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http://www.fisme.uu.nl/nno/opgaven/bestanden/Ronde1-Theorie-2009.pdf, question 5:

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You have to calculate Id when the switch at S is opened, when its closed, the Id = 0.5 A. All resistors are the identical in the circuit.

So, the answer is B, Id = 0.67 A. But why? I don't get it, maybe its elementary, but I don't get it. Could you explain to me, in steps, how to solve this problem?

Thanks.

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closed as off-topic by Emilio Pisanty, akhmeteli, dmckee Sep 22 '13 at 16:47

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Please note that Homework questions are supposed to show more effort. For more details, please see the Homework Policy. –  Dimensio1n0 Sep 22 '13 at 10:19

1 Answer 1

up vote 1 down vote accepted

Start with the switch closed. You're told that all the resistors have the same value - let's call this $R$. You can use the usual rules for calculating the total resistance to calculate $R_{total}$. Now the battery voltage is $V = IR_{total}$ and you know $I$ so you can calculate $V$ in terms of $R$.

With the switch open you calculate $R_{total}$ again (obviously it's higher than with the switch closed), and you calculated $V$ above, so now you can calculate $I$.

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Could you also do the calculation with the data given? Since when I do that I get a much higher value than the answer in the key ;p Thanks btw, I didn't know about the homework policy, apologies for that. –  user209347 Sep 22 '13 at 12:16
    
I solved it thank you! –  user209347 Sep 22 '13 at 18:17

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