What is heat energy?
“Heat is a form of energy that flows between a system and its environment by virtue of a temperature difference between them.”Heat is therefore,called as the energy in transient.Once the heat enters a body,it becomes its internal energy and no longer exists as heat energy.
It is a common observation that if you place a hot object (say a cup of tea) or a cold object (say a glass of ice water) in an environment at ordinary room temperature,the object will tend toward thermal equilibrium with its environment.That is,the hot cup of tea gets colder and the ice water gets warmer;the temperature of each approaches the temperature of the room.
It seems clear that such approaches to thermal equilibrium must involve some sort of exchange of energy between the system and its environment.
Heat is an important form of energy.It is necessary for our survival.We need it to cook our food and to maintain our body temperature.Heat is also needed in various industrial processes.How to protect ourselves from high as well as low temperature,needs knowledge of how heat travels.
Misconceptions about heat:
Heat is similar to work in that both represent a means for the transfer of energy.Neither heat nor work is an intrinsic property of energy.Neither heat nor work is an intrinsic property of a system;that is ,we cannot say that a system “contains” a certain amount of heat or work.Instead,we say that it can transfer a certain amount of energy as heat or work under certain specified conditions.
some of the confusion about the precise meaning of heat results from the popular usage of term.Often heat is used when what is really meant is temperature or perhaps internal energy.When we hear about heat in relation to weather ,or when cooking instructions indicate”heat at 300 degrees ,”it is temperature that is being discussed.On the other hand ,we we also hear references to the “heat generated ” by the brake linings of an automobile or by briskly rubbing the palms of your hands together.In this case ,as we shall see,it is usually internal energy that is meant.A clue to the proper usage comes from the definition of heat:When you rub your hands together ,they do work on one another ,thereby increasing their internal energy and raising their temperature.This excess energy can then be transferred to the environment as heat,because the hands are at a higher temperature than the environment.
Unit of heat:
Because heat is a form of energy ,its units are those of energy,namely,the joule (J) in the SI system.Before it was recognized that heat is a form of energy,other units were assigned to it.In some cases these units ,specifically the calorie(cal) and the British thermal unit (Btu) ,are still in use today.They are related to the joule according to:
1 cal =4.186 J
1 Btu=1055 J.
The “calorie” in common use as a measure of nutrition (Cal) is in reality a kilo calorie;that is,
1 Cal = 1000 cal = 4186 J.
The Btu is still commonly found as a measure of the ability of an air conditioner to transfer energy (as heat) from a room to the outside environment.A typically room air conditioner rated at 10,000 Btu/h can therefore remove about 10 7 J from a room every hour and transfer it to the outside environment.
The Mechanical Equivalent of Heat:
In the past ,when the calorie was independently defined as a unit of heat,it was necessary to determine an empirical relationship between the calorie and the joule.This was first done by James Joule in 1950 in an experiment to determine the mechanical equivalent of heat.The result of Joule’s experiment,and others that followed,provided for nearly 100 years a conversion between the Joule and the Calorie.Today ,following the adoption in 1948 of the joule as the SI unit of heat and work ,in J, and so this conversion factor has lost the importance it had in Joule’s time. Nevertheless .joule’s work is still noteworthy for the skill and ingenuity of his experiments,for its precision (joule’s results differ by only 1 % from the SI defined relationship between the joule and the calorie),and for the direction it provided in showing that heat,like work,could property be regarded as a means of transferring energy.
What are the main sources of heat energy?
- The sun is the biggest source of heat.Sun’s heat reaches the Earth in the form of radiations.Solar radiations keep the Earth environment warm at a suitable temperature for the survival of life.
- We keep our body warm and alive by the heat produced from the food during its metabolism in the body cells.
- Heat is also produced by burning of Fossil fuels(wood,coal,oil and gas etc).We cook food and warm our rooms by the heat produced by burning of wood and natural gas, etc.Heat produced by the burning coal and oil etc. is used to produce electricity in thermal power stations.
- Electricity is also used to produce heat.
- Heat is obtained from nuclear reactions like Fission and Fusion.
- Bio mass gas is also a source of heat.
Differentiate between thermal expansion and contraction?
All kinds of material objects are made up of tiny particles such as atoms and molecules.When an object is heated,the object expands.This expansion of material objects on heating is called thermal expansion.On the other hand ,when an object is cooled ,the object contracts.This contraction of material objects is called thermal contraction.
Applications of Conduction of Heat:
Conduction plays an important role in our daily lives:
- Cooking utensils,electric kettles,irons,soldering irons etc.are made of metals to conduct heat quickly.their handles are made of plastic or wood which are bad conductors.
- Birds have feathers which keep their bodies warm because feathers are bad conductor of heat.
- woolen clothes and blankets slow down the transfer of heat.It happens so because the wool traps air in it.The air is a bad conductor of heat.
- Ice is covered with jute rugs ton reduce its melting speed.jute is the bad conductor of heat.
- An insulating material (e.g. Styrofoam) is filled between the double walls of a refrigerator.it reduces the transfer of heat across the walls of the refrigerator.
- Double plane windows are used in buildings to slow the transfer of heat.Air between the two layers of glass acts as an insulator.
- Thermos bottles use air or a vacuum to slow the transfer of heat by conduction.
How many ways by which heat transfer?
There are three ways by which heat can flow,which are given in the list below: