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# PHYSICS question a 60 watt bulb is connected to a 12 v car

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PHYSICS question

a 60 watt bulb is connected to a 12 v car batter. When another 60 watt bulb is connected in parallel with the first bulb, the battery's output energy -- a: halves; b:remains the same; c:doubles.

Please explain BOTH conceptually and quantitatively. In particular, is there a relationship between power and resistance implicit in this question? If so, how does it play out.
Submitted: 7 years ago.
Category: Homework
Expert:  David replied 7 years ago.

Conceptually the power doubles. Look, you just add another source of light, so you need to consume double energy compared to the case of a single bulb, so the power is obviously doubles.

Quantitatively the current through each bulb is I=12/R, where 12 is the voltage, since you have to bulbs in parallel the current you take from the battery doubles I(tot) = 2*12/R

Since the power is given P=IV, the increase of the current by a factor of 2 means that the power increases as well by the same amount. by factor of 2.

From another side you can say that in parallel the net resistance is halved, hence P=V^2/R and once R is halved the power obviously doubles.

Please let me know if you have more Physics questions, I can help. Just type "For David" before your post

Customer: replied 7 years ago.

This answer makes sense, especially the Power/Resistance inverse relationship.

Just one follow up: I never thought of a battery as changing it's power to accomodate more bulbs in parallel. I guess I just thought that a battery always supplies, say, 12 Volts regardless of resistors. But, from your answer, I should probably think that since voltage is constant, power is an inverse function of resistance in parallel and a direct function of current. So any time you add more resistors to a parallel circuit powered by a battery, the power goes up since the resistance goes down; and, conversely, anytime you add more resistors to a series circuit powered by a battery, the power goes down since the resistance goes up. Is that correct?

Thx
Tom
Expert:  David replied 7 years ago.