# Homework Help: HELP! drift velocity question

1. Jan 10, 2006

### rakal

In a circuit, if the voltage is doubled, what happens to the drift velocity?

2. Jan 10, 2006

### Tide

It will essentially double. The Coulomb scattering cross section decreases with speed but for typical voltages the change is very slight.

3. Jan 10, 2006

### rakal

thanks. Does the drift speed half if the length of wire doubles?

4. Jan 10, 2006

### lightgrav

If you couldn't tell from the previous response,
You are supposed to at least tell us how you're thinking ...

How would a tiny charge deep inside an opaque wire find out
how long the wire was?

5. Jan 10, 2006

### rakal

OK i just need to know what factors vary drift velocity. I am not very smart, my teacher is rubbish and I have exams in a week so im quite frustrated. So length doesnt change the speed? Voltage increases the drift speed as electrons gain more energy? Increasing cross-sectional area decreases the drift speed as there is a larger area for more collisions therefore slowing down the electrons

Last edited: Jan 10, 2006
6. Jan 10, 2006

### lightgrav

No, the drift velocity is causally determined by LOCAL quantities:
the Electric Field, the charge carrier density, the density, size,
and mass of the NON-mobile atoms (and the Temperature).

Voltage only has an effect on drift velocity if it influences the local E-field.
It's the E-field that accelerates electrons, before they collide and start again.

Cross-sectional Area only affects drift velocity if it decreases R thereby decreasing V (if current is held constant) thereby decreasing the local E-field.

Increasing the length might (if Voltage is held constant) change the local E-field, in what way?

Last edited: Jan 10, 2006
7. Jul 27, 2007

### pbfred

I have a question regarding drift velocity. Paul Drude originally had the drift velocity given by

v =(q*E/m*2)*t (1)

but then the factor of 2 was dropped and now most textbooks write the drift velocity as

v = (q*E/m)*t (2)

My question is was it measurement of the actual current in a wire that led to the dropping of the factor of two ? In other words Drude's formula disagreed with observation so equation (2) was adopted.