Physics General Knowledge Questions and Answers for all Competitive Examinations.

1. How do we see colours?
Ans. The retina of the eye has a large number of cells which are sensitive to light. These cells are of two shapes — rods and cones. The rod shaped cells are sensitive to the intensity of light, i.e. these cells respond to the degree of brightness or darkness of the light. The cone shaped cells are sensitive to the colour of light. They respond differently to lights of different colours.

2. Why does a dentist use a concave mirror to examine the teeth of a patient?
Ans. A concave mirror converges light on the teeth and the dentist can see the affected tooth clearly.

3. Can a liquid be evaporated and solidified at the same time? If so, how?
Ans. Yes, it is possible to evaporate a liquid and also to solidify it at the same time. If a liquid is allowed to evaporate continuously it becomes colder and colder as evaporation causes cooling. A stage may reach when the temperature of the liquid may fall below its freezing point and it may solidify.

4. Why do all places lying on the same parallel of latitude not have the same temperature?
Ans. The temperature of a place does not depend upon its latitude alone. There are other factors also which affect the temperature of a place. These factors include altitude, distance from the sea, the presence of mountains and the type of wind blowing etc.

5. Is there heat in ice?
Ans. Yes, there is heat in ice. Temperature of ice is 0°C and when it melt,1, it absorbs heat that is known as the latent heat of fusion which is equal to 80 cal/gm. it is due to this heat that ice is not extremely cold.

6. Why are there usually two separate circuits in a house?
Ans. Usually there are two separate circuits in a house for drawing different currents. The two circuits are the lighting circuit with a 5A fuse and the power circuit with a 15A fuse. The lighting circuit is for running electrical bulbs, tube-lights, fans and radio etc. which draw small current. On the other hand, power circuit is for running electric iron, room heater, electric stove, refrigerator, etc. which draws heavy current.

7. Explain why it snows on high hills, while it rains lower down?
Ans. The temperature of the atmosphere at higher altitude is generally below the freezing point of water. Hence, the water vapour present in the air at higher altitudes gets converted into snow which collects on the hills. Lower down, the temperature of the atmosphere is above the freezing point of water. Hence, the water vapour is converted into liquid water which comes down as rain in those regions.

8. A copper wire should never be used as a fuse wire. Why?
Ans. A copper wire should never be used as a fuse wire because a copper wire has a very high melting-point due to which a cooper wire fuse cannot protect the wiring against short - circuiting or overloading.

9. What causes fog?
Ans. The cause of fog and cloud is the same, because fog is cloud at ground level. When damp air near the ground becomes cold enough for the water vapour present in the atmosphere to condense into droplets of water, then fog and mist occur.

10. It is advised that one should not stand near an approaching, fast moving train on the railway platform. Why?
Ans. The atmospheric pressure around the fast moving train becomes quite low. If one stands near that train, one may be pushed towards the train by some external force.

11. How does the reading of a thermometer change?
Ans. The thermometres utilise the expansion property of a material which may be a solid, liquid or a gas. With the rise of temperature the matter inside the thermometer expands and the marking on the glass tube of thermometer where the matter reaches after expansion, gives the reading.

12. Why a diver cannot hear the sound produced on the land, when he is inside water?
Ans. Sound has different wavelengths in the two mediums i.e. air and water. Therefore, the sound produced on the land cannot travel through water. Most of the sound waves get reflected from the surface of the water.

13. Why is a vessel of water heated more quickly when heat is applied at the bottom instead of at the top?
Ans. When the bottom is heated the particles of water become lighter and rise up through conduction while the heavier particles from the surface come down by the sides of the vessel. In this way whole of the water is heated. On the other hand, when it is heated at the top, higher water which is hot remains so and the rest of the water does not heat up as water is bad conductor of heat.

14. Why small particles of camphor dance on the surface of water?
Ans. If a small particle of camphor is floated upon the surface of clean water a part of it is gradually dissolved. The surface tension of solution is less than that of pure water hence camphor particles are pulled towards pure water i.e. towards the region of higher surface tension. Since comphor is continuously dissolving, the force of surface tension is not uniform all around. As a result camphor pieces move in different directions.

15. Why is a tube light considered to be better than a tungsten filament bulb?
Ans. In a tungsten-filament bulb only 2% to 3% of total electric current supplied is converted into light energy and the maximum electricity (98%) gets wasted in the form of heat whereas in a tube light about 9% of the total electric current gets converted into light energy. Therefore, a tube-light is considered better than an electric bulb.

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