Electric charge in motion is the heart of electricity. When tiny charged particles (electrons) drift through a conductor like copper wire, we say a current flows. The amount of charge is measured in coulombs (C), where one electron carries a charge of about $1.6 \times 10^{-19}$ C.
Electric current is the rate of flow of charge. If a charge $Q$ flows through any cross-section of a wire in time $t$, then the current is $I=\frac{Q}{t}$. The SI unit of current is the ampere (A), where $1\ \text{A}=1\ \text{C/s}$. By the old convention, the direction of conventional current is taken opposite to the direction of electron flow — current goes from the positive terminal to the negative terminal in the external circuit.
Potential difference is what pushes charge around a circuit. It is the work done to move a unit charge between two points: $V=\frac{W}{Q}$. Its SI unit is the volt (V), where $1\ \text{V}=1\ \text{J/C}$. A cell or battery acts like a pump that maintains this potential difference. We measure current with an ammeter (connected in series) and potential difference with a voltmeter (connected in parallel across the component).
Ohm's law connects current and potential difference. At constant temperature, the current through a conductor is directly proportional to the potential difference across it:
- $V \propto I$, so $V=IR$, where $R$ is the resistance of the conductor.
- Resistance is the property that opposes the flow of current; its SI unit is the ohm ($\Omega$), where $1\ \Omega = 1\ \text{V/A}$.
- Rearranging gives $I=\frac{V}{R}$ and $R=\frac{V}{I}$.
For an ohmic conductor (like a metal wire at constant temperature), the V–I graph is a straight line through the origin, and its slope equals $R$. Non-ohmic devices such as a filament bulb, diode or LED do not obey Ohm's law — their V–I graph is curved because resistance changes with temperature or voltage.