AP Physics C Electricity : Calculating Current

Study concepts, example questions & explanations for AP Physics C Electricity

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Example Questions

Example Question #1 : Current

A lamp has a \(\displaystyle 60W\) bulb. If the house wiring provides \(\displaystyle 110V\) to light up that bulb, how much current does the bulb draw?

Possible Answers:

\(\displaystyle 0.72\ \text{A}\)

\(\displaystyle 0.55\ \text{A}\)

\(\displaystyle 1.37\ \text{A}\)

\(\displaystyle 5.83\ \text{A}\)

\(\displaystyle 0.03\ \text{A}\)

Correct answer:

\(\displaystyle 0.55\ \text{A}\)

Explanation:

The formula for power is ,\(\displaystyle P=IV\)and we are given the following values.

\(\displaystyle P=60\ \text{W}\)

\(\displaystyle V=110\ \text{V}\)

Solve for the current, \(\displaystyle I\).

\(\displaystyle I=\frac{P}{V}=\frac{60\ \text{W}}{110\ \text{V}}=0.55\ \text{A}\)

Example Question #13 : Electric Circuits

A particle accelerator with a radius of 500 meters can have up to \(\displaystyle 10^{15}\) protons circulating within it at once.

\(\displaystyle q_{proton}=1.60*10^{-19}C\)

How fast must the protons in the accelerator move in order to produce a current of 1A?

Possible Answers:

\(\displaystyle 9.21*10^{7}\frac{m}{s}\)

\(\displaystyle 3.00*10^8\frac{m}{s}\)

\(\displaystyle 2.04*10^8\frac{m}{s}\)

\(\displaystyle 1.95*10^6\frac{m}{s}\)

\(\displaystyle 1.96*10^7\frac{m}{s}\)

Correct answer:

\(\displaystyle 1.96*10^7\frac{m}{s}\)

Explanation:

The current produced is the total charge that circulates the particle accelerator per unit time.

\(\displaystyle I=\frac{dQ}{dt}\)

We calculate this by the equation:

\(\displaystyle I = \frac{nqv}{2\pi R}\)

\(\displaystyle n\) is the number of protons, \(\displaystyle q\) is the charge per proton, \(\displaystyle v\) is the velocity of each proton, and \(\displaystyle R\) is the radius of the particle accelerator.

Using the given current, we then solve for the velocity:

\(\displaystyle 1A = \frac{(10^{15})(1.60*10^{-19}C)v}{2\pi (500m)}\)

\(\displaystyle v=\frac{(1A)2\pi(500m)}{(10^{15})(1.60*10^{-19}C)}= 1.96*10^7\frac{m}{s}\)

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