How do we calculate electric current?

Imagine standing next to a road and counting the cars that drive past in one minute. Twenty cars? Two hundred? That number tells you how busy the road is.

Scientists measure electric current in the same way. They “count” how much electricity passes a point in the wire every second. Let’s learn the three rules that let you work it out — no counting needed.

Rule 1: current is charge per second

Electricity is made of moving charge, carried by electrons. Charge is counted in coulombs, written C. One coulomb is a huge crowd of electrons: about 6.24 billion billion of them.

If one coulomb passes every second, the current is one ampere (1 A). As a formula:

$$I = \frac{Q}{t}$$

• $I$ is the current, in amperes (A),
• $Q$ is the charge that passed, in coulombs (C),
• $t$ is how long it took, in seconds (s).

Example: 30 coulombs pass a point in 10 seconds. The current is $30 \div 10 = 3$ A.

Rule 2: current = push ÷ resistance

Counting coulombs is hard in real life. Luckily there is an easier way. The current depends on two things:

• how hard the electricity is pushed — the voltage, in volts (V),
• how hard the path is — the resistance, in ohms (Ω).

This rule is called Ohm’s law:

$$I = \frac{V}{R}$$

Push harder (bigger $V$), and more current flows. Make the path harder (bigger $R$), and less current flows.

Example 1: a torch bulb has a resistance of about 10 Ω. With a 3 V battery, the current is $3 \div 10 = 0.3$ A.

Example 2: a car headlight has about 2.6 Ω and runs on the car’s 12 V battery. The current is $12 \div 2.6 \approx 4.6$ A. (The sign ≈ means “about”.)

Ohm’s law playground

Change the push and the resistance. The current is always the push divided by the resistance.

Push (voltage)

12 V

Resistance

6 Ω

Current 2.00 A

Power 24 W

Try a real device: Torch bulbPhone chargingLED lampToasterKettle

Rule 3: power = push × current

The power of a device tells you how much energy it uses every second. It is measured in watts (W):

$$P = V \cdot I$$

Example: a phone charger gives 5 V and 2 A. Its power is $5 \cdot 2 = 10$ W.

You can also turn the rule around to find the current: $I = P \div V$. A 2,000 W kettle on a 230 V socket takes $2000 \div 230 \approx 8.7$ A.

What does your electricity bill count?

The bill counts energy in kilowatt-hours (kWh). One kilowatt-hour is the energy a 1,000 W device uses in one hour.

Example: the 2,000 W kettle runs for 3 minutes (that is 0.05 hours). It uses $2 \text{ kW} \cdot 0.05 \text{ h} = 0.1$ kWh. Boil it ten times, and you have used one whole kilowatt-hour.

Using the rules to stay safe

Most sockets and power strips in Europe are made for at most 16 A. Let’s check a busy kitchen with the rules:

Device	Power	Current ($P \div 230$ V)
Kettle	2,000 W	8.7 A
Heater	2,000 W	8.7 A
Microwave	1,000 W	4.3 A
All three on one power strip	5,000 W	21.7 A

21.7 A is much more than 16 A. The cable would get dangerously hot, and a safety switch in your home should cut the power.

Go deeper: Two more power formulas for grown-ups and the extra curious

Put Ohm’s law into $P = V \cdot I$ and you get two more useful forms:

$$P = I^2 \cdot R \qquad \text{and} \qquad P = \frac{V^2}{R}$$

The first one explains why power lines use very high voltages. The wires waste energy as heat, and that loss grows with the square of the current. To send the same power with 10 times higher voltage, you need only a tenth of the current — and the heat lost in the wires drops to a hundredth. That is one reason power grids use alternating current: its voltage is easy to change up and down.

Go deeper: How current and voltage are measured for grown-ups and the extra curious

An ammeter measures current. It must be put into the loop, so the current has to flow through it. A voltmeter measures voltage between two points, so it is connected across a device, side by side with it. A multimeter can do both — but only a grown-up should use one on anything connected to the mains.

Check yourself

A heater uses 2,300 W from a 230 V socket. What current flows through it?

1 A 10 A 100 A

Show the answer

10 A — Current = power ÷ voltage = 2,300 ÷ 230 = 10 A.