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Power

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Energy Measured in Joules, Power Measured in Watts

One Watt is the same as One Joule per Second.

Circuits consume energy from the power supply and produce energy at the output in the form of movement, light, sound, heat and so on. Circuits also produce waste heat energy.

Power Calculations

In the diagram, the power in Watts is dissipated as heat by the resistor.

To prevent overheating, the resistor power rating must be sufficient.

Direct Current Calculations

P = I V

P = V² / R

P = I² R

Alternating Current Calculations

To calculate RMS power, use RMS values for the voltage and current.

V_rms = 0.7 V_peak

I_rms = 0.7 I_peak

P_rms = I_rms V_rms

P_rms = V_rms² / R

P_rms = I_rms² R

Deriving Power Formulas

Ohm's law states that current, voltage and resistance are related in the following three ways ...

R = V / I
V = I R
I = V / R

Starting with

P = V I
V = IR

and by substituting V you get ...

P = I² R

Starting with

P = V I
I = V / R

and by substituting I you get ...

P = V² / R