I am making an assignment in my electricity book, and I got an issue solving it. Two lightbulbs are in parallel circuit. The total current is 1.16A. The voltage drop across one of the bulbs is 2.27V. I need to calculate the total resistance for the light bulbs. Since the light bulbs are in parallel, the voltage drop across both bulbs is 2.27V. So the total resistance should be $R=\frac{U}{I}=\frac{2.27}{1.16}=1.96$ohms. But the right answer should be 2.96 ohms according to the book. I cant figure out what the problem is....

Setup of the circuit

The assignment text is as following:

The voltage source is shown on the left of the picture. The plus pole of the voltage is connected to the circuit via the red wire(1). The minus pole is connected via the black wire (2). One of the multimeter shows the current in ampere, and the other shows the voltage in volt.

a) Calculate the total resistance for the two light bulbs.

  • $\begingroup$ It sounds like either you've misunderstood the question, or there's a typo in the book. It's hard to know which one is the case without seeing the actual problem in the book. $\endgroup$ – Red Act Sep 28 '17 at 15:56
  • $\begingroup$ Hey Red Act. Thanks for response. I edited the post and uploaded an image of the setup of the circuit. $\endgroup$ – Elias S. Sep 28 '17 at 16:02
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    $\begingroup$ Also, are you using the exact same equipment (power supply, lamps, meters, wires, etc) that the book authors used when they predicted what the result would be? $\endgroup$ – The Photon Sep 28 '17 at 16:08
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    $\begingroup$ Looks like a typo. $\endgroup$ – A. C. A. C. Sep 28 '17 at 16:26
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    $\begingroup$ After going through this with 2 different methods I got $1.96$ twice. And as 1 & 2 are very close I'd say it was a typo as well. $\endgroup$ – CooperCape Sep 28 '17 at 16:32

The resistance of a light bulb depends on the current passing through it so it may well be that the book value was found with a higher current passing through the bulbs.
Change the current through the bulbs, but do not make it too high, and see what you get.


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