= R I 2
= U 2 / R (4)
P = electrical power (watts, W)
The power consumed in the 12V electrical circuit above can be calculated as
P = (12 volts) 2 / (18 ohm)
= 8 W
A 100 W electric light bulb is connected to a 230 V supply. The current flowing can be calculated by reorganizing (4) to
I = P / U
= (100 W) / (230 V)
= 0.43 ampere
The resistance can be calculated by reorganizing (4) to
R = U 2 / P
= (230 V) 2 / (100 W)
= 529 Ω
This nomogram can be used to estimate power vs. voltage and ampere.
The default values in the nomogram above indicates 240 volts , resistance 10 amps and power 2.4 kW for DC or single phase AC - and 4 kW for three phase AC.
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