# TPRH physics circuit question

#### podyon18

10+ Year Member
7+ Year Member
Having a difficult time with this question - any help would be great!
I attached a pic of the circuit

How much energy is provided by the 12V battery in 3 seconds?

A. 9 joules
B. 12 joules
C. 27 joules
D. 36 joules

Thanks!

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5+ Year Member
Is the answer B?

#### Zjodak

10+ Year Member
7+ Year Member
I'm not 100% sure but this would be my guess:

P=IVt
V=12V
I=1
--> Batteries oppose each other so V of circuit is 3 V, R is 3ohms, I is therefore 1A by Ohm's law.
t=3s

P=36J

OP
P

#### podyon18

10+ Year Member
7+ Year Member
Book answers:

P = I^2R or P=V^2/R would give wrong answers since they give total power supplied by resistors, which is not equal to the power supplied by the 12V battery, since the 9V battery eats up some of the power. so only P=IV would give the correct answer -
1 x 12 x 3 seconds = 36

what i dont understand: we only know that I = 1 since I=V/R = 3/3. but to get that we had to use the effect of the 9V battery either way.

So we are ignoring the 9V battery when calculating the 12V power, but we are using it to calculate the current which tells us the power?

#### ChickasawPup

Great example, Thanks!

Having a difficult time with this question - any help would be great!
I attached a pic of the circuit

How much energy is provided by the 12V battery in 3 seconds?

A. 9 joules
B. 12 joules
C. 27 joules
D. 36 joules

Thanks!

#### chiddler

5+ Year Member
i forgot to multiply by 3 :<

P=IV tells us the power that a given resistor uses. the question asks how much power does the battery provide? thus we look at all the voltage that the battery provides to the resistors. Thus you only consider 12 V - 9 V = 3V since that is all the voltage that gets to the resistors.

So to answer your question, no we are not ignoring any batteries.

.

#### chiddler

5+ Year Member
yes. if i'm not mistaken, the voltage that is wasted actually does work on the 9V battery and is just converted to heat, right?

a better question would ask "how much work is done by the 12 V battery onto the resistors" i think that would be more specific.

OP
P

#### podyon18

10+ Year Member
7+ Year Member
i forgot to multiply by 3 :<

P=IV tells us the power that a given resistor uses. the question asks how much power does the battery provide? thus we look at all the voltage that the battery provides to the resistors. Thus you only consider 12 V - 9 V = 3V since that is all the voltage that gets to the resistors.

So to answer your question, no we are not ignoring any batteries.
if so then P=IV = 3
3 X 3seconds = 9
but the correct answer is 36

#### chiddler

5+ Year Member
i have an idea but instead of being wrong again i'm hoping somebody will just give the correct answer.

sorry mr op

screw it. the idea is that the battery does indeed do work, like i wrote above. so you get 9 volts going to battery which is 9*1 = 9 and then 3 volts going to resistors which is 3*1 = 3. Adding both of these, you get 12. Three seconds, 36.

also i realize how much credibility I have right now lol Last edited: