Charge flowing round a circuit carries energy. Potential difference, or voltage, between two points is the electrical energy transferred by each coulomb of charge as it moves from one point to the other. One volt means one joule is transferred per coulomb.
Across a battery, potential difference is the energy each unit of charge gains, drawn from chemical energy. Across a bulb, it is the energy each unit of charge gives up, turned into light and heat. A voltmeter connected across a component reads this energy transfer per unit charge.
A helpful picture is water flowing downhill: the height difference sets how much energy each litre releases, just as potential difference sets how much energy each coulomb delivers. Without a potential difference there is nothing to drive a current.
In SPM you should define potential difference as energy per unit charge and connect voltmeters across components, not in series.
Common misconceptions
- Potential difference is the same as current -> Current is the rate of flow of charge; potential difference is the energy given to each unit of charge.
- A voltmeter is connected in series like an ammeter -> A voltmeter is connected in parallel, across the component whose voltage you want.
- Voltage flows through a component -> Charge flows; voltage is the energy difference between two points, it is not itself something that flows.
The physics behind it
Potential difference (p.d.), or voltage, between two points is the electrical energy transferred by each coulomb of charge passing between them. In symbols, V = W/Q, where W is energy in joules (J) and Q is charge in coulombs (C).
One volt is one joule per coulomb: 1 V = 1 J C⁻¹.
If a bulb transfers W = 12 J of energy while a charge of Q = 4 C flows through it, the p.d. across it is V = W/Q = 12 J / 4 C = 3 V. Potential difference is a per-charge quantity, so it does not depend on how long the current flows, only on the energy each coulomb carries. In a simple circuit it is also linked to current and resistance by V = IR, so a p.d. of 3 V across a 6 Ω resistor drives a current I = V/R = 3 V / 6 Ω = 0.5 A. Because voltage measures energy transfer between two points, it is always stated across a component, never through it.
See it in daily life
Look at any charger or battery and you see a voltage printed on it: a phone charger may say 5 V, a torch cell 1.5 V, a car battery 12 V. That number tells you how much energy each coulomb of charge is given as it leaves the source. A 12 V car battery hands twice the energy per coulomb that a 6 V lantern battery does, which is why the same bulb glows brighter on the larger voltage.
A helpful picture is water flowing downhill: a bigger drop in height gives each litre more energy to do work at the bottom, just as a bigger p.d. gives each coulomb more energy to release in a bulb or motor. When a torch battery runs low, its p.d. falls, less energy reaches each coulomb, and the bulb dims.
The voltage is not used up by sitting still; it only delivers energy when charge actually flows through a component.
How this comes up in SPM
This belongs to the Form 5 Electricity chapter, alongside current, resistance and Ohm's law. Paper 2 asks you to Define potential difference as the energy transferred per unit charge, and to State that its unit is the volt.
You are often asked to Calculate p.d. using V = W/Q or V = IR, so units on every line matter.
Practical questions require you to Draw or Describe a voltmeter connected in parallel across a component, and to Explain why it must not be placed in series. Data questions may give an energy and a charge and ask you to Determine the p.d., or give a current-voltage graph and ask you to Find the resistance from its gradient.
Neighbouring standards include electric current as charge per unit time, resistance, and series and parallel circuits, so command words such as Define, State, Calculate and Explain recur. Confusing potential difference with current is a common error, so keep p.d. as energy per charge between two points.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)