In a laboratory demo, a 12 V car battery powers two 12 V, 24 W lamps in parallel.

The current is measured using an ammeter (of resistance 10 ohms) which is connected in series with each of the bulbs in turn. What will the ammeter read? Comment on your answer.

I got two answers, not sure which is right (if any)

1) 1.2 A because I = V/R, therefore 12/10 = 1.2 A

2) 0.75 A because:
I = P/V, therefore current of bulbs normally is 2A.
R = V/I, therefore R = 12/2 = 6 Ohms
Total resistance = 6+10 = 16 Ohms
I = V/R, therefore 12V/16Ohms = 0.75 A

Question

21-10-2014
Physics

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In a laboratory demo, a 12 V car battery powers two 12 V, 24 W lamps in parallel.

The current is measured using an ammeter (of resistance 10 ohms) which is connected in series with each of the bulbs in turn. What will the ammeter read? Comment on your answer.

I got two answers, not sure which is right (if any)

1) 1.2 A because I = V/R, therefore 12/10 = 1.2 A

2) 0.75 A because:
I = P/V, therefore current of bulbs normally is 2A.
R = V/I, therefore R = 12/2 = 6 Ohms
Total resistance = 6+10 = 16 Ohms
I = V/R, therefore 12V/16Ohms = 0.75 A

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AL2006 AL2006 · Answer 1 · 2014-10-21T20:14:24-04:00

AL2006 AL2006

21-10-2014

Your second solution is excellent, and well done. Ammeters are usually assumed to have zero resistance, and that would be your first solution. Your second one shows how much difference it can make when the meter itself has substAntial resistance. / / / Notice also that the poor meter has to dissipate a little more than 5 watts. That's a lot.

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