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# Drift velocity watch

1. Hello,

I have a question; that when you decrease the cross sectional area of a wire, does the 'drift velocity' of electrons in a wire increase or decrease? From the quation I=nAqv, if we keep the current & n constant, we see that the velocity decreases as you increase the cross sectional area & vice versa Why? I thought that as you decrease the cross sectional area, the collisions between the free electrons & the ions in the conducting wire increase, therefore impeding the 'smooth' movement of electrons i.e decreasing their velocity & hence leading to an overall fall in the average drift velocity. This is not the case, however. What am I missing?

Help appreciated.
2. Oh how I hated this topic...

When an external potential is applied across the conductor, it creates an electric field inside which produces a force on the electron. This is what causes the electrons to move.
As the cross sectional area of a wire decreases, the drift velocity must increase to carry the same value of current. This is due to the electrical field lines being compressed into a smaller area, thereby increasing the strength of the electric field.
When the cross sectional area is increased, the opposite takes place. In order to keep the rate of charge flowing at any point constant, the drift velocity decreases. The Electric field lines occupy a larger area, decreasing the strength of the field, and thus decreasing the force applied on the electrons.

I only understood it fully after doing unit 5...

I hope that helps...
3. Thanks Muhiba I understood the bit about the electrical fields being created, but why does the strength of an electric field increases when it gets compressed i.e occupy a smaller area?
4. I think that when they are closer to each other, they sort of reinforce each other.
Think of it this way, each electric field line produces a field strength. So when any two are brought close to each other, their electrical fields overlap, increasing the overall strength of the two.

United we stand, divided we fall…sort of thing.
5. Ah OK thanks!
6. You're welcome!
7. That orange book is useless
8. Yes it is. All it does is make statements, without actually explaining why. I can't stand books or teachers like that. I want to know WHY. How else am I supposed to remember a bunch of what to me would be random statements?
9. Ditto! & to think its written by 'senior, experienced' examiners
10. Lol...you think that's messed up, you should see the C1 math book. More than 6 authors and yet one of the most inaccurate books ever written.
More than 50% of the answers are wrong, including alot of the answered examples...
Edexcel sucks...
11. REally?
12. My rather famous math tutor is thinking of making his own book. He's a bloody genius, MashAllah but a rip off too.
13. hehe
14. (Original post by Neo~)
Hello,

I have a question; that when you decrease the cross sectional area of a wire, does the 'drift velocity' of electrons in a wire increase or decrease? From the quation I=nAqv, if we keep the current & n constant, we see that the velocity decreases as you increase the cross sectional area & vice versa Why? I thought that as you decrease the cross sectional area, the collisions between the free electrons & the ions in the conducting wire increase, therefore impeding the 'smooth' movement of electrons i.e decreasing their velocity & hence leading to an overall fall in the average drift velocity. This is not the case, however. What am I missing?

Help appreciated.
Sigh...
You have forgotten one thing. Think about resistivity. As you half the cross sectional area you double the resistance. If the current remains the same, then remember V = IR, hence voltage doubles, and it is voltage that accelerates the electrons through the wire.
15. I this Can be related to fluid dynamics. By considering flowing e- as fluids
then applying equation of continuity.

Av=constant
16. (Original post by Mehh)
Sigh...
You have forgotten one thing. Think about resistivity. As you half the cross sectional area you double the resistance. If the current remains the same, then remember V = IR, hence voltage doubles, and it is voltage that accelerates the electrons through the wire.
um OK...thanks
17. (Original post by driving_seat)
I this Can be related to fluid dynamics. By considering flowing e- as fluids
then applying equation of continuity.

Av=constant
What?

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