Figure below shows a thick copper rod  and a thin copper wire  joined in series. They carry a current which is sufficient to make  much hotter than

Which one of the following is correct?

Number density of         Mean time between

Conduction                       collisions of the

   electrons                         electrons

a)	Same in              less in  than in
b)	Same in              same in   and
c)	Same in              more in  than in
d)	more in              less in  than in

(c)

The number density (  of conduction electrons in the copper is a characteristic of the copper and is about  at room temperature for both the copper rod  and the thin copper wire .

Both  carry the same current  since they are joined in series

From

Where  is the electron charge of C,

 is the drift velocity in the conductor and  is the cross-sectional area of the conductor.

We may conclude that rod  has a lower drift velocity of electrons compared to wire  since rod  has a larger cross-sectional area. This is so because the electrons in  collide more often with one another and with the copper ions when drifting towards the positive end. Thus, the mean time between collisions of the electrons is more in  than .