Calculating Current

Help Questions

AP Physics C: Electricity and Magnetism › Calculating Current

Questions 1 - 2

1

A lamp has a bulb. If the house wiring provides to light up that bulb, how much current does the bulb draw?

$0.55\ \text{A}$

$0.72\ \text{A}$

$1.37\ \text{A}$

$0.03\ \text{A}$

$5.83\ \text{A}$

Explanation

The formula for power is ,and we are given the following values.

$P=60\ \text{W}$

$V=110\ \text{V}$

Solve for the current, .

$I=\frac{P}{V}=\frac{60\ \text{W}}{110\ \text{V}}=0.55\ \text{A}$

2

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

$q_{proton}=1.60*10^{-19}C$

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

$1.96*10^7\frac{m}{s}$

$2.04*10^8\frac{m}{s}$

$3.00*10^8\frac{m}{s}$

$9.21*10^{7}\frac{m}{s}$

$1.95*10^6\frac{m}{s}$

Explanation

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

$I=\frac{dQ}{dt}$

We calculate this by the equation:

$I = \frac{nqv}{2\pi R}$

is the number of protons, is the charge per proton, is the velocity of each proton, and is the radius of the particle accelerator.

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

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

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

Return to subject