Introduction
Take two beakers one with hot water and another with cold water. Dip your fingers in hot water your finger experience hotness. Then dip your fingers in cold water. Your fingers experience coldness.
Sources of Heat
Sun is a natural source of energy. Solar heat energy is essential for the survival of all forms of life on earth. Solar energy is considered as an alternate source of energy.
Heat and Temperature
Heat energy is present in every object above absolute zero. How do we know that heat is present in every object? Temperature of the body is an indication.
Measurement of Temperature
Temperature is a physical quantity and hence, measurable. In fact in SI system, temperature is a fundamental physical quantity.
Scales of Temperature
A reference scale with respect to which the temperatures can be measured is known as 'scale of temperature'.
Kelvin Scale of Temperature
Lord Kelvin (1824-1907) devised this scale of temperature. Hence, this is called the Kelvin scale, and in this scale, temperature is expressed in Kelvin. In SI system, Kelvin is the unit of temperature.
Heat and Motion (Kinetic Theory of Heat)
Matter is made up of atoms and molecules. These atoms in molecules are in a state of continuous motion. For example, molecules and atoms of a solid can have only vibrational motion, because they are held in their positions by strong intermolecular forces.
Unit of Heat Energy
In CGS system, heat is measured in calories. Calorie is defined as 'the heat energy required to increase the temperature of 1gm of water through one degree Celsius'.
Heat Capacity or Thermal Capacity
When same amount of heat is supplied to different substances of equal mass at the same initial temperature, rise in temperature for different substances is different.
Specific Heat
The specific heat of a substance is an important physical property because it tells us the suitability of a given substance for a specific purpose. Aluminium vessels are used in cooking because aluminium is a light metal. Hence, for a given volume, its thermal capacity will be less than that of vessels made of steel of same volume.
Calorimetry
Calorimetry is the measurement of the amount of heat evolved or absorbed in a chemical reaction, change of state, or formation of a solution.
Effects of Heat
We heat raw food to soften it. We freeze water into ice by cooling it in refrigerator. When we boil water in a kettle we observe the steam oozing from the mouth of the kettle.
Expansion of Matter
Matter expands on heating, i.e., solids, liquids and gases expand on heating.
Linear Expansion
The ratio of increase in length to its original length per every degree rise in temperature is known as 'the coefficient of linear expansion'.
Superficial or Area Expansion
Expansion in area of a Laminar surface due to heating is known as superficial expansion.
Cubical or Volume Expansion
The coefficient of cubical expansion is defined as the ratio of the increase in volume to its original volume for every degree increase in temperature. It is denoted by 'g'.
Expansion of Liquids
Liquids do not have a definite shape. They take the shape of the container. Thus, we can specify a liquid by its volume. Hence, we can speak of volume expansion only for liquids. Expansion of liquids is much greater than that of solids.
Expansion of Gases
Gases also expand on heating. Their volume expansion is very much greater than that of liquids. If a gas is heated at constant pressure, its volume increases and if a gas is heated at constant volume, its pressure increases due to expansion. Similarly, if we have to study the variation of pressure with temperature, its volume must be kept constant.
States of Matter
Heat energy can bring about a change of state in matter from solid to liquid and liquid to gaseous state.
Latent Heat
Take a few pieces of ice in a beaker. Insert a thermometer. Note the temperature. You will observe it to be 00C.
Evaporation
In liquids, the molecules of the liquid are always in a state of random motion, within its surface. Some molecules may have sufficient kinetic energy to escape from the surface of the liquid. This process is known as evaporation. Evaporation takes place at all temperatures. Rate of evaporation increases with rise in temperature and becomes maximum at the boiling point of the liquid. The process of evaporation also increases with increase in surface area of the liquid.
Humidity
Humidity is the amount of moisture (water vapor) present in the atmosphere. This moisture is due to the continuous evaporation of river, lakes, and oceans, and also from various life activities.
Relative Humidity
Relative humidity is defined as the ratio of the actual amount of water vapor in the atmosphere to the absolute humidity at that temperature.
