⚡️ The potential difference (p.d.) between two points in an electric circuit is the amount of energy per unit charge that moves from one point to the other.
It is measured in volts (V), just like we measure height in metres or weight in kilograms.
Think of it like the difference in water height between two points in a pipe – the higher the water level, the more “push” it has to flow downwards.
In a circuit, a higher p.d. means a stronger “push” for electrons to flow.
🔋 EMF is the energy supplied by a source (like a battery) that drives electrons through a circuit.
It is the maximum potential difference the source can provide when no current is flowing.
In a simple battery, the EMF is the voltage you read on a voltmeter when the battery is not connected to a load.
📌 Remember: The symbol for potential difference is \$V\$, and its unit is the volt (V).
When you see a question about “voltage across a resistor”, they are asking for the p.d. between its two ends.
Always check whether the circuit is open (no current) or closed (current flowing) – this tells you whether you should use EMF or actual p.d.
Imagine a battery with EMF \$E = 1.5\,\text{V}\$ and internal resistance \$r = 0.2\,\Omega\$.
If a resistor \$R = 5\,\Omega\$ is connected, the current \$I\$ is:
\$I = \frac{E}{R + r} = \frac{1.5}{5 + 0.2} \approx 0.29\,\text{A}\$
The p.d. across the resistor is:
\$V_R = I \times R \approx 0.29 \times 5 = 1.45\,\text{V}\$
Notice \$V_R\$ is slightly less than the EMF because of the voltage drop across the internal resistance.
| Quantity | Symbol | Unit |
|---|---|---|
| Potential Difference | \$V\$ | Volt (V) |
| Electromotive Force | \$E\$ | Volt (V) |
| Internal Resistance | \$r\$ | Ohm (\$\Omega\$) |
📐 When you calculate the p.d. across a component:
Double‑check units – volts for p.d., ohms for resistance, amperes for current.