Wheatstone Bridge Circuit

[MedPR]

Ok maybe I'm tired (I'm not), but this entire passage is way over my head. Can someone with TBR Physics look at book 2, page 216 (Chapter 9 passage 4).

The only question I was sure of was question 24, and I was kind of sure of 25. I also got 23 right, but only because I guessed based on the Wheatstone bridge circuit.

I especially don't understand 26, 27, and 28. For 26, how does the resistance of the different metals affect the distance between AZ and ZC?

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[411309]

Ok maybe I'm tired (I'm not), but this entire passage is way over my head. Can someone with TBR Physics look at book 2, page 216 (Chapter 9 passage 4).

The only question I was sure of was question 24, and I was kind of sure of 25. I also got 23 right, but only because I guessed based on the Wheatstone bridge circuit.

I especially don't understand 26, 27, and 28. For 26, how does the resistance of the different metals affect the distance between AZ and ZC?

resistance is equal to pL/A. Three different metals would give three different constants for p. The book says AZ is shorter making it less resistive but I dont see how it reasoned that it's shorter?

This passage owned me as well. My problem is I dont fully understand the experiment either. I think this way harder then any circuit problem well see on the mcat.

 

[MedPR]

resistance is equal to pL/A. Three different metals would give three different constants for p. The book says AZ is shorter making it less resistive but I dont see how it reasoned that it's shorter?

This passage owned me as well. My problem is I dont fully understand the experiment either. I think this way harder then any circuit problem well see on the mcat.

Yea, I can't make the connection between the length of AZ or ZC and the resistivity of the metals or resistance of the resistors...

I mean, I have a pretty good grasp on circuits, but I think this passage might require some outside knowledge that I just don't have.

 

[411309]

Yea, I can't make the connection between the length of AZ or ZC and the resistivity of the metals or resistance of the resistors...

I mean, I have a pretty good grasp on circuits, but I think this passage might require some outside knowledge that I just don't have.

I have to agree. I owned the circuits problems but when I got to this passage I also got completely destroyed. I feel like they left something out of the passage because I have no idea how they even reasoned one is shorter, nothing infers that in the passage?

 

[BerkReviewTeach]

resistance is equal to pL/A. Three different metals would give three different constants for p. The book says AZ is shorter making it less resistive but I dont see how it reasoned that it's shorter?

This passage owned me as well. My problem is I dont fully understand the experiment either. I think this way harder then any circuit problem well see on the mcat.

The Wheatstone Bridge is in the AAMC practice materials, so it's fair game for the MCAT. I'm sure it's one of the curve busters they need to offset some easier passage.

The trick here is to think of it in terms of proportions.

Upper pathway of a parallel circuit: __________ . . __________
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .a. . . . . . . . . . .b

Lower pathway of a parallel circuit: __________ . . __________
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .c. . . . . . . . . . .d


Let's say it has a 12V battery. The bridge is set up so the first segment in each pathway drops the same voltage as the other one. So let's say that the first segment drops 4V and the second drops the reminaing 8V.

Upper pathway of a parallel circuit: 12V __________ 8V __________ 0V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .a. . . . . . . . . . . .b

Lower pathway of a parallel circuit: 12V __________ 8V __________ 0V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .c. . . . . . . . . . . .d

The facts we know are that (1) iaRa = icRc, (2) ibRb = idRd, (3) ia = ib, and (4) ic = id.

Using the 8V and 4V drops, we know that idRd divided by icRc = 2, so given that ic = id, we know that Rc is half of Rd. By symmetry, we also know that Ra is half of Rb. If you know three of the R-values, you can solve for the fourth one.

It's a complicated looking circuit with a simple purpose. To find the resistance of the unknown segment when you know the resistance of the other three.