Confusion about Ohm's Law example

2011 April 6
by roma

I'm teaching myself the basics of electricity with Stan Gibilisco's Electricity Demystified.

I'm stumped by a problem in Chapter 3 (pg. 50).

4. Suppose that the timer above, which has no resistance, is replaced by another timer that has resistance equal to that of the lamp. If nothing else in the circuit changes, how will the voltage across the lamp (not the voltmeter reading) be different when the switch is closed, as compared with the voltage across the lamp in the original circuit when the switch is closed?
a. It will be the same.
b. It will be half as great.
c. It will be 1/4 as great.
d. More information is needed to answer this question

I chose a, but the answer is b. This really confused me, because the lamp and the voltmeter are wired in parallel. According to, all voltage remains the same throughout a parallel circuit.

Seems like I'm missing something.

2 Responses leave one →
  1. April 7, 2011

    Hmm.. Yeah, this is a tricky question.

    It's best to imagine the path that your current is going to make ( + to - ). Because the circuit is parallel, it is intuitive that the current is going to bipass the extra resistance, and go to the lamp through the reverse..

    ..But the lamp is not the end-game. The electrons are trying to get back to the battery anode. So, to get back to the battery in a shorter circuit, it will pass through the timer before the lamp.

    Hope that makes sense!

  2. roma permalink*
    April 20, 2011

    I explained the solution to this problem in my April 7 Voltage post.

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