METRONMETHOD. REASON. LEARN.
Electricity · 12 min

Current, voltage and Ohm’s law

These quantities help explain why different components allow different currents under the same voltage.

01Why this matters

These quantities help explain why different components allow different currents under the same voltage.

02See it

6 V3 ΩI = 2 A
One ideal resistor makes a closed path across a 6 V supply. Current is 6/3 = 2 A.
supply 6 Vresistor 3 ΩI = 6/3current 2 A
Read left to right: supply 6 V → resistor 3 Ω → I = 6/3 → current 2 A. Each panel connects one part of the example.

03Understand it

Current is the rate of flow of charge. Voltage is energy transferred per unit charge. Resistance relates voltage to current for a component.

For an ohmic resistor at constant temperature, V = IR and resistance stays approximately constant. Voltage is measured in volts, current in amperes and resistance in ohms. The relationship is a model for suitable components, not a universal constant-resistance rule for every device.

04Guided example

A 6 V supply is connected across a 3 Ω resistor in an ideal circuit. Find current.

  1. Use V = IR and isolate current: I = V/R.
  2. Substitute the voltage across this resistor and its resistance: I = 6/3.
  3. The current is 2 A. Use the same current through this single series path.

05Try it

Voltage 12 V, resistance 4 Ω. Current?
A little hint

Divide voltage by resistance.

06Practise it

Current 2 A, resistance 5 Ω. Voltage?
A little hint

Multiply current by resistance.

Voltage 9 V, current 3 A. Resistance?
A little hint

Divide voltage by current.

07Use it in the real world

The circuit explorer uses virtual low-voltage components. Increasing resistance from 3 Ω to 6 Ω at 6 V halves current from 2 A to 1 A.

Go a little deeper

A filament lamp heats as current rises, changing resistance. Its current–voltage graph is curved, so treating its resistance as one fixed number is an approximation.

08Quick check

Voltage 6 V, resistance 12 Ω. Current?
A little hint

Use I = V/R.

09Summary

Current is the rate of flow of charge. Voltage is energy transferred per unit charge. Resistance relates voltage to current for a component. The current is 2 A. Use the same current through this single series path.

Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.

10Key idea

V = IR

V: potential difference in V; I: current in A; R: resistance in Ω.

I = V/R for R > 0; R = V/I for I ≠ 0

Find this in the formula library →