Q26 of 26 Page 1

(i) In the circuit diagram given below, AB is a uniform wire of resistance 15 Ω and length 1 m. It is connected to a cell E1 of emf 2V and negligible internal resistance and a resistance R. The balance point with another cell E2 of emf 75 mV is found at 30 cm from end A. Calculate the value of R.


(ii) Why is potentiometer preferred over a voltmeter for comparison of emf. of cells?


(iii) Draw a circuit diagram to determine internal resistance of a cell in the laboratory.


OR


(i) Plot a graph showing variation of voltage vs the current drawn from the cell.


How can one get information from this plot about the emf of the cell and its internal resistance?


(ii) Two cells of emf’s E1 and E2 and internal resistance r1 and r2 are connected in parallel. Obtain the expression for the emf and internal resistance of a single equivalent cell that can replace this combination?


Given:


Resistance of the wire, r=15 Ω


Length of potentiometer wire, l = 1 m


EMF of cell 1, E1 = 2 V


EMF of cell 2, E2 = 75 mV


Balancing point, L=30 cm=0.3 m


Calculation EMF per unit length is given by,



Current is given by,



Potential is given by,



EMF per unit length of the wire is given by,



(ii) The reasons for selecting potentiometer over voltmeter for comparison are as follows:


• The potentiometer is more sensitive in comparison with the voltmeter.


• The voltmeter measures the terminal voltage whereas the potentiometer measures the actual emf.


• The potentiometer is used to measures the internal resistance of the cell but the voltmeter cannot measure the internal resistance.


(iii)



OR


(i) The graph showing variation of voltage vs the current drawn from the cell is given by.



The potential of the cell is given by,



Where,


E=EMF of the cell


I = Current


r = internal resistance


(ii) When two cells are connected in parallel combination, Applying Kirchhoff’s law


I =I1 +I2


The equivalent emf of two cells connected in parallel is given by,



The equivalent internal resistance,



More from this chapter

All 26 →
22

(i) Define self-inductance. Write its SI units.

(ii) A long solenoid with 15 turns per cm has a small loop of area 2.0 cm2 placed inside the solenoid normal to its axis. If the current carried by the solenoid changes steadily from 2.0 A to 4.0 A in 0.1 s, what is the induced emf in the loop while the current is changing?


 23

Gautam went for a vacation to the village where his grandmother lived. His grandmother took him to watch ‘nautanki’ one evening. They noticed a black box connected to the mike lying nearby. Gautam’s grandmother did not know what that box was. When she asked this question to Gautam, he explained to her that it was an amplifier.

(i) Which values were displayed by the grandmother? How can inculcation of these values in students be promoted?


(ii) What is the function of an amplifier?


(iii) Which basic electronic device is used in the amplifier?


 24

(i) State the essential conditions for diffraction of light.

(ii) Explain diffraction of light due to a narrow single slit and the formation of pattern of fringes on the screen.


(iii) Find the relation for width of central maximum in terms of wavelength ‘l’, width of slit ‘a’, and separation between slit and screen ‘D’.


(iv) If the width of the slit is made double the original width, how does it affect the size and intensity of the central band?


OR


(i) Draw a labelled schematic ray diagram of astronomical telescope in normal adjustment.


(ii) Which two aberrations do objectives of refracting telescope suffer from? How are these overcome in reflecting telescope?


(iii) How does the resolving power of a telescope change on increasing the aperture of the objective lens? Justify your answer.


 25

A 2 μF capacitor, 100 W resistor and 8 H inductor is connected in series with an AC source.

(i) What should be the frequency of the source such that current drawn in the circuit is maximum? What is this frequency called?


(ii) If the peak value of e.m.f. of the source is 200 V, find the maximum current.


(iii) Draw a graph showing variation of amplitude of circuit current with changing frequency of applied voltage in a series LRC circuit for two different values of resistance R1 and R2 (R1 > R2).


(iv) Define the term ‘Sharpness of Resonance’. Under what condition, does a circuit. become more selective?


OR


(i) With the help of a neat and labelled diagram, explain the principle and working of a moving coil galvanometer.


(ii) What is the function of uniform radial field and how is it produced?


(iii) Define current sensitivity of a galvanometer. How is current sensitivity increased?