UPSC Physics Heat NCERT Extracts - Heat

 Heat

1.           Temperature and Heat

• Heat is the form of energy transferred between two (or more) systems or a system and its surroundings by virtue of temperature difference.
• A measure of temperature is obtained using a thermometer. Many physical properties of materials change sufficiently with temperature to be used as the basis for constructing thermometers.
• The commonly used property is variation of the volume of a liquid with temperature.

2.           Thermal Expansion

• We observed that sometimes sealed bottles with metallic lids are so tightly screwed that one has to put the lid in hot water for ometime to open the lid. This would allow the metallic cover to expand, thereby loosening it to unscrew easily.
• In case of liquids, you may have observed that mercury in a thermometer rises, when the thermometer is put in a slightly warm water. If we take out the thermometer from the warm water the level of mercury falls again.
• Similarly, in the case of gases, a balloon partially inflated in a cool room may expand to full size when placed in warm water.
• A fully inflated balloon when immersed in cold water would start shrinking due to contraction of the air inside.
• It is our common experience that most substances expand on heating and contract on cooling. A change in the temperature of a body causes change in its dimensions. The increase in the dimensions of a body due to the increase in its temperature is called thermal expansion. The expansion in length is called linear expansion. The expansion in area is called area expansion. The expansion in volume is called volume expansion.
• Water exhibits an anomalous behavour; it contracts on heating between \[0{}^\circ C\] and \[4{}^\circ C.\] The volume of a given amount of water decreases as it is cooled from room temperature, until its temperature reaches \[4{}^\circ C.\]Below \[4{}^\circ C.\]the volume increases, and therefore the density decreases.
• This means that water has a maximum density at \[4{}^\circ C.\] This property has an important environmental effect: Bodies of water, such as lakes and ponds, freeze at the top first. As a lake cools toward \[4{}^\circ C.\] water near the surface loses energy to the atmosphere, becomes denser, and sinks; the warmer, less dense water near the bottom rises. However, once the colder water on top reaches temperature below \[4{}^\circ C.\] it becomes less dense and remains at the surface, where it freezes. If water did not have this property, lakes and ponds would freeze from the bottom up, which would destroy much of their animal and plant life.
• Gases at ordinary temperature expand more than solids and liquids. For liquids, the coefficient of volume expansion is relatively independent of the temperature. However, for gases it is dependent on temperature.

3.           Specific Heat Capacity

• The change in temperature of a substance, when a given quantity of heat is absorbed or rejected by it, is characterized by a quantity called the heat capacity of that substance.
• Every substance has a unique value for the amount of heat absorbed or rejected to change the temperature of unit mass of it by one unit. This quantity is referred to as the specific heat capacity of the substance.
• The specific heat capacity is the property of the substance which determines the change in the temperature of the substance (undergoing no phase change) when a given quantity of heat is absorbed (or rejected) by it.
• Water has the highest specific heat capacity compared to other substances. For this reason water is used as a coolant in automobile radiators as well as a heater in hot water bags.
• Owing to its high specific heat capacity, the water warms up much more slowly than the land during summer and consequently wind from the sea has a cooling effect. That is why in desert areas, the earth surface warms up quickly during the day and cools quickly at night.

4.           Change of State

• The temperature at which the solid and the liquid states of the substance in thermal equilibrium with each other is called its melting point. It is characteristic of the substance. It also depends on pressure.
• The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point.
• Cooking is difficult on hills. At high altitudes, atmospheric pressure is lower, reducing the boiling point of water as compared to that at sea level. On the other hand, boiling point is increased inside a pressure cooker by increasing the pressure. Hence cooking is faster. The boiling point of a substance at standard atmospheric pressure is called its normal boiling point.
• All substances do not pass through the three states : solid-liquid-gas. There are certain substances which normally pass from the solid to the vapour state directly and vice versa. The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.
• Dry ice (solid CO₂) sublimes, so also iodine. During the sublimation process both the solid and vapour states of a substance coexist in thermal equilibrium.

5.           Latent Heat

• Certain amount of heat energy is transferred between a substance and its surroundings when it undergoes a change of state. The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process.
• For example, if heat is added to a given quantity of ice at -\[10{}^\circ C\] the temperature of ice increases until it reaches its melting point \[\left( 0{}^\circ C \right).\]At this temperature, the addition of more heat does not increase the temperature but causes the ice to melt, or changes its state. Once the entire ice melts, adding more heat will cause the temperature of the water to rise.
• A similar situation occurs during liquid gas change of state at the boiling point. Adding more heat to boiling water causes vaporisation, without increase in temperature.
• The latent heat for a solidliquid state change is called the latent heat of fusion (L_f), and that for a liquid-gas state change is called the latent heat of vaporisation (L_V).
• For water, the latent heat of fusion and vaporisation are L_f \[=3.33\times {{10}^{5}}\] J kg^-1 and L_V = \[22.6\times {{10}^{5}}\] J kg-1 respectively. That is \[3.33\times {{10}^{5}}\] J of heat are needed to melt 1 kg of ice at\[0{}^\circ C\], and \[22.6\times {{10}^{5}}\]J of heat are needed to convert 1 kg of water to steam at \[100{}^\circ C\].

6.           Heat Transfer

• Heat is energy transfer from one system to another or from one part of a system to another part, arising due to temperature difference. There are three distinct modes of heat transfer : conduction, convection and radiation –

• Conduction
• Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference.
• We noticed that some cooking pots have copper coating on the bottom. Being a good conductor of heat, copper promotes the distribution of heat over the bottom of a pot for uniform cooking. Plastic foams, on the other hand, are good insulators, mainly because they contain pockets of air.
• Houses made of concrete roofs get very hot during summer days, because thermal conductivity of concrete (though much smaller than that of a metal) is still not small enough. Therefore, people usually prefer to give a layer of earth or foam insulation on the ceiling so that heat transfer is prohibited and keeps the room cooler.
• In some situations, heat transfer is critical. In a nuclear reactor, for example, elaborate heat transfer systems need to be installed so that the enormous energy produced by nuclear fission in the core transits out sufficiently fast, thus preventing the core from overheating.

• Convection
• Convection is a mode of heat transfer by actual motion of matter. It is possible only in fluids. Convection can be natural or forced.
• In natural convection, gravity plays an important part. When a fluid is heated from below, the hot part expands and, therefore, becomes less dense. Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated, rises up and is replaced by the colder part of the fluid. The process goes on.
• In forced convection, material is forced to move by a pump or by some other physical means. The common examples of forced convection systems are forced-air heating systems in home, the human circulatory system, and the cooling system of an automobile engine.
• In the human body, the heart acts as the pump that circulates blood through different parts of the body, transferring heat by forced convection and maintaining it at a uniform temperature.
• Natural convection is responsible for many familiar phenomena. During the day, the ground heats up more quickly than large bodies of water do. This occurs both because the water has a greater specific heat and because mixing currents disperse the absorbed heat throughout the great volume of water.
• The air in contact with the warm ground is heated by conduction. It expands, becoming less dense than the surrounding cooler air.
• As a result, the warm air rises (air currents) and other air moves (winds) to fill the space-creating a sea breeze near a large body of water. Cooler air descends, and a thermal convection cycle is set up, which transfers heat away from the land.
• The other example of natural convection is the steady surface wind on the earth blowing in from north-east towards the equator, the so called trade wind. A reasonable explanation is as follows: the equatorial and polar regions of the earth receive unequal solar heat. Air at the earth's surface near the equator is hot while the air in the upper atmosphere of the poles is cool. In the absence of any other factor, a convection current would be set up, with the air at the equatorial surface rising and moving out towards the poles, descending and streaming in towards the equator.
• The rotation of the earth, however, modifies this convection current. Because of this, air close to the equator has an eastward speed of 1600 km/h, while it is zero close to the poles. As a result, the air descends not at the poles but at \[30{}^\circ \]N (North) latitude and returns to the equator. This is called trade wind.

• Radiation
• Conduction and convection require some material as a transport medium. These modes of heat transfer cannot operate between bodies separated by a distance in vacuum. But the earth does receive heat from the sun across a huge distance and we quickly feel the warmth of the fire nearby even though air conducts poorly and before convection can set in.
• The third mechanism for heat transfer needs no medium; it is called radiation and the energy so radiated by electromagnetic waves is called radiant energy.
• In an electromagnetic wave electric and magnetic fields oscillate in space and time. Like any wave, electromagnetic waves can have different wavelengths and can travel in vacuum with the same speed, namely the speed of light i.e., \[3\times {{10}^{8}}\]m s¹.
• Heat is transfered to the earth from the sun through empty space. All bodies emit radiant energy, whether they are solid, liquid or gases. The electromagnetic radiation emitted by a body by virtue of its temperature like the radiation by a red hot iron or light from a filament lamp is called thermal radiation.
• When this thermal radiation falls on other bodies, it is partly reflected and partly absorbed. 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.
• Similarly, a Dewar flask or thermos bottle is a device to minimise heat transfer between the contents of the bottle and outside. It consists of a double-walled glass vessel with the inner and outer walls coated with silver. Radiation from the inner wall is reflected back into the contents of the bottle. The outer wall similarly reflects back any incoming radiation. The space between the walls is evacuted to reduce conduction and convection losses and the flask is supported on an insulator like cork. The device is, therefore, useful for preventing hot contents (like milk) from getting cold, or alternatively to store cold contents (like ice).