Voltage, current, and resistance

Understand the idea

Voltage is the electrical potential difference between two points. Current is the rate charge flows through a path. Resistance describes how strongly a component opposes that flow. These are related by Ohm's law: V = I × R.

A resistor in series with an LED limits current. The LED does not safely set its own current: as it warms, a small voltage change can cause a large current change. The resistor makes the operating point predictable.

Power matters too. P = V × I tells you how quickly energy becomes heat. A value can be electrically correct and still overheat a component if its power rating is too low.

Worked example

A 5 V source, red LED with about 2 V forward drop, and 220 Ω resistor gives an estimate of (5 − 2) / 220 = 0.0136 A, or about 14 mA. The resistor dissipates roughly I²R ≈ 0.041 W, so a common 0.25 W resistor has margin for this simple example.

Try it at the bench

1. With power disconnected, identify the LED's anode and cathode and place a resistor in series.
2. Measure the supply and LED voltage, then calculate current from the resistor voltage divided by its resistance.
3. Compare the measured value with your estimate. Record meter range, resistor tolerance, and any difference.

Check your understanding

In the example, why calculate current from the voltage left across the resistor rather than use all 5 V?