Electric potential difference, commonly called voltage, is the energy transferred per unit charge when charge moves between two points. It is measured in volts, where .
This is an SL and HL concept in IB Physics B.5, Current and circuits. It is expressed as
where is the potential difference in volts, is the work done or energy transferred in joules, and is the charge in coulombs.
A potential difference exists when two points have different electric potentials. As positive conventional charge moves through a component from higher to lower potential, electrical potential energy is transferred into other forms. For example, a lamp transfers electrical energy into thermal energy and light.
If passes through a lamp connected across , then
Therefore, the lamp transfers of energy. Equivalently, every coulomb passing through the lamp transfers .
A source such as a cell does the reverse: it transfers chemical energy to the charges, raising their electrical potential energy. The source's electromotive force (emf) is the energy supplied per unit charge, whereas potential difference usually describes energy transferred per unit charge across a circuit component.
The common misconception is that “voltage flows.” Voltage does not flow; electric current is the rate of flow of charge, while voltage measures energy transferred per unit charge. A voltmeter measures potential difference and is connected in parallel across the component.
In an ideal voltmeter, resistance is extremely large, so it draws negligible current and minimally changes the circuit being measured. The reading compares electric potential at its two terminals; it does not measure the potential at one point alone. In a series circuit, component potential differences add to the supply's terminal potential difference.
Exam technique: For a “Calculate” question, show the equation, substitution and answer. For an “Explain” question, link potential difference to energy transfer per unit charge and state the unit or volt.
