Circuits

[moonlit]

I have two physics problems that I'm not sure how to do, can someone help me?

1) A defibrillator is used during a heart attack to restore the heart to its normal beating pattern. A defibrillator passes 16 A of current through the torso of a person in 1.6 ms. (a) How much charge moves during this time? (b) How many electrons pass through the wires connected to the patient?

2) A resistor is connected across the terminals of a 9.0-V battery, which delivers 2 x 105 J of energy to the resistor in 4 hours. What is the resistance of the resistor?

Should I use the equation delta q=I(delta t) for both these problems?

[chroot]

The first one is just \Delta Q = I \Delta t.

The second one involves the definition of power, namely

P = \frac{d E}{d t} = \frac{V^2}{R}.

- Warren

[moonlit]

So the answer to part A would be .01 C correct? What about part B?

[chroot]

So the answer to part A would be .01 C correct?
I got 0.0256 C: http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=16+amps+*+1.6+milliseconds&btnG=Google+Search

What about part B?
What about it? All you need to do is find out how much power was delivered to the resistor, which is just the energy over the time, and then solve that equation (P = V2/R) for R.

- Warren

[moonlit]

Ok, not sure what to do with part b. P=E/T so wouldn't that be .01? Also, how do I convert amps to volts?

[chroot]

Power is energy per unit time. You are given energy (2 x 105 joules) and time (4 hours). Divide them. You get around P = 13.9 Joules per second delivered to the resistor:

http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=2+*+10%5E5+J+%2F+4+hours+in+J%2Fs&btnG=Google+Search

Now, since you know the power, you can use P = V2/R. Plug in P and V, and find R.

- Warren

[paul11273]

Moonlit-
When you first mentioned you were not sure what to do with part (b), I think you were referring to the second part of question #1.

If that is correct, than to find the number of electrons that pass through one of the electrodes, you divide the total charge (what you found in part a) by the charge of each electron.

Number of Electrons = Total Charge / Elementary Charge
n = Q/e
n = .0256C / 1.6*10^-19C
n = 1.6*10^17 electrons

[chroot]

I apologize, even though you were talking about part b, I was thinking about question 2! :eek:

- Warren