CBSE Class 12 · 2013 · Delhi · Current ElectricityShort Answer

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Explain the term 'drift velocity' of electrons in a conductor. Hence obtain the expression for the current through a conductor in terms of the drift velocity. \\ \textbf{OR} \\ Describe briefly, with the help of a circuit diagram, how a potentiometer is used to determine the internal resistance of a cell.

Solution

\textbf{Drift velocity:} the average velocity with which free electrons drift opposite to the applied field. In time

t = ℓ / v_{d}

the charge crossing a section is

Q = (n A ℓ) e

, so

I = \frac{Q}{t} = \frac{n A ℓ e}{ℓ / v _{d}} = n e A v_{d} .

\textbf{OR (internal resistance by potentiometer):} with key

K_{2}

open, balance the cell's EMF:

E = k ℓ_{1}

. Then close

K_{2}

to connect a known

R

across the cell and balance its terminal voltage:

V = k ℓ_{2}

. Since

\frac{E}{V} = \frac{ℓ _{1}}{ℓ _{2}}

and

r = (\frac{E}{V} - 1) R

r = (\frac{ℓ _{1}}{ℓ _{2}} - 1) R .

CBSE Class 12 Board · 2013 · Delhi · Subjective

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CBSE Class 12 · 2013 · Delhi · Current ElectricityShort Answer

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