Physics Help Forum problem whith circuit

 Oct 27th 2017, 03:42 AM #1 Junior Member   Join Date: Oct 2017 Posts: 2 problem whith circuit I need help to solve this problem. Can anyone help me to solve this exercise? I can not really solve it. The source is Halliday. Thanks in advance. Prove that if a resistor of resistance r is inserted between points $a$ and $b$ of Figure 27.39 (attached figure), the current throug it is given by $i = \frac{E(R_s - R_x)}{(R+2r)(R_s+R_x)+2R_sR_x}$ where $E$ is e.m.f. of the ideal battery and $R=R_1=R_2$ Assume that $R_0=0$ Attached Thumbnails
Oct 27th 2017, 07:22 PM   #2
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 Originally Posted by Jeff18 I need help to solve this problem. Can anyone help me to solve this exercise? I can not really solve it. The source is Halliday. Thanks in advance. Prove that if a resistor of resistance r is inserted between points $a$ and $b$ of Figure 27.39 (attached figure), the current throug it is given by $i = \frac{E(R_s - R_x)}{(R+2r)(R_s+R_x)+2R_sR_x}$ where $E$ is e.m.f. of the ideal battery and $R=R_1=R_2$ Assume that $R_0=0$
The rules of this forum only allow us to help you once you've shown what you've done so far or to give you a nudge if you're unable to get anywhere at all. The later seems to be the case here so here's a hint: Draw the circuit another way, in a way that makes it look very easy to work with. Once you've done that you can use what you already know, i.e. expressions for the resistance of resistors in and parallel and in series, to solve the problem. Take it step by step.

Oct 27th 2017, 07:27 PM   #3

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 Originally Posted by Jeff18 I need help to solve this problem. Can anyone help me to solve this exercise? I can not really solve it. The source is Halliday. Thanks in advance. Prove that if a resistor of resistance r is inserted between points $a$ and $b$ of Figure 27.39 (attached figure), the current throug it is given by $i = \frac{E(R_s - R_x)}{(R+2r)(R_s+R_x)+2R_sR_x}$ where $E$ is e.m.f. of the ideal battery and $R=R_1=R_2$ Assume that $R_0=0$
This circuit is called a "Weatstone bridge." You might find this link to be helpful.

-Dan
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 Oct 28th 2017, 02:28 AM #4 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 820 One of the reasons for asking for information about your thoughts, besides the rules, is that there are several ways to solve this circuit. I would not want to ofer a method that you have not (yet) been introduced to as that would just cause confusion. At this moment we don't know what methods you have been taught so even just saying Could I use parallel circuits, or perhaops Kirchoff's laws, but I can't see how to apply them? would be helpful in this respect. Here is a hint. Label the other two corners c and d. What do you know about the voltages at c and d? topsquark likes this.

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