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Science / Electronics basics / Grade 7 / se7w3

Watts: Convert Before You Multiply

Keep current units visible while explaining an energy-transfer rate.

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In a steady ideal resistor model, P = V x I. Use volts and amperes to obtain watts. Divide milliamps by 1000 to convert to amperes. A watt means a joule transferred each second, not the total energy for an unspecified run. Read the model on paper only: no batteries, wall outlets or physical circuit are needed.

Worked example

A model has 3 V across a resistor and 10 mA through it. First 10 / 1000 = 0.01 A. Then 3 x 0.01 = 0.03 W. At this steady power, 0.03 J is transferred each second. The number 30 from multiplying 3 by 10 is not a power in watts.

Worked example in three labeled steps: 10 mA becomes 0.01 A; 3 V times 0.01 A equals 0.03 W; 0.03 W means 0.03 joule each second.
Convert current first. A power value describes a rate, not an unspecified total.
Question 1 Convert 80 mA to amperes.
Question 2 A model resistor has 5 V across it and 80 mA through it. What is its power?
Question 3 What does a constant power of 0.4 W mean?
Question 4 Two ideal resistors each have 4 V across them. One carries 25 mA and the other 75 mA. How do their powers compare?
Question 5 Does a power calculation of 0.4 W alone tell you the resistor temperature or whether a real build is safe?