Edexcel GCSE Combined Science · 1SC0

Potential difference and energy transfer

What you need to know

Edexcel 1SC0 · Current, voltage, and resistance Check the specification (PDF) (opens in a new tab)

Describe the basic structural model of the atom, focusing specifically on the relative positions, masses, and charges of protons, neutrons, and electrons.
Construct and interpret standard electric circuit diagrams utilizing appropriate conventions for terminals and standard component symbols.
Analyze the fundamental operational differences between series and parallel electrical circuits.
Understand that a voltmeter must be connected in parallel with a component to accurately measure its potential difference.
Define potential difference as the energy transferred per unit of passed charge, equating one volt to one joule per coulomb.
Calculate energy transferred utilizing the relationship between moved charge and potential difference.

$E = Q \times V$

Understand that an ammeter must be connected in series with a component to measure the current flowing through it.
Explain electric current as the fundamental rate of charge flow, specifically detailing that current in metals consists of flowing electrons.
Calculate electrical charge by relating current and time.

$Q = I \times t$

Understand that a closed circuit containing a source of potential difference will consistently sustain an electric current.
Understand the principle of current conservation at junctions within electrical circuits.
Explain the relationship between circuit resistance and current flow, and demonstrate how a variable resistor can manipulate this relationship.
Calculate potential difference, current, or resistance using Ohm's Law.

$V = I \times R$

Analyze how connecting resistors in series increases net resistance, while connecting them in parallel decreases net resistance.

Potential difference measures energy per charge

A cell has a positive terminal and a negative terminal. It establishes a potential difference, also called voltage, between these terminals. Components connected in the circuit can therefore have a potential difference across them.

Potential difference tells us how much energy is transferred for each coulomb of charge that passes. Charge is measured in coulombs (C) and energy in joules (J). A potential difference of 6 V across a lamp means that 6 J of energy is transferred in the lamp for every 1 C of charge passing through it.

This gives the meaning of the unit volt:

1\text{ V} = 1\text{ J/C}

A larger potential difference means more energy transferred per coulomb. It does not, by itself, tell us the total energy transferred: that also depends on how much charge passes.

Measuring across a component

A voltmeter measures potential difference between two points. To measure the potential difference across a resistor, connect one voltmeter lead to each side of the resistor. The voltmeter is then in parallel with it: both are connected between the same two circuit points.

Series circuit with a cell and one path containing, in order: Resistor. The voltmeter is connected across component 1 (resistor), not in series. — The voltmeter connects to both ends of the resistor, so it measures the potential difference across it.

In a circuit diagram, the voltmeter symbol is a circle containing V. Follow its two connections in the diagram: they join the circuit on opposite sides of the resistor. This is what measuring voltage across a component means. The voltmeter is not inserted into the main path through the resistor.

Calculating the total energy transferred

Because voltage is energy transferred per coulomb, multiplying the charge moved by the potential difference gives the total energy transferred:

E = Q \times V

Here, $E$ is energy transferred in joules (J), $Q$ is charge moved in coulombs (C), and $V$ is potential difference in volts (V).

For example, a lamp operates at a potential difference of 6 V, and 4200 C of charge passes through it. The energy transferred is:

E = 4200 \times 6 = 25\,200\text{ J}

Each coulomb transfers 6 J, so all 4200 C together transfer 25 200 J in the lamp.

Finding voltage or charge

The same relationship can be rearranged depending on the unknown quantity:

V = \frac{E}{Q} \qquad Q = \frac{E}{V}

Dividing energy by charge gives the energy transferred per coulomb—the potential difference. Dividing energy by potential difference gives the charge that must have passed to transfer that amount of energy.

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