Electronics Basics: Power and Energy

Distinguish the rate of energy transfer from total energy.

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For steady DC, electrical power P = V I, measured in watts. Energy transferred over time t is E = P t, measured in joules when t is seconds. For an ohmic resistor, P also equals I squared R or V squared / R. Use diagrams and calculations only. Never use wall outlets, short batteries, open powered equipment or handle charged capacitors.

Worked example

A 6 V source supplying 0.02 A delivers 0.12 W. Over 30 s it transfers 3.6 J. A resistor can convert electrical energy to thermal energy; charge is not used up.

At six volts and 0.02 amp, power is 0.12 watt. Over 30 seconds the transferred energy is 3.6 joules, and over 60 seconds it is 7.2 joules.

Would you believe it?

Conventional current uses the direction positive charge would move. In metal wires, negatively charged electrons drift in the opposite direction.

OpenStax: electrical current

Question 1 What is power for 6 V and 0.02 A? 0.12 W 300 W 6.02 W

Question 2 What energy is transferred at 0.12 W for 30 s? 0.004 J 30.12 J 3.6 J

Question 3 With the same power for 60 s, what energy is transferred? 0.12 J 7.2 J 3.6 J

Question 4 At fixed resistance, doubling voltage changes resistor power by what factor? 4 2 1/2

Question 5 Which quantity is a rate rather than a total amount? Energy Elapsed time Power

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