Class11 physics Note(Thermodynamics)

THERMODYNAMICS:

 Q.Explain why water remains cool in earthen pot in summer.

Ans: 

In earthen vessel, vessel's water o0zes out through the pores andevaporated. Evaporation of oozed water takes heat from the water in thevessel and hence, cools it. Such type of oozing does not occur in metal vesselHence, the water in the earthen vesscl is cooler than that in a metal vessel.

Q.Why do you get a more severe burn from steam at 100°C than from water at 100°C ?

Ans: 

The latent heat of steam is 540 calorie per gram, which implies that lg ofsteam at 100°C gives out 540 calorie of heat energy while converting into 1of boiling water at 100°CClearly, lg of steam at 100°C contains 540 calories additional energy(heat) than 1g of water at 100°C. Hence, the burns caused by steam are moresevere than those caused by boiling water.

Q.Why does food cook faster in a pressure cooker than in an open pot?

Ans:

Ans: The steam generated inside the pressure cooker increases pressure over watersurface in it. This increased pressure causes water to boil at a temperaturegreater than 100°C. As such the boiling water provides more heat to the food,the food is cooked in much less time.

Q.What will extinguish fire more effectively-hot water or cold water ?

Ans: 

Hot water will extinguish fire more effectively. When the hot water comes incontact with fire, it changes into steam in much less time. For this, it requireslatent heat, which is taken from the burning object and hence, temperature ofbuming object falls.

Q.Why is value of relative humidity more near sea water

Ans:

 Relative humidity can be defined as the ratio of the saturated vapour pressure at dew point to the saturated vapour pressure at room temperature. Saturated vapour pressure at dew point Saturated vapour pressure at room temperatureie. R.H.The amount of water vapour present in the atmosphere near the sea water 1scomparatively more. Due to this, the value of relative humidity is more near sea water

Q.Why are dews formed in the early morning hours ?

Ans:

Dew point is the temperature at which the water vapour actually present in the atmosphere is just sufficient to saturate it. In early morning, the atmospheric temperature goes down and when the temperature is below the dew point, the air becomes saturated and condensation begins forming droplets on grass,window pan etc. which are called dews.

Q.Relative humidity increases at night. Why?

Ans: For the same amount of water vapour present in air, the relative humidity increases at night and decreases in the day. At night, as the temperature decreases and comes closer to the dew point, the water vapour pressure comescloser to the saturation vapour pressure. Hence, relative humidity increases at

night.

Q.Explain why dews are formed in the clear night but not in the cloudy night.

Ans:

In clear nights, the earth's surface is cooled down by radiating heat. As theatmospheric temperature falls below dew point, the excess vapour iscondensed and deposited as dew.But in cloudy night, the heat radiated by the earth can't escape from the cloudand hence, can't be cooled upto the dew point as the cloud acts as a blanket.So, water vapour can't condense to form dew.

Q.Why are good absorbers always good emitters?

Ans:

When a body reaches in thermal equilibrium with its surroundings, the body must emnit radiant energy at the same rate at which it absorbs it. Thus, a bodythat is a good emitter must also be a good absorber and vice-versa.

Q.Good absorbers are good radiators, why? Give reason.

Ans: Good absorbers absorb most of the radiations falling on it. So theirtemperature rises very largely. Since the radiation radiated (emitted) by a body depends on the temperature and the temperature is high for good absorbers,the good absorbers are good radiators.

Q.What is a black body ? How is it realized in practice?

Ans:

perfectly black body is one which absorbscompletely the radiations of all wavelength falling on itand emits radiation of all possible wave lengths whenheated. A perfectly black body can't be realized inpractice. However, the nearest approach to a perfectly black body is a surfaceCoated with lamp black or platinum black. Such a surface absorbs 96% to 98% of the incident radiation