Watts: Energy Each Second

Convert milliamps to amps, then connect voltage and current to power.

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Power is an energy-transfer rate, measured in watts (W). One watt means one joule each second. For these steady ideal resistor examples, P = V x I. Use volts and amperes: divide milliamps by 1000 first. Voltage is across a resistor; current is through it. These are paper calculations, not wiring instructions. Do not use batteries, wall outlets or powered equipment.

Worked example

A model resistor has 4 V across it and 25 mA through it. Convert 25 / 1000 = 0.025 A. Then P = 4 x 0.025 = 0.1 W. Multiplying 4 by 25 without converting would not give watts.

Would you believe it?

Different powers can transfer the same energy over different times: 1 W for 6 seconds and 2 W for 3 seconds each transfer 6 J. Compare both rate and duration.

OpenStax: electric power and energy

Question 1 Convert 50 mA to amperes. 0.5 A 0.05 A 50 A

Question 2 A model resistor has 6 V across it and 50 mA through it. What is its power? 0.3 W 300 W 3 W

Question 3 Which statement correctly describes a constant power of 0.3 W? 0.3 joule is the energy for every possible duration 0.3 ampere flows through the resistor 0.3 joule is transferred each second

Question 4 Two ideal resistors each have 6 V across them. One carries 20 mA and the other carries 40 mA. How do their powers compare? Their powers are equal because voltage is equal The 40 mA resistor has twice the power The 20 mA resistor has twice the power

Question 5 A calculation gives a resistor power of 0.3 W. Does that alone establish its temperature or a safe physical build? Yes; its temperature must be 0.3 degrees Yes; every resistor can safely use this power No; more physical information and qualified safety guidance are needed

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