Q23 of 26 Page 1

Ram is a student of class X in a village school. His uncle gifted him a bicycle with a dynamo fitted in it. He was very excited to get it. While cycling during night, he could light the bulb and see the objects on the road. He, however, did not know how this device works. He asked this question to his teacher. The teacher considered it an opportunity to explain the working to the whole class. Answer the following questions:

(a) State the principle and working of a dynamo.


(b) Write two values each displayed by Ram and his school teacher.

(a) Principle of dynamo:


A dynamo is an electrical generator that produces direct current on the principle of Faraday’s law of induction.


Working principle:


The dynamo uses rotating coils of wire attached to a turbine fitted with a plastic cap. It uses magnetic fields to convert the mechanical rotation to current and When kept in a magnetic field there is a change in the flux through the coil continuously . This induces a pulsating direct electric current in the coil. If the coil is rotated as a steady rate, we get a steady current.


b)


The values shown by Ram:


(i) He was not reluctant to ask the question and quench his curiosity.


(ii) He was indeed curious to know the scientific principle behind the working of a dynamo.


The values shown by the school teacher:


(i) He had a reasonable command over his subject and


(ii) He readily help ram understand the working and principle of dynamo, thus he is a understanding teacher.


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21

Define the term wave front. State Huygens’s principle.

Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focuses on the focal point of the lens, giving the shape of the emergent wave front.


OR


Explain the following, giving reasons:


(i) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.


(ii) When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave?


(iii) In the wave picture of light, intensity of light is determined by the square of the amplitude of the wave. What determines the intensity in the photon picture of light?

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Use Biot-Savart law to derive the expression for the magnetic field on the axis of a current carrying circular loop of radius R.

Draw the magnetic field lines due to a circular wire carrying current I.

 24

(i) Draw a labelled diagram of a step-down transformer. State the principle of its working. (ii) Express the turn ratio in terms of voltages. (iii) Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.

(iv) How much current is drawn by the primary of a transformer connected to 220 V supply when it delivers power to a 110 V— 550 W refrigerator?


OR


(a) Explain the meaning of the term mutual inductance. Consider two concentric circular coils, one of radius r1 and the other of radius r2 (r1 < r2) placed coaxially with centres coinciding with each other. Obtain the expression for the mutual inductance of the arrangement.


(b) A rectangular coil of area A, having number of turns N is rotated at ' f ' revolutions per second in a uniform magnetic field B, the field being perpendicular to the coil. Prove that the maximum emf induced in the coil is 2 rf NBA.

 25

(i) Derive the mathematical relation between refractive indices n1 and n2 of two radii and radius of curvature R for refraction at a convex spherical surface. Consider the object to be a point since lying on the principle axis in rarer medium of refractive index n1 and a real image formed in the denser medium of refractive index n2. Hence, derive lens maker's formula.

(ii) Light from a point source in air falls on a convex spherical glass surface of refractive index 1.5 and radius of curvature 20 cm. The distance of light source from the glass surface is 100 cm. At what position is the image formed?


OR


(a) Draw a labeled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position. Define its magnifying power.


(b) You are given three lenses of power 0.5 D, 4 D and 10 D to design a telescope.


(i) Which lenses should he used as objective and eyepiece? Justify your answer.


(ii) Why is the aperture of the objective preferred to be large?