How much power is produced if a voltage of #3 V# is applied to a circuit with a resistance of #21 Omega#?
Let P be power dissipated from 21 ohm resistor to ambient.
Let V be the voltage applied across 21 ohm resistor.
Let I be the current passing through 21 ohm resistor.
We know that power defined as the product of voltage across resistor and current through resistor
By ohm's law,
from (1) and (2)
substitute V=3 volts and R=21 ohms