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AP Physics C: E & M

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Unit 3

3.2 Power in a Circuit ๐Ÿ”ฆ

2 min readโ€ขnovember 3, 2020

caroline-koffke

Caroline Koffke

peter57616

Peter Apps


Power in a Circuit

Power ๐ŸŒŠ

Electrical circuits are often used to convert electrical energy into other types of energy. Recall from your previous physics classes that
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Using this, we can derive an equation for power in electrical terms.
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-RFxlOyF6OUJP.png?alt=media&token=d31f677c-0eb6-467d-b99e-3ea5c373756f
Because of Ohm's Law we can also write this several different ways by subbing in
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Choose the one that best fits the information that's given in the problem.
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-p8arDyRvfkql.png?alt=media&token=0e6384c3-cd1e-4c99-b493-332fdb0d3dbb

Practice Questions

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

Answer

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2. A student assembles the circuit shown below
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-44cW2Ii033wz.png?alt=media&token=268cf677-9e2b-43fd-b2fa-7f152faec05b

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
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where t is the time elapsed from the instant the switch is closed and I_0 and k are constants.
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-vIwv5JnspCE9.png?alt=media&token=4f76699d-5fba-45a1-87d2-13e32addfa95

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

Answers

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
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ahFwlBHf4b24.png?alt=media&token=a06b8b20-c3cf-4a39-a1cb-9ddc8e210441
c)
https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-nQtINNcUAuCO.png?alt=media&token=7dcadd1f-6750-4100-bbb3-b19ab0336cd8

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