0
$\begingroup$

enter image description here

The voltmeter has infinite resistance. What are the readings on the voltmeter when the switch is open and when it is closed?

I thought that the reading on the voltmeter would be E/2 regardless of whether the switch was open or closed:

When open: The two resistors are identical, so voltage splits evenly;

When closed: The voltmeter has infinite resistance and so it serves as an open switch for that part of the circuit. Even if the voltmeter was connected just around the resistor to the top left of the circuit, while a parallel circuit went around both this resistor and the voltmeter, I would say that the voltage is E/2.

My answer is right for when the switch is opened, but the answer to when the switch is closed is "less than E/2." Why so?

$\endgroup$

closed as off-topic by Emilio Pisanty, AccidentalFourierTransform, ACuriousMind, user36790, Qmechanic May 8 '16 at 19:34

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Emilio Pisanty, AccidentalFourierTransform, ACuriousMind, Community, Qmechanic
If this question can be reworded to fit the rules in the help center, please edit the question.

1
$\begingroup$

When the switch is closed the two resistors on the left are in parallel. The current from the battery will split up half passing through one resistor and half passing through the other. The result will be a smaller voltage drop across each resistor, think $V=IR$.

The measured voltage will less than it was with the switch open but not half as much because by adding a second resistor in parallel the total resistance of the circuit is decreased and so the new total current will increase.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.