Charge carrier drift velocity of wire

  • #1

Homework Statement


1.JPG


Homework Equations




The Attempt at a Solution



Current is I = nqvA so drift velocity V is: V = I/nqA

Drift velocity for x is: Vx = I/nqA

Drift velocity for y is: Vy = 2I/nqA

So the ratio of Vx : Vy should be 2:1 since Vy is equal to 2 lots of Vx?? (but correct answer is B)
 

Answers and Replies

  • #2
gneill
Mentor
20,925
2,867
So the ratio of Vx : Vy should be 2:1 since Vy is equal to 2 lots of Vx?? (but correct answer is B)
You may want to rethink your logic there.

Edit: My mistake. I presumed that their table presented the ratios as ##V_y:V_x##, but looking again at the text I see that it's the other way around. So the book's suggested answer is incorrect as you found.

Edit2: I retract my retraction! See later postings.
 
Last edited:
  • #3

Homework Statement


View attachment 111644

Homework Equations




The Attempt at a Solution



Current is I = nqvA so drift velocity V is: V = I/nqA

Drift velocity for x is: Vx = I/nqA

Drift velocity for y is: Vy = 2I/nqA

So the ratio of Vx : Vy should be 2:1 since Vy is equal to 2 lots of Vx?? (but correct answer is B)

Nothing seems wrong with your calculation. Answer should be 2:1. But books sometimes give wrong answers.
 
  • #4
You may want to rethink your logic there.

Edit: My mistake. I presumed that their table presented the ratios as ##V_y:V_x##, but looking again at the text I see that it's the other way around. So the book's suggested answer is incorrect as you found.

Nothing seems wrong with your calculation. Answer should be 2:1. But books sometimes give wrong answers.

I've done this question a few times now and ended up with the same answer, but the question is from an official exam paper in the UK so I don't believe there's been a mistake.

FYI, here's what the mark scheme says:

1.JPG


How can it still be B?
 
  • #5
gneill
Mentor
20,925
2,867
I'm retracting my edit from before. Clearly the coffee isn't working this morning :smile:

If the cross sectional area is halved the velocity doubles. So Vy is twice Vx as you originally pointed out:
since Vy is equal to 2 lots of Vx
So ##V_y## is twice the size of ##V_x##. That makes the ratio ##V_x:V_y = 1:2## which is indeed answer B.
 
  • #6
I'm retracting my edit from before. Clearly the coffee isn't working this morning :smile:

If the cross sectional area is halved the velocity doubles. So Vy is twice Vx as you originally pointed out:

So ##V_y## is twice the size of ##V_x##. That the ratio ##V_x:V_y = 1:2## which is indeed answer B.

Indeed. Vy/2=Vx , so Vx: Vy should be 1:2 because then Vy÷2=1.
 
  • #7
I'm retracting my edit from before. Clearly the coffee isn't working this morning :smile:

If the cross sectional area is halved the velocity doubles. So Vy is twice Vx as you originally pointed out:

So ##V_y## is twice the size of ##V_x##. That makes the ratio ##V_x:V_y = 1:2## which is indeed answer B.

Indeed. Vy/2=Vx , so Vx: Vy should be 1:2 because then Vy÷2=1.


Vx = I/nqA

and

Vy = 2[I/nqA] = 2Vx

so doesn't that mean that for every 1 part of Vx we have half a part of Vy? So that 2 parts of Vx gives 1 part of Vy so the ratio is still C, 2:1?? Argh!
 
  • #8
gneill
Mentor
20,925
2,867
Pick a value for the magnitude of Vx, say 1. What value would you assign to Vy using your expressions?
 
  • #9
Vx = I/nqA

and

Vy = 2[I/nqA] = 2Vx

so doesn't that mean that for every 1 part of Vx we have half a part of Vy? So that 2 parts of Vx gives 1 part of Vy so the ratio is still C, 2:1?? Argh!

2Vx=Vy. So , if the value of Vx is 1, value of Vy should be 2 right?
 
  • #10
gneill
Mentor
20,925
2,867
2Vx=Vy. So , if the value of Vx is 1, value of Vy should be 2 right?
Right. So, Vx = 1, Vy = 2. What's Vx:Vy?
 
  • #11
Right. So, Vx = 1, Vy = 2. What's Vx:Vy?

Then Vx:Vy should be 1:2
 

Related Threads on Charge carrier drift velocity of wire

  • Last Post
Replies
3
Views
3K
Replies
19
Views
3K
Replies
2
Views
5K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
1
Views
626
  • Last Post
Replies
6
Views
2K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
4
Views
14K
  • Last Post
Replies
2
Views
3K
N
Top