understand that the charge on charge carriers is quantised

What is Electric Current?

Electric current, denoted by I, is the rate at which electric charge flows past a point in a circuit. It is measured in amperes (A), where 1 A = 1 C s⁻¹.

Think of it as a river of tiny cars (charge carriers) moving through a road (the conductor). The faster the cars move or the more cars there are, the higher the current.

Charge Carriers

In most conductors (like copper wires), the charge carriers are electrons. Each electron carries a charge of $-e$, where $e = 1.602\times10^{-19}\,\text{C}$.

In electrolytes, ions (e.g., $Na^+$ or $Cl^-$) act as carriers. Their charges are integer multiples of the elementary charge.

Quantisation of Charge

Charge is not continuous; it comes in discrete packets. Every charge carrier carries an integer multiple of the elementary charge $e$:

1. Electrons and protons: each carries exactly $e$ (with opposite signs).
2. Ions: a $Na^+$ ion carries +$e$, a $Cl^-$ ion carries –$e$, a $Ca^{2+}$ ion carries +2$e$, etc.
3. Quarks: carry fractional charges (±1/3 $e$, ±2/3 $e$), but they never exist freely in nature.

Because of this quantisation, the total charge in any closed system is always an integer multiple of $e$.

Analogy: The Parking Lot

Imagine a parking lot where each car represents an electron. Cars can only enter or leave in whole numbers; you can't have half a car. The current is like the number of cars passing a checkpoint per second.

Because cars are discrete, the flow rate (current) changes in steps, not smoothly. This is similar to how charge is quantised.

Exam Tip Box

Key point to remember: In any circuit, the charge transferred in time $t$ is $Q = I\,t$, and $Q$ must be an integer multiple of $e$. Use this to check if your calculated charge makes sense.

When answering questions about current, always state the unit (A) and remember that 1 A = 1 C s⁻¹.

Quick Quiz

1️⃣ If a 2 A current flows for 3 s, how many elementary charges pass a point?

2️⃣ Which of the following particles carries a charge of +2 $e$?
a) Electron
b) Proton
c) Calcium ion
d) Chloride ion

Answers: 1️⃣ $Q = I\,t = 2\,\text{A}\times3\,\text{s} = 6\,\text{C}$, so $N = Q/e = 6/(1.602\times10^{-19}) \approx 3.75\times10^{19}$ charges. 2️⃣ c) Calcium ion