# Quick Revision for Class X Physics SA1

(iv) The cold water from the depth is pumped up and condensed water again changes into

vapour.

1. Though wood is renewable source of energy, but the use of wood as a fuel is not wise decision. Explain.

A.  Wood is obtained by cutting trees.  A newly plant sapling usually takes about 10-15 years to grow and mature into a tree.  Cutting of trees on a large scale cases depletion of forests leading to disturbance of balance in nature. The balance between Oxygen & Carbon dioxide is also disturbed.  Therefore the use of wood as a fuel is not wise decision

1. Explain, why a sheet of glass is used in solar heating devices?

A. Glass has a property that allows the infra red rays of short wavelength from the sun to get in the device but does not allow infra red rays of long wavelength to leave the solar heating device.   Therefore, heat is trapped inside the heating device.  A kind of green house effect is produced, which helps to increase the temperature inside the heating device.

1. Explain, why is a reflector used in a solar cooker?

A.  The reflector is used to increase the area over which the solar energy is collected so that more and more heat rays of the sun enter the solar cooker. It increases the efficiency of solar cooker.

HOTS QUESTIONS (UNSOLVED)

1. What is the use of black painted surface in solar heating devises.
2. Why are bio gas plants considered to be boon to the farmers?  Give reason.
3. Hydroelectricity generated at a dam may be considered another form of solar energy.  Why?
4. How is the slurry left over after the generation of biogas in biogas plant used?
5. Why is charcoal considered to be a better fuel than wood?
6. Why a solar cooker cannot be used for frying or making chapattis?
7. In parabolic reflector type coolers, even temperature up to 1800C- 2000C can be attained.  How?
8. Modern chulahs are more efficient than traditional chulahs.  Why?
9. How is hydro energy converted into electrical energy?
10. Explain, why only a part of the solar energy that strikes the upper regions of atmosphere reaches the surface of the earth?

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