AAMC 4 Circuit problem

[folktale]

The passage indicates a cathode and an anode separated by distance L and there's a resistor R that is in series with the circuit. The question is:

Which of the following changes to the circuit will decrease the electric field between the electrodes by the greatest amount?

A. Increasing L by a factor of 2
B. Decreasing L by a factor of 2
C. Increasing R by a factor of 2
D. Decreasing R by a factor of 2

Answer: A

I picked C. But the explanation is that they used the equation:
E = (V-IR)/L

I have no idea how that equation is derived. I know V=IR and E = kq/r^2 = F/q but that equation just made me confused.

---

Members don't see this ad.

 

[misseskwee]

So in the passage they mention: The electric field at all points between the electrodes is equal to the electrode voltage difference divided by L.

So I simply took that as the electric field E is inversely proportional to L.
And from there, you'll immediately see that the greatest decrease in E is caused by choice A. You don't necessarily need to know the formula they show you in the solution, although I'm also curious as to where it is derived from.

 

[folktale]

Upon further review, I noticed that the equation is described word by word in the passage, but the actual equation is not given. That's sneaky.

 

[folktale]

So in the passage they mention: The electric field at all points between the electrodes is equal to the electrode voltage difference divided by L.

So I simply took that as the electric field E is inversely proportional to L.
And from there, you'll immediately see that the greatest decrease in E is caused by choice A. You don't necessarily need to know the formula they show you in the solution, although I'm also curious as to where it is derived from.

Yep I noticed this just now too. I was trying to rely on background knowledge instead xP. Lesson learned I guess.

 

[HollowSuperet36]

Yeah this is just V=E*d.

Just play around with the units and the relationships start to make sense.

Any questions on Physics, Chemistry, or Orgo check this guy out.

He's got me 40's on the practice AAMC's. Hopefully do the same on Thursday.

http://www.youtube.com/watch?v=lB3Wgxe4A6Q&feature=relmfu

 

[Class1P]

I may have over-thought this one.

If V/d=E I would erraneously assume that IR/d=E as well. Then I'd get confused between increaing R and decreasing d. I know that it is wrong but I am not sure why.

 

[pfaction]

The resistor would have no thing in here. Electric field doesn't have to do with resistance, unless I remember wrong. So I just remember it as V=Ed, so if we decrease increase D at a constant V, E must decrease.

 

[Buttafuoco]

All you really need to think about is E=kQ/r^2.

Increase r, decrease E.

 

[SaintJude]

All you really need to think about is E=kQ/r^2.

Increase r, decrease E.

No, that equation is only valid for an electric field due to a point charge.

The equation to use here is the definition of an electric field, E = Voltage/ distance b/w plates. The equation that AAMC cites E = (V-IR)/ L is just a modified a version to account for internal resistance.

HollowSuperet, thanks for the link..I will try to watch one video every day until my test date. Wait, there's so many...!

 

[Buttafuoco]

No, that equation is only valid for an electric field due to a point charge.

The equation to use here is the definition of an electric field, E = Voltage/ distance b/w plates. The equation that AAMC cites E = (V-IR)/ L is just a modified a version to account for internal resistance.

HollowSuperet, thanks for the link..I will try to watch one video every day until my test date. Wait, there's so many...!

Oh shoot, I didn't even see the resistor thing. I have been loopy for the past couple days.

 

[GoldenSage]

http://physics.stackexchange.com/qu...ence-and-is-confusing-me-what-i/303603#303603