Heat - BPSC (Bihar) PDF Download

Heat

• 0^oC = 273.16 K or, 100⁰C = 273.16 + 100 = 373.16 K.
• A theoretical perfect or ideal gas is that which obeys all gas laws.
• In a barometer, if water is used in place of mercury, then the length of the barometric tube has to be increased because water occupies more spaces as compared to mercury.
• In northern Canada and Russia, the choice of liquid to be used in thermometer is alcohol instead of mercury because winter temperatures in those regions are -40oC and mercury freezes at -39^oC whereas the freezing point of alcohol is -115^oC.
• The value of absolute Zero (Zero Kelvin) on the Fahrenheit scale is –273.16°C or, -459.67°F.

Absolute Zero is the lowest temperature theoretically possible.

• A liquid boils when its internal vapour pressure equals the external pressure on its surface.
• A calorie is the unit of heat and not of temperature.
• –40°C is the temperature at which Celsius and Fahrenheit's scales show the same reading.
• Charle’s law deals with the relationship of volume and temperature, whereas Boyle’s law, the relationship of volume and pressure of gases.
• The gas thermometers are more sensitive than liquid thermometers because gases expand more than liquids.
• A gas thermometer is an apparatus used for measuring temperature by the alteration in pressure produced by a change in gases kept at constant volume.
• A bolometer is an extremely sensitive instrument for measuring heat rays or, radiation.
• The energy change at constant pressure and temperature is called the enthalpy change. It is equal to the amount of heat exchanged with the surroundings at constant pressure and temperature.
• In general, substances expand on melting and contract on freezing. The exceptions are ice, cast iron, bismuth, etc. which contract on melting.
• It is not easy to produce a good casting of a substance, which contracts on freezing, and this is the reason that copper and silver coins are stamped and not cast.
• The printer’s type (an alloy of bismuth and lead) and water expand on freezing.
• If two pieces of ice are pressed together for a few seconds, they form one single piece on releasing the pressure. The reason is that the pressure lowers the melting point.
• The same reason applies while skating on ice. The pressure due to the weight of the body melts ice forming a thin film of water that enables the skate to bite.
• The presence of impurities in the solid substance decreases its melting point.

If one part of salt is mixed with three parts of ice, the freezing temperature falls as low as –22oC. Such a mixture is called a freezing mixture. It is very commonly used for making ice-creams or Kulphies.

• In conduction, occurring in solids, transfer of heat is from one molecule to another without themselves moving from their original position.
• Convection is the property of liquids (with the exception of mercury and molten metals) and gases. In convection, the molecule themselves move from one place to another, carrying heat with them
• In Radiation, for the transfer of heat, the medium is not necessary. It occurs even in a vacuum (like solar radiation).
• A fireman’s helmet is kept highly polished to reflect the heat of the fire.
• Pyrex glass is used for making glass vessels because it expands only slightly on heating. Vitreosil and quartz glass, having the same property as Pyrex, are used in making cooking utensils.
• To prevent cracking of bulbs, platinite (an alloy of nickel 45% and steel) is used through which wires connecting the filament to the outside circuit pass. The platinite expand to the same extent as glass thus prevent cracking from unequal expansion.
• The latent heat of fusion of ice is 80 calories per gram. This means 80 calories of heat are needed to convert 1 gram of ice into water (0^oC) without a change in temperature.
• The latent heat of vaporization of a substance is the amount of heat in calories required to convert 1 gram of the substance from liquid to vapour state without any change in temperature.

Thus, in vapour and water at 100°C, the vapour is hotter than the water because it contains latent heat.

• The volume of steam formed, when 1 cc of water boils is 1670 cc.
• A large iceberg melts at the base, but not at the top because of high pressure of ice at the base has a lower melting point.
• If the water in a lake were to behave like other liquids, then in extremely cold weather it would freeze from bottom to top.
• Drinks remain hot or cold for a long time in a thermos flask because of the vacuum between the double walls; and silver coating inside the double walls.
• A hygrometer is used in measuring relative humidity.
• At the time of rain, the relative humidity of air becomes 100 per cent.
• The amount of heat required to raise the temperature of 1000 lb of water through 1000of is called therm. Therm is the practice unit of quantity of heat.