Question 39

[steven.won3962]

Answer says to do:
480/(480+1000)x 2 to get Volts, and then to plug it into the power equation. I'm wondering why you need to do 480/(480+1000) because I chose A.

Thanks!

[mcat_premed3832]

There are several ways to do this calculation, here is one:

We know that for the thermistor, P = (V^2)/R

We know R for the thermistor but we need to figure out the voltage just for the thermistor. The easiest way to do this is to ask: of the total voltage for the whole circuit (2 V), how can we calculate the fraction of voltage drop that occurs only at the thermistor? Well, check the fraction of resistance caused by the thermistor. That fraction is, of course, given by (note that R1 in Figure 1 is a 1 kilohm resistor):

480/(480+1000)

The correct answer is A.

The long way: use Kirchoff's second law to calculate the current:

2 V (epsilon) = i R1 + i R2
2 = 1480 i
i = 1/740

thus the voltage for the thermistor:

V = i R2 = (1/740)(480) = approx. 2/3 V.

[wesley.pau5578]

Why do the divide I^2 by 500? Isn't the power equation P=(I^2)R or P=IV (either one)?

[jellywing_2058]

As mcat_premed3832 explained,