Question

electricity practice problems: voltage difference (v), current (i), resistance (r),
electrical power (p) & electrical energy (e)
1 a toaster oven is plugged into an outlet where the voltage difference is 120v. how much power does the
toaster oven use if the current in the oven is 10 a?

2 a video - disc player that is not playing still uses 6.0 w of power. what is the current into the video - disc player if
it is plugged into a standard 120 v outlet?

3 a flashlight bulb uses 2.4 w of power when the current in the bulb is 0.8 a. if the flashlight uses two batteries,
what is the voltage difference supplied by each battery?

4 a hair dryer is rated at 1.2 kw. when it is plugged into a standard outlet (120v) and turned on, what is the
current in the hair dryer?

5 a refrigerator operates an average of 10.0 hours a day. if the power rating on the refrigerator is 700 w, how
much electrical energy does the refrigerator use in one day?

6 a tv with a power rating of 200 w uses 0.8 kwh of electrical energy in one day. for how many hours was the
tv on during this day?

7 a hair dryer has a power of 1,200 w. how much electrical energy does it use in 3 min?

Explanation:

Problem 1

Step1: Use power formula $P=V \times I$

Given $V=120\ \text{V}$, $I=10\ \text{A}$
$P = 120 \times 10$

Step2: Calculate power value

$P = 1200\ \text{W}$

Problem 2

Step1: Rearrange power formula for $I$

$I = \frac{P}{V}$, given $P=6.0\ \text{W}$, $V=120\ \text{V}$
$I = \frac{6.0}{120}$

Step2: Solve for current

$I = 0.05\ \text{A}$

Problem 3

Step1: Rearrange power formula for $V_{\text{total}}$

$V_{\text{total}} = \frac{P}{I}$, given $P=2.4\ \text{W}$, $I=0.8\ \text{A}$
$V_{\text{total}} = \frac{2.4}{0.8}$

Step2: Find voltage per battery

Divide total voltage by 2: $V_{\text{per battery}} = \frac{3}{2} = 1.5\ \text{V}$

Problem 4

Step1: Convert power to watts

$1.2\ \text{kW} = 1200\ \text{W}$, use $I = \frac{P}{V}$
$I = \frac{1200}{120}$

Step2: Calculate current

$I = 10\ \text{A}$

Problem 5

Step1: Use energy formula $E=P \times t$

Convert $P=700\ \text{W}=0.7\ \text{kW}$, $t=10.0\ \text{h}$
$E = 0.7 \times 10.0$

Step2: Compute daily energy

$E = 7.0\ \text{kWh}$

Problem 6

Step1: Rearrange energy formula for $t$

$t = \frac{E}{P}$, convert $P=200\ \text{W}=0.2\ \text{kW}$, $E=0.8\ \text{kWh}$
$t = \frac{0.8}{0.2}$

Step2: Find runtime

$t = 4\ \text{hours}$

Problem 7

Step1: Convert time to hours

$3\ \text{min} = \frac{3}{60} = 0.05\ \text{h}$, $P=1200\ \text{W}=1.2\ \text{kW}$
$E = 1.2 \times 0.05$

Step2: Calculate energy

$E = 0.06\ \text{kWh}$ (or $216000\ \text{J}$ if using $E=P \times t$ in seconds: $1200 \times 180 = 216000$)

Answer:

1. $1200\ \text{W}$
2. $0.05\ \text{A}$
3. $1.5\ \text{V}$
4. $10\ \text{A}$
5. $7.0\ \text{kWh}$
6. $4$ hours
7. $0.06\ \text{kWh}$ (or $216000\ \text{J}$)