Transfer of Heat and Heat Capacities, Physics tutorial

Conduction of Heat:

It is a daily experience that if you dip a silver spoon into a hot tea, the handle of spoon much rapidly feels hotter than it was before. The heat from the hot tea has been transferred all along the metal handle to the other end of the spoon through the procedure of conduction.

Conduction of heat is a procedure by which heat energy is transferred via a material, the average position of the particles of the material remaining similar.

Heat energy is for all time transferred when various parts of a solid body are at dissimilar temperatures. The direction of heat transfer is always from the hotter to the cooler portions of the solid.

Good and Bad Conductors of Heat:

Most of the metals like aluminum, copper, silver, iron, let heat energy to pass via them much easily. We refer to these materials as good conductors of heat. They are employed where heat is needed to travel quickly (example: the cooking utensil). On the other hand, we refer to such materials that don't let heat to pass simply via them as poor conductors of heat or insulators. Illustrations of such materials are most non-metals like water, wood, air, plastic, cloth, cotton wool and cork.

The handle of a cooking vessel is made up of wood or plastic that is insulators so that heat can't be conducted quickly via them. The capability of a metal to conduct heat is termed as its thermal conductivity.

Description of Conduction in terms of Kinetic Molecular Theory:

Heat is transferred via solids mostly through conduction because of the nature of solid molecules that are much close altogether. Whenever we heat a part of solid directly, the solid molecules which are directly heated absorb the heat and vibrated faster than before the heat was applied. They don't move from place to place because of the strong forces of attraction between them. As they vibrate, they pass on a few of their heat energy to their neighbors that is turn vibrate faster and pass on some of the heat to the subsequent layer of molecules and so forth. The heat carries on moving in this manner until it reaches the end of the solid. All the molecules of the solid ultimately vibrate more quickly regarding their fixed positions. We state that thermal energy is transferred all along the solid via the average position of the molecules remains unchanged. Whenever heat is transferred in this manner, it is stated to be conducted all along the body.

Thermal Conductivity of Liquids and Air:

Thermal conductivities of liquids usually reduce with rise in temperature (example: glycerine, water and so on over some ranges of temperature), however are almost independent of pressure. Leaving aside liquid metals, water has the highest thermal conductivity of all liquids. Thermal conductivities of gases rise with temperature and reduce with molecular weight; pressure dependence becomes important only at high pressure. Among the gases, hydrogen consists of the highest thermal conductivity.

Practical applications of Good and Bad Conductors:

The practical applications of good and bad conductors are as follows:

1) Cooking pots and frying pans are made of metals to make sure fast transfer of heat from the fire to the food being cooked. The handles of cooking vessels are made up of insulators in such a way that the vessels when hot can be held at ease by the handles.

2) Home cooling in the tropics such as asbestos ceiling and thatched-roof houses prevent heating up of houses as their materials are insulators and don't conduct the heat from the sun to the room.

3) The use of cloth to keep warm - Clothes keep us warm by means of trapping air (that is, poor conductors) around the body. Clothing made up from woolen and fur materials are worn in the cold climates to hold heat and keep the body warm.

4) Use of rugs on floors - Heat is transferred from one's foot to the floor is conducted away rapidly by the floor and the foot feels cold however the rug doesn't conduct away heat rapidly. It heats up to the temperature of the foot.

5) Cotton wool and air employed as insulators to decrease to the minimum loss of heat in calorimeters. This is termed as lagging.

Convection of Heat through Liquids and Gases:

Liquids and gases are generally fluids. The particles in such fluids can move from place to place. Convection takes place if particles having a lot of heat energy in a liquid or gas move and take the place of particles having less heat energy. Heat energy is transferred from hot places to cooler places through convection.

Liquids and gases expand whenever they are heated. This is due to the reason that the particles in liquids and gases move faster if they are heated than they do if they are cold. As an outcome, the particles take up more volume. This is due to the reason of the gap between particles widens, whereas the particles themselves stay in similar size.

The liquid or gas in hot regions is less dense than the liquid or gas in cold regions, so it increases into cold regions. The denser cold liquid or gas falls into the warm regions. In this manner, convection currents which transfer heat from place to place are set up.

Convection in Terms of Kinetic Molecular Theory:

If the water is heated below, its molecules at the point of heating put on more kinetic energy, expand through vibrating farther away from their fixed position, become less dense and thus rise. The denser molecules from the top sink to the place of such that have risen above. Such molecules in turn get heated and increase. By the heating, upward warm currents and downward cooler currents are established till the water boils.

Applications of Convection Current:

1) Car engines: Cooled through convection currents in the water pipes. Water is an extremely good substance to carry the surplus heat away from the engine to the radiator.

2) The sun can cause extremely large convection currents of air. This flow of air is wind. In day-time, the land consists of a higher temperature than the sea. The warm air increases over the land and the cool air falls over the sea. Therefore we sense a breeze from the sea.

3) Increasing air over the land is convection currents and is employed by glider pilots to keep their gliders in the sky.

4) Air conditioners are installed close to the ceiling of the room, to let setting up of convection currents. The air-conditioner discharges cool dry air into the room. As cool air is denser, it sinks. The warm air, being less dense, will mount. The air circulated and the temperature of the air will ultimately fall to the desired value.

Radiation of Heat:

Radiation is a process of heat transfer which doesn't rely on any contact among the heat source and the heated object as is the case by means of conduction and convection. Heat can be transmitted via empty space through thermal radiation often termed as infrared radiation. This is a kind of electromagnetic radiation. No mass is exchanged and no medium is needed in the method of radiation. Illustrations of radiation are the heat from the sun, or heat discharged from the filament of a light bulb.

Applications of Radiation:

1) We obtain or receive heat from the sun by the procedure of radiation.

2) House roofs in the tropics and also factory roofs are brightly painted in shiny aluminum color to decrease the absorption of heat throughout the day. It as well decreases the heat loss through radiation at night.

3) It is not suitable to wear a dark colored jacket in hot sunshine as it will absorb heat and make the wearer feel hot and uncomfortable. As well, a brightly painted car is favored to a black-painted car in hot tropical countries. The black-painted car will absorb and hold heat from the sun and inside the car will be much hot.

4) Petrol storage tanks are sprayed by silver paints to reflect the heat rays falling on it.

5) A teapot consists of a silvery surface and an electric iron consists of a silvered surface base in order to make each capable of retaining its heat for a long time period.

6) A fire-fighting suit is shiny and bright so that it doesn't absorb much heat to burn the fire man.

Heat capacity:

As we are familiar that, the supply of heat to a substance rises the average kinetic energy of the molecules of the substance therefore a raise in temperature of the substance. The average kinetic energy attained by the molecules per degree Celsius increase in temperature is the heat (that is, thermal) capacity of the substance.

Define: It is the heat capacity of a substance stated as the heat energy needed to increase the temperature of the substance by 1oC (or 1K).

Symbol:

The heat capacity is denoted by 'C'.

S.I. unit: joule per Kelvin (J/K).

Specific Heat Capacity:

We are familiar that equivalent masses of various substances need different quantities of thermal (heat) energy to increase them via similar temperature range. To take care of such differences and to make it possible to compute the amount of heat energy needed to increase the temperature of a substance via a definite temperature range, the idea of specific heat capacity was introduced.

Define:

The Specific heat capacity of a substance is the amount of heat needed to increase the temperature of unit mass of the substance by 1oC (or IK).

Symbol:

The Specific heat capacity is denoted by 'C'.

S.I unit joule per kilogram per Kelvin (Jkg-1K-1)

Heat capacity of a substance:

C = Mass of the substance x specific heat capacity of the substance

If the temperature of a substance changes, then heat energy given or received

Q = mass x specific heat capacity x temperature change.

In symbols:

C = mc

Q = mc (θ2 - θ1)

Here,

C = heat capacity of the substance

Q = heat energy

M = mass of substance

C = specific heat capacity of the substance

θ2 = higher temperature

θ1 = lower temperature

2 - θ1) =   temperature change

'Q' is the quantity of heat - it is at times represented by H

Newton's Law of Cooling:

Newton's law of cooling defines that for a small difference of temperature between body and its surrounding; the rate at which a body loses heat is proportional to the temperature difference between the body and its surroundings.

The law is approximately true in still air only for a temperature excess of around 200oC or 300oC. This is true for all excess temperature in conditions of forced convection of the air, that is, in a draught. At low excess temperature less than 10oC, radiation is the main contributing factor to the rate of cooling of an object.

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