current chapter 3 class 12 objective dpp2

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1.    A wire of length 6 m and area of cross section 1 m m ² carries a current of 2 A. if unit cubic meter of the material of a wire contains 10²⁹        free electrons ,find the average time taken by an electron to cross the length of wire .

16 hr            b. 13.33 hr           c. 14.44 hr                      d. 17 hr

2.    The number of density of electrons in coppers is 8.5 x 10 ²⁸ m^-3. What is the time taken by an electron to drift from one end of the wire 2.2 m long to its other end if area of cross section of the wire is 2.0 x 10^-6 m ² and it is carrying a current of 6.0 A.

a.     10⁴ s               b. 10 s                 c. 100 s                             d.400 s

3.    A wire of length 2.0 m ,diameter 1.0 mm has 50 mΩ resistance . calculate resistivity of the material of the wire ?

a.     1.963 x 10^-8Ω m.    b. 1.2 x 10^-8Ω m     c. 3 x 10^-8Ω m   d. 6 x 10^-8Ω m

4.    A wire of resistance 1000Ω and length l is increased to twice its original length .calculate its new resistance ?

a.     3000Ω              b.  4000Ω            c. 5000Ω                       d. none of these

5.    A cylindrical metallic wire is stretched to increase its length by 10%. Calculate the percentage change in its resistance .

a.     20%                 b. 30%                   c. 45%                           d. 21%

6.  

1.    (a) Estimate the average drift speed of conduct in electrons in a copper wire of cross sectional area 1.0×10^-7m^-2 carrying a current of 1.5 A. Assume that each copper atom contributes roughly one conduction electron. The density of copper is                9  .0 × 10^-3 kg m^-3  and its atomic mass is           6 3 .5 u.${\displaystyle \mathrm{ <span\; style="font-family:\; "times\; new\; roman",\; serif;\; font-size:\; 14pt;\; line-height:\; 115\%;"><o:p></o:p></span>}}$

(b) Compare the drift speed obtained with

(i) thermal speed of electron carrying the current and

(ii)the speed of propagation of electric field along the conductor, which causes the drift motion.                                     Ans . a. 1.1 x 10^-3m/s b.  10^-8, 3.6 x 10^-12

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7.    The area of cross section , length and density of a piece of metal of atomic weight 60 are 10^-6 m ², 1 m and 5 x 10³ kg m^-3 respectively . find the number of free electrons per unit volume if every atom contributes one free electron. Also find the drift velocity of electrons in metal the metal when a current of 16 A passes through it . given Avogadro number 6 x 10²³ mol^-1.

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Ans 5 x 10²⁸, 2 x 10^-3 m/s