NCERT Science Major Test Model Paper Class 7 2020 Questions and Answers Part 2

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10. Explain the construction and working of electric bell with help of labelled diagram.

Ans:

  • Electric bell: In an electric bell, a small hammer is attached to the iron strip called armature. The armature is vibrated back and forth several times a second, striking a metal bell (or gong).

  • Fig. shows the circuit that causes the armature to move. When a button is pushed, the switch is closed. An electric current flow through the contacts and the spring attached to the coils, and the iron core becomes magnetized.

  • The core attracts the iron armature, which moves toward the electromagnet, causing the hammer to strike the bell.

  • As the hammer strikes the bell, the movement of the armature opens the contacts. The electric current stops flowing to the coils and the soft iron core becomes demagnetized, releasing the armature back to its original position.

  • A spring pulls the armature back to re-establish contact, thereby completing the circuit, and the entire cycle begins again.

Electric Bell

Electric Bell

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11. Draw a ray diagram for following cases:

(i) When object placed between pole and focus in case of concave mirror.

(ii) When object placed at infinity in case of convex lens.

(iii) Formation of image of point objects by plane mirror.

OR

(i) Write any four uses of curved lenses.

(ii) Explain dispersion of white light.

Ans:

(i)

case of concave mirror - 1

Case of Concave Mirror - 1

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(ii)

case of concave mirror - 2

Case of Concave Mirror - 2

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(iii)

case of concave mirror - 3

Case of Concave Mirror - 3

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OR

  • Uses of curved lenses (any four): Lenses are used in spectacles to correct short-sightedness and long sightedness, optical instruments like cameras, binoculars, microscopes, telescopes etc.

  • The phenomenon of splitting up of white light into its constituent Colours is called .dispersion of light...

case of concave mirror

Case of Concave Mirror

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  • When light is made to pass through prism, the lights of different Colours travel with different speeds.

  • The red light travels the fastest, thus bending the least, while violet light is the slowest, bending the most.