Question

1. an electric light is plugged into a 120 v outlet. if the current in the bulb is 0.5 a, how much electrical energy does the bulb use in 15 min?
2. calculate the current into a desktop computer plugged into a 120 v outlet is the power used is 180 w
3. a circuit breaker is tripped when the current in the circuit is greater than 15 a. if the voltage difference is 120 v, estimate the power being used when the circuit breaker is tripped.
4. estimate the monthly cost of using a 700 w refrigerator that runs for 10 hours a day if the cost kwh is $0.20 (assume a month is 30 days).
5. a toy car with a resistance of 20 ω is connected to a 3 v battery. what is the current in the car?
6. the current in an appliance connected to a 120 v source is 2 a. approximately how many kilowatt - hours of electrical energy does the power company provide for the appliance in 4 hours?
7. a calculator uses a 9 v battery and draws 0.1 a of current. what is the power of the calculator?

Explanation:

Step1: Calculate bulb power

$P = V \times I = 120\ \text{V} \times 0.5\ \text{A} = 60\ \text{W}$

Step2: Convert time to seconds

$t = 15\ \text{min} \times 60 = 900\ \text{s}$

Step3: Find energy used

$E = P \times t = 60\ \text{W} \times 900\ \text{s} = 54000\ \text{J}$

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Step1: Rearrange power formula for current

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

Step2: Substitute values

$I = \frac{180\ \text{W}}{120\ \text{V}} = 1.5\ \text{A}$

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Step1: Calculate tripping power

$P = V \times I = 120\ \text{V} \times 15\ \text{A} = 1800\ \text{W}$

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Step1: Convert power to kilowatts

$P = 700\ \text{W} = 0.7\ \text{kW}$

Step2: Calculate daily energy use

$E_{\text{daily}} = 0.7\ \text{kW} \times 10\ \text{h} = 7\ \text{kWh}$

Step3: Calculate monthly energy use

$E_{\text{monthly}} = 7\ \text{kWh} \times 30 = 210\ \text{kWh}$

Step4: Calculate monthly cost

$\text{Cost} = 210\ \text{kWh} \times \$0.20/\text{kWh} = \$42.00$

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Step1: Use Ohm's Law for current

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

Step2: Substitute values

$I = \frac{3\ \text{V}}{20\ \Omega} = 0.15\ \text{A}$

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Step1: Calculate appliance power

$P = V \times I = 120\ \text{V} \times 2\ \text{A} = 240\ \text{W} = 0.24\ \text{kW}$

Step2: Calculate energy in kWh

$E = 0.24\ \text{kW} \times 4\ \text{h} = 0.96\ \text{kWh}$

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Step1: Calculate calculator power

$P = V \times I = 9\ \text{V} \times 0.1\ \text{A} = 0.9\ \text{W}$

Answer:

1. $54000\ \text{J}$
2. $1.5\ \text{A}$
3. $1800\ \text{W}$
4. $\$42.00$
5. $0.15\ \text{A}$
6. $0.96\ \text{kWh}$
7. $0.9\ \text{W}$