Heat

Heat is a form of energy that flow from body of higher temperature to the body with lower temperature. Every body is composed of large numbers of particles posses certain kinetic energy.the total sum of kinetic energy of all constituent particles of body is equal to the heat contained by the body.

Newton’s law of cooling: the rate of loss of heat by a body is directly proportional to the difference in temperature between the body and the surrounding.

Temperature: it is the hotness and coldness of the body. it is just a scale to measure the thermal state of body. Temperature is measured in

1.     Degree centigrade or degree Celsius

2.    Degree Fahrenheit

3.    Kelvin

Absolute zero = there is no upper limit of temperature but there is limit of lower temperature. The lowest value of temperature is called absolute zero it is 0 Kelvin or -273.15 degree Celsius.

Clinic thermometer= it is made of mercury, detect temperature of human body. A Clinical thermometer read temperature between 35°C to 42°C or 95°F to 110°F. Normal human temperature is 98.6°F (37°C).mercury is good conductor of heat. It does not stick on the wall of tube and also does not vaporized more. Mercury freezes at –39°C. In cold countries alcohol thermometer is used.

Heat transfer: heat transfer through conduction, convection, radiation.metals is best conductor of heat and gases are worst conductor of heat. But diamond is the best conductor of heat.

1.   conduction:

 Heat transfer because of temperature difference. Heat is transferred from one place to another place by the successive vibration of the particles of the medium without bodily movement of the particles of the medium. In solids heat transfer placed by conduction. For example when one rod is heated from one end the second end is heated by conduction.

-Cooking utensils are made of metals whereas their handles are made of plastic or wood because metals are better conductor and plastic & wood are poor conductors. Hence, hot utensils can be easily held by their insulating handles.

-In winter, metallic handle of a wooden door appear colder because it is a good conductor. Heat flows from our body to the handle which gives the sense of cold.

-Eskimos live in double walled snow huts called igloos. Air enclosed between the walls of ice is a bad conductor and ice is also a bad conductor. It prevents the heat they produce from escaping and keeps them warm.

-Saw dust is a poorer conductor of heat than the wood from which it is made because of air trapped in the saw dust. That is why ice is kept in sawdust to prevent them from melting.

-In winter, birds often swell their feathers. Air enclosed between their body and feathers does not allow flow of heat from the body of the birds to the cold surroundings.

-Ice is packed in gunny bags or sawdust. Air trapped in the sawdust blocks the transfer of heat between the cold ice and the surroundings. It prevents the melting of ice.

-A new quilt is warmer than an old one. The new quilt encloses more air than the old one. This air, being bad conductor of heat, does not allow heat of our body to flow to the surroundings

-Two thin blankets together are warmer than one blanket of their combined thickness. The air contained in between the two blankets is a bad conductor and it prevents heat from escaping and keeps the body warm.

-In air conditioned rooms, double layered windows are preferred than the single layered windows because a thin layer of air is contained between the two layer of glass panes in the window.

-In winter, a stone floor feels cold to the bare feet, but a carpet on the same floor feels warm even though they, are at the same temperature. Since the stone is a better conductor of heat than the carpet, hence the feet feel cold on the stone but not on the carpet.

-Due to the continuous working of a refrigerator, a thick layer of ice deposits on the outside and the inside of the freezer and hence, it has to be switched off for defrosting. The cooling action of the freezer slows down because ice is a bad conductor of heat. Hence, defrosting improves the functioning a refrigerator.

2.   Convection:

Convection is a mode of heat transfer by actual motion of particles. It is possible only in the fluids. When air is hot is density decrease hence its float upward and cool air take place of that air. The heating of atmosphere is because of convection.

-Function of ventilation in houses is based on the convection. During winter, air comes out from houses because the inside temperature is high whereas in summer air comes in because the outer temperature is high. It involves the movement of air from high in temperature to lower temperature by convection currents.

-In refrigerators, freezer is set at the top. Temperature of air at the bottom is higher than that inside the freezer. The warm bottom air moves up and cold freezer air moves down. It results convection current of air in the refrigerators that cools the entire inner space.

-In geysers and waters heater, the heating elements are fitted at the bottom. It causes hot-bottom water to move up and cold top-water to move down and thereby setting up convection current. Consequently entire water content gets heated.

-In electric ovens, the heating elements are fitted near the bottom to heat the entire enclosed air by convection.

-Monsoons, trade wind in sea, water streams in sea, winds, etc. are due to the convection.

-The main mechanism for heat transfer inside a human body is forced convection through blood which is circulating by the pumping action of heart.

3. Radiation:

 In radiation the heat transfer does not need any medium ( conduction and convection require medium). It heat by electromagnetic waves from sun called radiant energy. Everything emits heat by radiation.

      When thermal radiation falls on body it absorbs, reflect and emit heat. When a body absorbs more and emits less radiation, its temperature goes up. When a body emits more and absorbs less radiation, its temperature goes down and when a body absorbs and emits equal amount of radiations, its temperature remains constant. A good absorber is a good emitter and a poor absorber is a poor emitter. The amount of heat that a body can absorb by radiation depends on the colour of the body.

     We find that black bodies absorb and emit radiant energy better than bodies of lighter colours. This fact finds many applications in our daily life. We wear white or light coloured clothes in summer so that they absorb the least heat from the sun. However, during winter, we use dark coloured clothes which absorb heat from the sun and keep our body warm. The bottoms of the utensils for cooking food are blackened so that they absorb maximum heat from the fire and give it to the vegetables to be cooked.

Black body will emit radiation at the fastest rate on heating. Lampblack is close to a blackbody. Bolometer and thermopile are close to a blackbody. Bolometer and thermopile are instruments used to measure thermal radiation.

  - A rough black surface is a good emitter as well as a good absorber of heat radiation while a bright polished surface is a bad emitter as well as a bad absorber. Hence, a bright polished cup keeps tea or coffee warm for a longer time in comparison a rough black cup. The base of an electric iron is highly polished so that it does not lose heat by radiation. White or light colours of houses keep cooler in summer because light coloured surface do not absorb much solar radiation.

 -Thermos Flask: It is a double walled glass bottle having a vacuum between the walls. The inner wall is silvered whose mouth is closed by a plastic stopper. The vacuum does not allow the loss of heat by convection and the stopper being insulator does not allow conduction of heat. The silvered wall prevents radiation as it is a poor emitter and a poor absorber of radiation. Thus, in a thermos flask transfer of heat by conduction, convection and radiation is minimized. Hence, a hot liquid in it remains hot and a cold liquid or ice in it remains cold for a longer time.

 -At a point in front of fire, heat is received due to radiation only, while at a point above the fire, heat is received both due to radiation and convection. Hence, it is hotter at the same distance over the top of a fire than in front of it.

 -When animals feel cold, they curl their bodies in a ball so as to decrease the surface area of their bodies. As total

Energy radiated by a body is proportional to the surface area of the body, the loss of heat due to radiation would be reduced.

 -A box or a house with glass walls or glass windows, acts as a greenhouse because it traps heat radiations entering it through glass. The glass allows the heat radiation inside but blocks the hot air from coming out. Hence, a car parked in the sun with its windows closed gets terribly warm in comparison to the outer atmosphere.

 -Cloudy nights are warmer than clear nights because clouds reflect the radiations emitted by the earth at night and keep it warm. Thus, clouds act as a blanket.

 -Glass, which is ordinarily used as the base in photographic plates, is transparent to light while opaque to thermal infrared radiation. But certain special kinds of glass have been prepared which are transparent only to thermal infra-red. Such glasses, used in the preparation of photographic plates, have made it possible to obtain long distance photographs, even under misty conditions using infra-red radiations.

 -The fact that good reflectors are bad absorbers and bad radiators is utilized in making firemen’s helmets and metallic tea-pots highly polished on the outside.

-Water can absorb 90% of heat radiation, which shows the importance of the existence of water vapour in the atmosphere. The water vapours in the atmosphere protect the earth from the more intense heat rays of the sun during the days and from the extreme cold during the nights. In very dry regions, where there is little water vapour in the air, the days are intensely hot and the nights are extremely cold.

-Gases are poor radiators of heat, hence in the construction of furnaces (heating device), the gases are made to fall on fire-bricks, which radiate heat freely.

 - Because black bodies are the best absorbers of heat radiations, a black dress is uncomfortable during the hot summer, while a white one is cool.

Law of thermodynamics.

First law= also know as law of conservation. It state that an energy cannot be destroyed nor be created. It just converted from one form to another.

Second law= it is impossible to convert all heat energy to work. Some energy is unavailable to doing work called entropy. And the entropy of a system is always increased in isolated system.

Third law= entropy of a system approach a constant value as the temperature reach absolute zero.

Types of processes

Isobaric Process: It involves constant pressure

Isochoric Process: It involves constant volume.

Isothermal Process: It involves constant temperature. However, heat changes.

Adiabatic Process: It involves constant heat. However, temperature changes

Laws of gas:

 v= volume, p=pressure, ∝=proportional, n =number of molecules

Ideal gas equation:

PV=nRT

All equations of gas made from this equation and All laws of gas comes from this equation

Boyle’s Law:

For a given mass of gas, the volume of gas is inversely proportional to the pressure applied to gas,at constant temperature.Mean if we apply pressure then volume of gas decreased.

v∝1/p   or VP=proportionally constant.

V1P1=V2P2

Here V1  and  P1 are initial volume and initial pressure and V2 And P2 are final volume and pressure.

Charles's Law:

  The volume of gas is directly proportional to temperature (in Kelvin scale) at constant pressure. Mean if temperature increase the volume will increase.

v/t= constant

v1/t1=v2/t2

Gay Lussac’s Law:

The volume remaining constant, the pressure of gas is directly proportional to temperature. Mean as the temperature increase if we increase the pressure then volume of gas become constant.

p/t=constant

p1/t1=p2/t2