Q23 SECTIO of 26 Page 1

Sunita and her friends visited an exhibition. The policeman asked them to pass through a metal detector. Sunita’s friends were initially scared of it. Sunita, however, explained to them the purpose and working of the metal detector. Answer the following questions:

(a) On what principle does a metal detector work?


(b) Why does the detector emit sound when a person carrying any metallic object walks through it?


(c) State any two qualities which Sunita displayed while explaining the purpose of walking through the detector.


(a) A metal detector works on the principle of resonance in an AC circuit.


(b)A metal detector has an AC circuit tuned to resonance condition. When a person carrying any metal walks through the gate of a metal detector, the impedance of the circuit changes. This results in a significant change in current which is immediately detected and the alarming sound is emitted.


(c) Sunita has a scientific temperament. She is knowledgeable and a responsible citizen.


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21

Define the terms ‘stopping potential’ and ‘threshold frequency’ in relation to photoelectric effect. How does one determine these physical quantities using Einstein’s equation?

 22

Three circuits, each consisting of a switch ‘S’ and two capacitors, are initially charged, as shown in the figure. After the switch has been closed, in which circuit will the charge on the left-hand capacitor (i) increase, (ii) decrease and (iii) remain same? Give reasons.

1.PNG


 24

(a) A point-object is placed on the principal axis of a convex spherical surface of radius of curvature R, which separates the two media of refractive indices n1 and n2 (n2 > n1). Draw the ray diagram and deduce the relation between the distance of the object (u), distance of the image (v) and the radius of curvature (R) for refraction to take place at the convex spherical surface from rarer to denser medium.

(b) Use the above relation to obtain the condition on the position of the object and the radius of curvature in terms of n1 and n2 when the real image is formed.


OR


(a) Draw a labelled ray diagram showing the formation of image by a compound microscope in normal adjustment. Derive the expression for its magnifying power.


(b) How does the resolving power of a microscope change when


(i) the diameter of the objective lens is decreased,


(ii) the wavelength of the incident light is increased? Justify your answer in each case.


 25

(a) State Faraday’s law of electromagnetic induction.

(b) Explain, with the help of a suitable example, how we can show that Lenz’s law is a consequence of the principle of conservation of energy.


(c) Use the expression for Lorentz force acting on the charge carriers of a conductor to obtain the expression for the induced emf across the conductor of length l moving with velocity v through a magnetic field B acting perpendicular to its length.


OR


(a) Using phasor diagram, derive the expression for the current flowing in an ideal inductor connected to an A.C. source of voltage, v = Vo sin ωt. Hence plot graphs showing variation of (i) applied voltage and (ii) the current as a function of ωt.


(b) Derive an expression for the average power dissipated in a series LCR circuit.