A learner wants to test a circuit containing an LED that has a 1.5 V forward-bias voltage, and a maximum current of 40 mA. She wishes to use four Cu/Zn cells connected in series as power source, giving a total voltage of 4 V. How should she connect the LED to the power source so that it will light up when a current of 10 mA flows through it?

Solution:

The source voltage is too high for the LED, and the resulting current through it will damage it. Thus, one has to put a resistance in series with the LED.
The value of the resistor R must be calculated bearing in mind the voltage that must be applied to the LED, The potential difference across the LED must be 1.5 V, which means that the potential drop across R must be 4.0 - 1.5 = 2.5 V.
Since we know the current (10 mA) through the LED, which must be the same as the current through R, the value of the resistor R can be calculated: it is V/I = 2.5/0.010 = 250 Ω.