Voltage, Current and Resistance

Every working circuit needs a source of electrical energy, a complete path, and a load that uses that energy. Three quantities describe what is happening: voltage, current, and resistance.

Voltage: the push

Voltage is the difference in electrical potential between two points. A 5 V battery provides about five volts between its positive and negative terminals. Think of voltage as the push that can move charge around a closed circuit. It is measured in volts (V).

Current: the flow

Current is the rate at which electric charge flows. It is measured in amperes (A), often in milliamperes (mA). One ampere is 1,000 milliamperes. A circuit must be closed for steady current to flow.

Resistance: the opposition

Resistance describes how strongly a component opposes current. It is measured in ohms (Ω). A resistor adds a known amount of resistance.

Ohm’s law

For many simple resistors: V = I × R. Rearranging gives I = V ÷ R and R = V ÷ I. For example, 5 V across a 1,000 Ω resistor gives I = 5 ÷ 1,000 = 0.005 A, or 5 mA. If resistance doubles while voltage stays the same, current halves.

Try it

Find the current through a 2,200 Ω resistor connected to 5 V: I = 5 ÷ 2,200 ≈ 0.00227 A, or 2.27 mA. Check your units.

Safety

Practice with low-voltage batteries or a current-limited classroom supply. Never experiment with wall outlets. Never connect a battery’s positive and negative terminals directly: a short circuit can heat wires and damage the battery.

Key idea

Voltage is the push, current is the flow, and resistance limits that flow. Ohm’s law connects all three.