Complete Electronics Self-Teaching Guide with Projects

Power

13 When current flows through a resistor, it dissipates power, usually in the form of heat. Power is expressed in terms of watts.

Question

What is the formula for power? _____

Answers

There are three formulas for calculating power:

14 The first formula shown in problem 13 allows power to be calculated when only the voltage and current are known.

Questions

What is the power dissipated by a resistor for the following voltage and current values?

A. V = 10 volts, I = 3 amperes

P = _____

B. V = 100 volts, I = 5 amperes

P = _____

C. V = 120 volts, I = 10 amperes

P = _____

Answers

A. 30 watts.

B. 500 watts, or 0.5 kW. (The abbreviation kW indicates kilowatts.)

C. 1,200 watts, or 1.2 kW.

15 The second formula shown in problem 13 allows power to be calculated when only the current and resistance are known.

Questions

What is the power dissipated by a resistor given the following resistance and current values?

A. R = 20 ohm, I = 0.5 ampere

P = _____

B. R = 560 ohms, I = 0.02 ampere

P = _____

C. V = 1 volt, R = 2 ohms

P = _____

D. V = 2 volt, R = 10 ohms

P = _____

Answers

A. 5 watts

B. 0.224 watts

C. 0.5 watts

D. 0.4 watts

16 Resistors used in electronics generally are manufactured in standard values with regard to resistance and power rating. Appendix D shows a table of standard resistance values for 0.25- and 0.05-watt resistors. ...