x Turn on thread page Beta
 You are Here: Home >< Physics

# The transport equation (AS physics)(Edexcel) watch

1. Hello there,
I came across this concept while studying Unit 2: DC Electricity. Its about the transport equation and obtaining an expression for the drift velocity of the charges. I just don't understand what its all about and especially how the expression is obtained.
Any help would be appreciated... Thanks
2. Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get responses.

I'm going to quote in Tank Girl now so she can move your thread to the right place if it's needed.

Spoiler:
Show
(Original post by Tank Girl)
x
3. (Original post by Hamoody96)
Hello there,
I came across this concept while studying Unit 2: DC Electricity. Its about the transport equation and obtaining an expression for the drift velocity of the charges. I just don't understand what its all about and especially how the expression is obtained.
Any help would be appreciated... Thanks
Do you mean I=nvqA therefore v=I/nqA ?

current I is the rate at which charge is passing through your sample. you have a current when charge carriers have a velocity... which is to say they're carrying their charge from one place to another over time.

now think about how the current would be affected by changing one of those parameters at a time...

if you doubled q, the charge per charge carrier, and kept everything else the same, how would that affect the current (which is the rate at which charge is passing) ?

same for n, changing the number of charge carriers per unit volume, keeping everything else constant.

same for A, cross sectional area of your sample

same for v, drift velocity of the charge carriers.
4. (Original post by Joinedup)
Do you mean I=nvqA therefore v=I/nqA ?

current I is the rate at which charge is passing through your sample. you have a current when charge carriers have a velocity... which is to say they're carrying their charge from one place to another over time.

now think about how the current would be affected by changing one of those parameters at a time...

if you doubled q, the charge per charge carrier, and kept everything else the same, how would that affect the current (which is the rate at which charge is passing) ?

same for n, changing the number of charge carriers per unit volume, keeping everything else constant.

same for A, cross sectional area of your sample

same for v, drift velocity of the charge carriers.
Thats fine, but what I don't understand is how the formula is derived :/
5. (Original post by Hamoody96)
Thats fine, but what I don't understand is how the formula is derived :/
say you worked in a canning factory that made baked beans in tomato sauce with a known, uniform quantity of beans per cubic meter of the mixture.
if you had a pipeline full of the baked beans in tomato sauce mixture, would you be able to come up with a formula for the number of beans per second coming out of the end of the pipe based on...
n: number of baked beans per cubic meter of the bean and sauce mixture
v: velocity of the bean and sauce mixture
A: cross sectional area of the pipeline

you can assume the pipeline is always full of the mixture - you don't need to wait for it to fill up or anything, and that the bean and sauce mixture is incompressible.

multiplying the cross sectional area of the pipe by the velocity perpendicular to the cross section gives you volume of the mixture per unit time.

multiplying the volume per unit time by the number of beans per unit volume of the mix gives you the number of beans per second.

---
in a material the charge carriers all carry the same amount of charge as each other... but that charge per carrier can be different for different materials. to get the amount of charge per second you multiply the number of carriers per second by the amount of charge per carrier... and charge per second is your current.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: October 25, 2015
Today on TSR

### Loughborough better than Cambridge

Loughborough at number one

### Can I date a girl with no boobs?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE