Heat

Heat is a form of energy, entry or exit of which correspondingly increases or decreases internal energy of a body when no work is done on the body or by the body. It is measured in joule or calories.

A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer. Unit of temperature is degree Celsius (°C), Fahrenheit and Kelvin (SI unit).

Different types of thermometers are used for different purposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the m a x i m u m - m i n i m u m thermometer.

Kink prevents mercury level from falling on its own.

The normal temperature of human body is 37°C.The temperature of human body normally does not go below 35oC or above 42oC. That is the reason that a clinical thermometer has the range 35oC to 42oC.

Heat flows from a hotter object to a colder object. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.

In solids, generally, the heat is transferred by the process of conduction because particles of solids are closely packed and heat is transferred from the hotter end to the colder end of an object.

It cannot reach us by conduction or convection as there is no medium such as air in most part of the space between the earth and the sun. From the sun the heat comes to us by a different process known as radiation which does not require any medium for heat transfer.

The water is poor conductor of heat so do not heated by conduction. When water is heated, the water became lighter. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises upward and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

The materials which allow heat to pass through them easily are conductors of heat. For examples, aluminium, iron and copper The materials which do not allow heat to pass through them easily are poor conductors of heat known as insulators such as plastic and wood.

At night, the water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

During the day, the land gets heated faster than the water. The air over theland becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

Dark surfaces absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter whereas light coloured clothes reflect most of the heat that falls on them and, therefore, we feel more comfortable wearing them in the summer.

There is a layer of air in between the blankets. Since air is bad conductor of heat prevent body heat to escape out, two thin blankets provides more heat than one thick blanket.

Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat and it has air trapped in between the fibres.

Since the range of temperature from ice-point to steam-point is equal in all the three scales, 100 centigrade degrees = (212 - 32) or 180 Fahrenheit degrees = (373 - 273) or 100 absolute degrees. We consider three thermometers in the above three scales are dipped simultaneously in a liquid of certain temperature. Let the temperatures recorded in the Celsius, Fahrenheit and Kelvin thermometers respectively be C, F and K. Now it can be proved that C / 5 = F- 32 / 9 = K- 273 /5.

Units of Heat

The quantity of heat absorbed or given out by a substance during a thermalchange depends on

1. mass:: A larger mass of a substance absorbs or gives out more heat than a smaller mass of it for a certain change of temperature. So, if 'H' be the quantity of heat absorbed or given out by a substance of mass 'm’ for a given change of temperature, H = m.
2. difference of temperature:The heat absorbed or released by a certain quantity of a given substance increases or decreases accordingly as the difference between the initial and final temperatures is large and small. Thus, H= (T-t) where, (T-t) is the difference between the initial and the final temperatures of the body.
3. Specific heat of the material of the substance:This is a fundamental property of matter. An equal mass of different materials absorb or give out different quantities of heat, even if they are heated or cooled through the same range of temperature.This is also called specific heat capacity (abbreviated as SHC) in S.I. system.

• Laboratory thermometer and the clinical thermometer consist of a long, narrow, uniform glass tube.
• Laboratory thermometer and the clinical thermometer have a bulb containing mercury at the end of the tube.
• Laboratory thermometer and the clinical thermometer are marked with Celsius scale on the glass tube.

In between the layers of cloths there is trapped air. As air is bad conductor of heat so the outside low temperature does not get transferred to body as well as prevent our body heat to escape outside. Hence more layers of cloths keep us warmer during cold winter.

In places of hot climate it is advised that the outer walls of houses be painted white because a light colour absorbs very less radiant heat and we feel comfortable inside such houses due to lower temperature inside house.

Heat is the total energy of the molecular motion of the substance and temperature is the measure of average energy of the molecular motion in the substance. When we heat a substance it increases the motion of vibrating molecules of the substance thereby increasing the temperature of the substances.

Specific heat is a fundamental property of matter. An equal mass of different materials absorb or give out different quantities of heat, even if they are heated or cooled through the same range of temperature.This is also called specific heat capacity (abbreviated as SHC) in S.I. system.

There is a layer of air in between the blankets. Since air is bad conductor of heat prevent body heat to escape out, two thin blankets provides more heat than one thick blanket.

Heat always from a body of higher temperature to a body of lower temperature, until the temperature of both bodies are equal, then the two bodies are said to be in thermal equilibrium.

The property of thermal expansion of liquids is used in making thermometers. It states liquid expands on heating and contracts on cooling.

Different temperature scales used are:
Kelvin scale (k)
Degree Celsius (°c)
Fahrenheit (F)