# What happens to the total power dissipated in the circuit ?

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1. Apr 9, 2016

### kamhogo

1. The problem statement, all variables and given/known data
What happens to the total power dissipated in the circuit once the switch is closed?

2. Relevant equations
Pr=Pbat=dEth/dt=(dq/dt)*Vr=I*Vr

3. The attempt at a solution
V total and I toral remain the same so the power dissipated by the circuit stays the same...?

2. Apr 9, 2016

### phyzguy

Why do you think Itotal remains the same? What is the resistance across the battery with the switch open and with it closed?

3. Apr 9, 2016

### Staff: Mentor

Can you show some equations to back up your guess?

4. Apr 9, 2016

### kamhogo

Well....for the current, it's current conservation. Switch open: I total goes through A, B and C ( they're in series) and then goes back to the battery. Switch closed: I total goes through A then B then splits into let's say I1( through the middle resustless wire) and I2 ( through C) at the upper junction. Then it becomes I total again at the lower junction. I think the closed switch short-circuits C but I am not sure this means I2 is null....As for the voltages, I think the voltage across C has to decrease once the switch is closed ( V=IR, I decreases; V decreases). But the total voltage has to stay the same ( equal to the voltage provided by the battery) so the voltages across A and B have to increase...but that would mean that the current through A and B increases.....?!?

5. Apr 9, 2016

### kamhogo

Pr=Pbat=dEth/dt=(dq/dt)*Vr=I*Vr
I got his from my textbook. We haven't learned it in class yet, so I am confused about it and the textbook does not help..

6. Apr 9, 2016

### cnh1995

What happens to the total resistance of the circuit when the switch is closed?

7. Apr 9, 2016

Right.

8. Apr 9, 2016

### kamhogo

But how is that possible??? A and B are right after the positive terminal of the battery so no matter the situation they should receive I total...?

9. Apr 9, 2016

### cnh1995

10. Apr 10, 2016

### phyzguy

Of course A and B receive all of Itotal, but Itotal is different when the switch is closed than when it is open.

11. Apr 10, 2016

### kamhogo

l think I see. Itotal=Vtotal*Req. Switch open: Req =3R (the bulbs are identical and in series). Switch closed: Req is smaller than 3R (C is now in parallel with A and B). So I is smaller when the switch is closed. Correct?

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12. Apr 10, 2016

### cnh1995

Yes.
No. C is eliminated from the circuit. Look up the term "short circuit".
Are you sure? Equivalent resistance decreased after closing the switch.

13. Apr 10, 2016

### cnh1995

I=V/R and not V*R.

14. Apr 10, 2016

### kamhogo

True! So the current increases.

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