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AP Physics C: E & M

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Unit 3

# 3.2 Power in a Circuit ๐ฆ

Caroline Koffke

Peter Apps

## Power in a Circuit

### Power ๐

Electrical circuits are often used to convert electrical energy into other types of energy. Recall from your previous physics classes that
Using this, we can derive an equation for power in electrical terms.
Because of Ohm's Law we can also write this several different ways by subbing in
Choose the one that best fits the information that's given in the problem.

## Practice Questions

1. A hair dryer is rated at 1200W when connected to 120V. What is the resistance of the dryer?

2. A student assembles the circuit shown below

Image created by the author

The switch is closed and the student records the current in the circuit as shown in the graph below. The equation for the graph is
where t is the time elapsed from the instant the switch is closed and I_0 and k are constants.

Image created by the author

a) Using the information in the graph, determine the potential difference Vo across the resistor immediately after the switch is closed.
b) Determine the value of k from this best-fit curve. Show your work and be sure to include units in your answer.
c) Determine the following in terms of R, k, I_o, and t.
i. The power delivered to the resistor at time t = 0 ii. The power delivered to the resistor as a function of time t iii. The total energy delivered to the resistor from t = 0 until the current is reduced to zero

a) According to the graph, the current at t=0 is approximately 9.3A. Using V = IR that gives us a Voltage of 31V across the 3.3ฮฉ Resistor
b) Using I_o=9.3A, I=7.5A, t=0.5hr
c)

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