Hutchoo's Unit 3 Physics Questions.

[Hutchoo]

Don't really get this: (see attached).

I know you'd have to use V_out  = R2/(r1+r2) * V_in. But I don't understand how you find R2 using another method?
(BTW, talking about the first row).

10 = r2/(1000+R2) * 20.
1/2 = R2 /(1000+R2) ..
r2 = 1000.

But, my teacher said that the 'V_out equation' is 'bad' and shouldn't be used in SACs/Exams because it makes students lose marks (Not sure how). Anyway, how can you do this question using another method?


Thanks

[mihir94]

I don't really see the problem with what you're doing. As long as you're getting the correct answer, while still showing appropriate working out, it shouldn't really matter how you do it. I don't even know if there is another way to work it out. Maybe your teacher means you should have a pretransposed formula for R2 to avoid silly mistakes. In this case it would be:

R₂=R₁/ [(V_in/V_out) -1]

To find out R1 it would be:
R₁=[(R₂/V_out) X V_in] - R₂

Hope that helps

[Hutchoo]

Thanks for the post.
Hmm, my teacher said to ditch the equation all together :/ If I do... WHAT DO I USE INSTEAD D=?

[Lasercookie]

The value you got out for R2 is correct.

There's nothing wrong with the voltage divider method, but you do have to be thorough with your algebra (opens up areas for simple mistakes). Also, it can be a somewhat lengthy process (you do end up getting pretty quick at it though).

I think the other method your teacher is talking about is to use ratios. Someone else will be able to explain this a lot better, I can't remember how to do it, but it was pretty much the same thing, except you do more of it in your head.

It was something like, , therefore ratio of the resistances is equal to the ratios of the voltage divided between them. So in that first row, you have 20V, two resistors. So in this case, you have of the input voltage divided between them, so 10 volts to each.

- so the resistances of the two are the same. Therefore resistor 2 is 1000 ohms.

You can check this method with one of the other rows. In row 2, you have a Vout/Vin ratio of . So 1 resistor gets 1/4 of the voltage, while the other gets 3/4 of the voltage. So . So R1 can be either . You can check which one would be correct by subbing into the the voltage divider equation and seeing which one gives you 1/4 (3000 in this case).

I don't remember having to do that much work for the ratio method though... what I just explained was just the voltage divider equation, but phrased differently. Yeah, I don't know what I'm going on about, I'll stop rambling now. Someone else should probably explain it.

[Hutchoo]

OOOOH. Yeah, ratios! Of course. That's genius

Ty man.

[Hutchoo]

Hey guys. How would you work this out?


-- see attached img.