Exercises: Electric Power

Work through each section in order. Show your work where indicated.

Grade 11·22 problems·~30 min·OpenStax Physics (High School)·section·sec-19-4

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Recall / Warm-Up

In mechanics, power is defined as the rate of doing work. Which expression correctly states this definition?

$P = \frac{W}{t}$

$P = W \cdot t$

$P = \frac{t}{W}$

$P = \frac{F}{d}$

A circuit element has a potential difference of $V$ across it and a current $I$ flowing through it. Which expression gives the power delivered to the element?

$P = V + I$

$P = \frac{V}{I}$

$P = IV$

$P = \frac{I}{V}$

Ohm's Law states the relationship between voltage, current, and resistance in a resistor. Which equation is correct?

$V = I \cdot R$

$V = \frac{R}{I}$

$I = V \cdot R$

$R = V \cdot I$

Fluency Practice

A hair dryer operates at $120\ \text{V}$ and draws a current of $12.5\ \text{A}$ . Calculate the power consumed by the hair dryer in watts.

A $60\ \text{W}$ light bulb operates at $120\ \text{V}$ . What current flows through the bulb? Give your answer in amperes.

A resistor has resistance $R = 40\ \Omega$ and carries a current of $3\ \text{A}$ . Calculate the power dissipated in the resistor using the formula $P = I^2R$ .

Decision tree showing which power formula to use: P=IV when given I and V, P=I-squared-R when given I and R, P=V-squared-over-R when given V and R, with the V and R row highlighted

A resistor has $V = 24\ \text{V}$ across it and $R = 8\ \Omega$ . Which formula should you use to calculate power directly from these two values, and what is the result?

$P = IV$ ; find $I$ first: result is $72\ \text{W}$

$P = I^2R$ ; result is $72\ \text{W}$

$P = V^2/R$ ; result is $72\ \text{W}$

$P = V^2/R$ ; result is $576\ \text{W}$

An electric heater has resistance $R = 20\ \Omega$ and is connected to a $240\ \text{V}$ outlet. Calculate the power dissipated using $P = V^2/R$ .

A $150\ \text{W}$ refrigerator runs for $8\ \text{hours}$ . How many kilowatt-hours of energy does it use?

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