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AS Physics Question
A copper wire joins a car battery to one of the tail lamps and carries a current of 1.8A.
The wire has a cross-sectional area of 1.0mm2 and is 6.0m long. Calculate how long it takes an electron to travel along this length of wire (use data).
---
Then the follow up question:
The answer in the previous question cannot seem right, it can't take that long? What do you have to say to this?
----
Any help is appreciated.
The wire has a cross-sectional area of 1.0mm2 and is 6.0m long. Calculate how long it takes an electron to travel along this length of wire (use data).
---
Then the follow up question:
The answer in the previous question cannot seem right, it can't take that long? What do you have to say to this?
----
Any help is appreciated.
Reply 1
Do you know the formula for the drift velocity of an electron in a wire? This should be in your class notes or book.
It is usually stated in terms of the current, I, in the wire, its cross section area A, the charge, e, on the electron, and the number, n, of "free" electrons in the wire.
You need a value of n to get an answer here. The usual assumption is that each copper atom in the wire provides one free electron. So you need to know, or calculate, this number first, then use it in the formula.
As for the second part, let's get an answer to the first part first!
It is usually stated in terms of the current, I, in the wire, its cross section area A, the charge, e, on the electron, and the number, n, of "free" electrons in the wire.
You need a value of n to get an answer here. The usual assumption is that each copper atom in the wire provides one free electron. So you need to know, or calculate, this number first, then use it in the formula.
As for the second part, let's get an answer to the first part first!
Stonebridge
Do you know the formula for the drift velocity of an electron in a wire? This should be in your class notes or book.
It is usually stated in terms of the current, I, in the wire, its cross section area A, the charge, e, on the electron, and the number, n, of "free" electrons in the wire.
You need a value of n to get an answer here. The usual assumption is that each copper atom in the wire provides one free electron. So you need to know, or calculate, this number first, then use it in the formula.
As for the second part, let's get an answer to the first part first!
It is usually stated in terms of the current, I, in the wire, its cross section area A, the charge, e, on the electron, and the number, n, of "free" electrons in the wire.
You need a value of n to get an answer here. The usual assumption is that each copper atom in the wire provides one free electron. So you need to know, or calculate, this number first, then use it in the formula.
As for the second part, let's get an answer to the first part first!
The equation we were given is ' I = AqnV '
So N would equal = AqV/I
Q is a constant we were told is 1x10^-19
We were told that the number of free electons in copper were 1.18 X 10^29
But I have no idea what V is.
(edited 14 years ago)
Reply 3
bob247
The equation we were given is ' I = AqnV '
So N would equal = AqV/I
Q is a constant we were told is 1x10^-19
We were told that the number of free electons in copper were 1.18 X 10^29
But I have no idea what V is.
So N would equal = AqV/I
Q is a constant we were told is 1x10^-19
We were told that the number of free electons in copper were 1.18 X 10^29
But I have no idea what V is.
V is the drift velocity. It's what you're trying to find, and then use the speed and the length to find the time.
Reply 5
n, in the formula, is the number of free electrons per meter cubed. V is the drift velocity of the electrons.
Is that the value of n you have been given?
Just rearrange the formula for V and substitute the values in. Make sure the units are correct. (The cross section area will need to be changed to m²
You have all the values.
When you have the drift velocity of the electrons it's a simple mattter to work out how long it will take them to travel the 6m length of wire.
Is that the value of n you have been given?
Just rearrange the formula for V and substitute the values in. Make sure the units are correct. (The cross section area will need to be changed to m²
You have all the values.
When you have the drift velocity of the electrons it's a simple mattter to work out how long it will take them to travel the 6m length of wire.
(edited 14 years ago)
Stonebridge
n, in the formula, is the number of free electrons per meter cubed. V is the drift velocity of the electrons.
Is that the value of n you have been given?
Just rearrange the formula for V and substitute the values in. Make sure the units are correct. (The cross section area will need to be changed to m²
You have all the values.
When you have the drift velocity of the electrons it's a simple mattter to work out how long it will take them to travel the 6m length of wire.
Is that the value of n you have been given?
Just rearrange the formula for V and substitute the values in. Make sure the units are correct. (The cross section area will need to be changed to m²
You have all the values.
When you have the drift velocity of the electrons it's a simple mattter to work out how long it will take them to travel the 6m length of wire.
It wasn't given to us in the questions, but my teacher told us that the number of free electrons in copper = 1.18 x 10 ^29
V = 1000 (A) x 1.18x10^29 (N) x 1.16x10^-19 (Q) / 1.8 (I)
= 7.604444444 x10^12
Now I don't get what I have to do with that? Divide it by 6.0 m?
The answer I'm supposed to get is 15 hours.
Reply 7
First, you have not rearranged the equation correctly.
If I=AqnV, then
V=I/(nqA)
I=1.8 ok
q=1.6 x10^-19
n= 1.18x10^29 per cubic meter ok
A= 1mm² which is 10^-6 m²
You should get a drift velocity, if you sub in to the formula.
Then if velocity is distance/time
time=distance / velocity
This will give the time taken to travel 6m
If I=AqnV, then
V=I/(nqA)
I=1.8 ok
q=1.6 x10^-19
n= 1.18x10^29 per cubic meter ok
A= 1mm² which is 10^-6 m²
You should get a drift velocity, if you sub in to the formula.
Then if velocity is distance/time
time=distance / velocity
This will give the time taken to travel 6m
Stonebridge
First, you have not rearranged the equation correctly.
If I=AqnV, then
V=I/(nqA)
I=1.8 ok
q=1.6 x10^-19
n= 1.18x10^29 per cubic meter ok
A= 1mm² which is 10^-6 m²
You should get a drift velocity, if you sub in to the formula.
Then if velocity is distance/time
time=distance / velocity
This will give the time taken to travel 6m
If I=AqnV, then
V=I/(nqA)
I=1.8 ok
q=1.6 x10^-19
n= 1.18x10^29 per cubic meter ok
A= 1mm² which is 10^-6 m²
You should get a drift velocity, if you sub in to the formula.
Then if velocity is distance/time
time=distance / velocity
This will give the time taken to travel 6m
Thank you!!!!
me messing up the equation was probably the biggest mistake.
Reply 9
Umm the final answer is not 15 hours not by a long shot bruh this is electricity ur talking about there is no way electrons will need 15 hours to travel 6 meters
Reply 10
Original post by Wronganswer
Umm the final answer is not 15 hours not by a long shot bruh this is electricity ur talking about there is no way electrons will need 15 hours to travel 6 meters
Drift velocity is actually fairly slow for an electron, and differs from what might be judged as the "velocity" of the signal through the wire or whatever this question is about.
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