Tag: drift velocity and mobility

Questions Related to drift velocity and mobility

Which of the following quantities do not change when an ohmic resistor connected to a battery is heated due to the current?

physics electric current drift velocity and mobility drift speed drift velocity & mobility

  1. drift speed

  2. resistivity

  3. resistance

  4. number of free electrons

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Correct Option: D
Explanation:

We know, for a conductor carrying current,
Drift speed $= neI =ne \dfrac{V}{R}$
Resistivity $= \dfrac {RA}{L}$
where,
$n$ is no. of electrons, 
$e$  is charge on electrons,
$V$  is applied voltage,
$R$  is resistance and 
$A$  and $L$  are area of cross-section and length of resistor.
From above equations it is clear that drift speed, resistivityand resistance of resistor will be affected due to heating of resistor

The drift of the electrons in a copper-wire of length 2 m under the application of potential difference of $ 200 V is 0.5 ms^{-1} $ . their mobility is $ (inm^2V^{-1}s^{-1} ) $

physics electric current drift velocity and mobility drift speed drift velocity & mobility

  1. $ 2.5 \times 10^{-3} $

  2. $ 2.5 \times 10^{-2} $

  3. $ 5 \times 10^{2} $

  4. $ 5 \times 10^{-3} $

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Correct Option: D

In semiconductor the concentrations of electrons and holes are $8 \times 10 ^ { 18 } / \mathrm { m } ^ { 3 }$and $5 \times 10 ^ { 8 } / \mathrm { m } ^ { 3 }$ respectively.If the mobilities of electrons and holes are 2.3$\mathrm { m } ^ { 2 } / \mathrm { Vs }$ and 0.01$\mathrm { m } ^ { 2 } / \mathrm { Vs }$ respectively then semi conductor is

physics electric current drift velocity and mobility drift speed drift velocity & mobility

  1. N-type with resistivity $0.34 \Omega - \mathrm { m }$

  2. P-type with resistivity $0.34 \Omega - \mathrm { m }$

  3. N-type with resistivity $0.034 \Omega - \mathrm { m }$

  4. P-type with resistivity $3.4 \Omega - \mathrm { m }$

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Correct Option: C

The mean free path of electrons in a metal is $44 \times 10 ^ { - 8 } \mathrm { m }$ . Theelectric field which can give on an average 2$e \mathrm { V }$ energy to an electron in the metal will be in units of VIm 

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. $8 \times 10 ^ { 7 }$

  2. $5 \times 10 ^ { - 11 }$

  3. $8 \times 10 ^ { - 11 }$

  4. $5 \times 10 ^ { 7 }$

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Correct Option: D

When 3 V potential difference is applied a wire of length 0.1 m. having resistivity $1.6 \times 10^{-5}$ $\Omega m$, the electrons started moving. If the electron density in the wire is $6 \times 10^{10} m^{-1}$, the drift speed of electrons is  

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. $1.94 \times 10^{-6}\ ms^{-1}$

  2. $1.94 \times 10^{-5}\ ms^{-1}$

  3. $1.94 \times 10^{-8}\ ms^{-1}$

  4. $1.94 \times 10^{-7}\ ms^{-1}$

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Correct Option: C

The drift velocity of free electron in a metal wire of a given potential gradient along it is $ V _d $ if this potential gradient is doubled the new drift velocity will be 

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. $ V _d $

  2. $ 2V _d $

  3. $ \frac {V _d}{2} $

  4. $4V _d $

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Correct Option: B

Unit of mobility :

physics electric current drift velocity and mobility drift speed drift velocity & mobility

  1. $Cskg$

  2. $m^2 V^{-1} s^{-1}$

  3. $Cskg^{-1}$

  4. $m^2 V^{-1} s$

Show answer
Correct Option: B
Explanation:

Mobility    $\mu = \dfrac{v _d}{E}$

Unit of  $v _d $ is  $m/s$  and  that of  $E$ is $V/m$
So, its unit is    $m^2 V^{-1}s^{-1}$

When a potential difference $V $  is applied across a conductor at a temperature $T,$  the drift velocity of electrons is proportional to

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. $\sqrt{V}$

  2. $V$

  3. $\sqrt{T}$

  4. $T$

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Correct Option: B
Explanation:

We know that Drift velocity $v _d = \displaystyle \dfrac{eE}{m} \tau = \dfrac{e}{m} \tau \left ( \dfrac{V}{l} \right ) $ ($\because E = \dfrac{V}{l})$

so for a particular conductor of a particular length the drift velocity will directly depend upon voltage
Hence $v _d \propto V$. option B is correct.

If temperature is decreased, then relaxation time of electrons in metals will

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. increase

  2. decrrease

  3. fluctuate

  4. remains constant

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Correct Option: A
Explanation:

With decrease in temperature, resistance of metal conductor decreases. But  $R \propto  \dfrac{1}{\tau}$, therefore time of relation $\tau$ increases. that's why the correct answer is option A.

A potential difference V is applied across a copper wire of diameter d and length L. when only d is doubled, the drift velocity :-

physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

  1. increases two times

  2. decreases $\displaystyle \frac{1}{2}$ times

  3. does not change

  4. decreases $\displaystyle \frac{1}{4}$ times

Show answer
Correct Option: C
Explanation:

Drift velocity $\displaystyle v _d=\frac{I}{neA}=\frac{V/R}{neA}=\frac{V}{neA(\frac{\rho L}{A})}=\frac{V}{ne\rho L}$
Thus, drift velocity is independent of diameter so it does not change.