Tag: electric current, potential difference and resistance

Quizzes (6) Questions (109)

Questions Related to electric current, potential difference and resistance

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

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

  1. increase

  2. decrrease

  3. fluctuate

  4. remains constant
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

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

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

  1. increases two times

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

  3. does not change

  4. decreases $\displaystyle \frac{1}{4}$ times
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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

The drift velocity of electrons in a conducting wire is of the order of $1:mm/s$, yet the bulb glows very quickly after the switch is put on because

  1. the random speed of electrons is very high of the order of $10^{-6}m/s$

  2. the electrons transfer their energy very quickly through collision.

  3. electric field is set up in the wire very quickly, producing a current through each cross section, almost intantaneously

  4. All the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Drift velocity $v _d= \frac{eEt}{m}$ so drift velocity is directly proportional  to the electric filed. When switch is on, the filed is quick set up wire  and produce current through wire instantaneously.   

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

A potential difference v exists between the ends of a metal wire of length l. The drift velocity will be doubled if

  1. v is doubled

  2. l is doubled

  3. The diameter of the wire is doubled

  4. The temperature of the wire is doubled
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The drift velocity is $v _d=\frac{eVt}{2ml}$
The relaxation time t changes with change of temperature. Actually it decreases with rise of temperature. Thus at a given temperature drift velocity  is directly proportional to the potential difference, inversely proportional to length and is independent of cross-sectional area.
So when drift velocity is doubled , V becomes double and l is half.

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

Assertion : A current flows in a conductor only when there is an electric field within the conductor.


Reason : The drift velocity of electron in presence of electric field decreases.

  1. Both (A) and (R) are true and (R) is the correct explanation of A

  2. Both (A) and (R) are true but (R) is not the correct explanation of A.

  3. (A) is true but (R) is false

  4. (A) is false but (R) is true
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Current flows when there in P.D.

The presence of P.D implies the presence of Electric field. 
Drift velocity $\propto$ Electric field.

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

 The electron drift speed is small and the charge of the electron is also small but still, we obtain large current in a conductor. This is due to 

  1. the conducting property of the conductor

  2. the resistance of the conductor is small

  3. the electron number density of the conductor is small

  4. the electron number density of the conductor is enormous
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Current in the conductor $I = An e v _d$

where $n$ is the number density of free electrons, $v _d$ is the drift velocity of electrons, $e$ is its charge and $A$ is the cross-section area of conductor.
Conductors have very large number density of free electrons, nearly $10^{28}$ $cm^{-3}$, which conduct electric current in the conductor. Thus inspite of having small drift velocity and electronic charge, we obtain large current in the conductor.

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

Two wires, each of radius r, but of different materials are connected together end to end. If the densities of charge carriers in the two wires are in the ratio 1:4, the ratio of the drift velocity of electrons in the two wires will be . .

  1. 1:2

  2. 2:1

  3. 1:4

  4. 4:1
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

We know that drift velocity is inversely proportional to density of charge carriers.


Hence, ratio of drift velocity is $4:1$.

Answer-(D)

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

The drift velocity of an electron is doubled if the applied electric field across the conductor is .....

  1. three times

  2. doubled

  3. half

  4. four times
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The flow of current current is nothing but drift velocity of electrons. Since current is directly proportional to voltage applied, drift velocity of electrons will also be directly proportional to the voltage applied. And it follows Ohm's law as V = I/R. So, if voltage is doubled, current is doubled and hence the drift velocity is doubled.

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

When there is an electric current through a conducting wire along its length then an electric field must exist

  1. inside the wire but normal to it

  2. inside the wire but parallel to it

  3. outside the wire but normal to it

  4. outside the wire but parallel to it
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

As current is flowing through it , it means charges are flowing along its length therefore heir must be some electric field parallel to the length of the wire. Hence correct option is B.

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

Copper contains $8.4\times 10^{28}$ free electrons$/m^3$. A copper wire of cross-sectional area $7.4\times 10^{-7}m^2$ carries a current of $1$A. The electron drift speed is approximately.

  1. $10^{-8}$m/s

  2. $10^3$m/s

  3. $1$m/s

  4. $10^{-3}$m/s
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$v=\dfrac{I}{ne}=\dfrac{1}{7.4\times 10^{-7}}\times 8.4\times 10^{28}$
$=\dfrac{10^{-2}}{8.4\times 1.6\times 7.4}=10^-3$.