Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    2
    ReputationRep:
    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.
    Offline

    12
    ReputationRep:
    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...:rolleyes:

    I hope that helps...
    • Thread Starter
    Offline

    2
    ReputationRep:
    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?
    Offline

    12
    ReputationRep:
    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. :p:
    • Thread Starter
    Offline

    2
    ReputationRep:
    Ah OK thanks!
    Offline

    12
    ReputationRep:
    You're welcome!
    • Thread Starter
    Offline

    2
    ReputationRep:
    That orange book is useless :mad:
    Offline

    12
    ReputationRep:
    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? :rolleyes:
    • Thread Starter
    Offline

    2
    ReputationRep:
    Ditto! & to think its written by 'senior, experienced' examiners :mad:
    Offline

    12
    ReputationRep:
    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...
    • Thread Starter
    Offline

    2
    ReputationRep:
    REally? :eek:
    Offline

    12
    ReputationRep:
    My rather famous math tutor is thinking of making his own book. He's a bloody genius, MashAllah but a rip off too.
    • Thread Starter
    Offline

    2
    ReputationRep:
    hehe
    Offline

    8
    ReputationRep:
    (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.
    Offline

    0
    ReputationRep:
    I this Can be related to fluid dynamics. By considering flowing e- as fluids
    then applying equation of continuity.

    Av=constant
    • Thread Starter
    Offline

    2
    ReputationRep:
    (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
    • Thread Starter
    Offline

    2
    ReputationRep:
    (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?
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: April 11, 2006
Poll
“Yanny” or “Laurel”

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

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.