ‘drop’ in energy as energy travels through a component and is converted to other forms.
3 V 3 V
The bulb is the only component converting energy in the circuit, so all the voltage of the battery ‘drops‘ across the bulb and is transformed to light and heat energy
6 V
Fig. 18.6.2 The circuit symbol for a voltmeter
18.6
6 V
3 + 3 = 6 V
Fig. 18.6.3 Potential difference in a circuit being measured using a voltmeter
Conservation of energy and mass in electric circuits
Using the law of conservation of energy and law of conservation of mass, discuss the following statement with your lab partner: ‘Energy and mass are not “lost” when electrons flow in a circuit.’
See Physical World 16.1 for a reminder about the law of conservation of energy and Chemical World 9.5 for a reminder about the law of conservation of mass.
