Voltage Drop, Current, Resistance and Power

In the last lesson we met voltage, current and resistance with a water tank. Now we use a different picture: charges (the green balls) travelling round a closed loop. We will see what a voltage drop is, how resistance controls current, and then learn a new quantity called power. Every step has a simulator: drag the sliders and watch what changes. All numbers on this page are teaching values.

+ WORDS USED ON THIS PAGE (OPEN THIS IF A WORD IS NEW)

charge: A tiny amount of electricity that moves round the circuit. We draw it as green balls.
voltage source, unit: volt (V): A battery or power supply that pushes the charges.
resistor, unit: ohm (Ω): A part that makes it harder for charges to flow.
voltage drop: The energy each charge gives up as it passes through a part.
current (I), unit: amp (A): How many charges pass a point every second.
power (P), unit: watt (W): How much energy is used every second.
power supply: A device that converts mains power into a steady voltage, such as 24 V DC for a control panel.
rating: The output voltage and current a supply is built to give.

PART 1 Charges, voltage drop, current and power

Press Next to move through seven steps. Back to return. The picture on the left stays in view while you work with the controls on the right.

PART 2 The four quantities

Keep this cheat-sheet in your head. The first three you already know. Power is new.

Quantity	Symbol	Unit	Formula
Voltage	V	volt (V)	V = I × R
Current	I	ampere (A)	I = V ÷ R
Resistance	R	ohm (Ω)	R = V ÷ I
Power	P	watt (W)	P = V × I

Power rule. Less resistance, more voltage and more current all mean more power.

PART 3 Worked examples

Read the question, work it out on paper, then press Show answer.

Example 1: A DC power supply has an output current of 5 A and an output voltage of 24 V. What power can this device supply?

P = V × I = 24 × 5 = 120 W

Example 2: A 5 V source drives a 100 Ω resistor. Find the current, then the power.

I = V ÷ R = 5 ÷ 100 = 0.05 A
P = V × I = 5 × 0.05 = 0.25 W

Example 3: A phone charger is rated 5 V and 2 A. How much power can it supply?

P = 5 × 2 = 10 W

Example 4: What is the unit of power?

The watt (W). 1 W = 1 V × 1 A.

PART 4 Try your own numbers

Type any two of voltage, current and resistance. The calculator finds the third one and the power.

Voltage V (volts)

Current I (amps)

Resistance R (ohms)

PART 5 Quick quiz

Twelve questions to check what you learned.