heat

Heat is a form of energy. The greater the heat energy, the hotter is the body.

In this lesson, we will learn about 

1. Difference between heat and temperature
2. Heat as a form of energy
3. Types of substances based on their ability to conduct heat energy or catch fire
4. Conversion of heat energy into other forms of energy and vice versa
5. Flow of heat from high temperature to low temperature
6. What are the different effects of heat? 
7. What happens when the heat is applied to different states of matter - solids, liquids, and gases? 

Let's start with understanding the difference between heat and temperature. 

  Difference between heat and temperature

Heat is a form of energy	Temperature is the degree of hotness or coldness of a body
Heat is the cause	Temperature is the effect
It is the combined energy of all the molecules moving inside a body	It is only the measure of how fast molecules are moving in a body. The faster the molecules vibrate, the hotter the body.
SI unit of heat is Joule (J); other units are calorie (Cal) and kilocalorie (KCal)	SI unit of temperature is Kelvin (K); other units are Celsius (°C) and Fahrenheit (°F)

 

Conversion of heat energy into other forms of energy

Heat energy can be converted into other forms of energy like mechanical energy, light energy, and electrical energy.

• Heat engine converts heat energy into mechanical energy
• Hot iron converts heat energy into light energy

Conversion of other forms of energy into heat energy

• Candle flame converts chemical energy into heat energy
• Electric bulb converts electrical energy into heat energy

Sources of heat – Sun (natural source of heat), fire, electricity

Types of substances

Inflammable substances are substances that can easily catch fire. E.g. LPG, wood, grass, kerosene, paper

Non-inflammable substances are substances that are fire-resistant. E.g. water, sand, stone, concrete

Conductors are substances through which heat is easily conducted. E.g. silver, gold, copper, aluminum

Insulators are substances through which heat is not easily conducted. E.g. wood, glass, wax, stone, water, air

Temperature

The temperature of a body is a measure of the degree of hotness or coldness of that body.

It is an indication of the amount of heat present in a body.

Flow of heat

If two substances at different temperatures are brought in contact, heat flows from the hotter body to the colder one until their temperatures are equalized.

For example, to cool a glass of hot milk, place it in cold water. Heat flows from hot milk to cold water.

Scales of temperature

Scale	Measured as	Lowest scale (freezing point of water)	Upper scale (boiling point of water)	The interval between the fixed points is divided into
Celsius	Degree Celsius	0°C	100 °C	100 parts
Fahrenheit	Degree Fahrenheit	32°F	212 °F	180 parts
Kelvin	Degree Kelvin	273 K	373 K	100 parts

Celsius to Fahrenheit = (°C ×(9/5)) + 32 = °F

Fahrenheit to Celsius = ((°F – 32 ) × (5/9)) = °C

Celsius to Kelvin = °C + 272 = K

When there are two objects at different temperatures, the one at a higher temperature will transfer heat to the other one until they have the same temperature.

When they have the same temperature, we say they are in thermal equilibrium.

Effects of Heat

Change in temperature of the body: When a body gains heat, temperature increases and when it is cooled temperature decreases.

Change in shape of the body: Length, volume and area of a substance increases when the heat is supplied to it. This is known as thermal expansion.

Change of state of matter:

• Fusion or Melting: Solid changes to liquid by absorbing heat at a constant temperature. E.g. ice into the water at 0°C
• Freezing: Liquid changes to solid by releasing heat at a constant temperature. E.g. water into ice at 0°C
• Vaporizing or Boiling: Liquid changes to vapor state by absorption of heat at a constant temperature. E.g. Water changes to steam at 100 °C
• Condensation: Vapor changes to a liquid state by releasing heat at a constant temperature. E.g. Steam converts into boiling water at 100 °C
• Sublimation: Solid directly converts into the gaseous state. E.g. naphthalene or camphor
• Deposition: Gas condenses into a solid-state. E.g. conversion of carbon dioxide into dry ice

Thermal Expansion of Solids

• Linear expansion – Increase in length of solid on heating
• Superficial expansion – Increase in area of solid on heating, i.e. increase in length and breadth of solid
• Cubical expansion – Increase in volume of solid on heating

Examples of precautions taken to take care of the thermal expansion

• The gap is given and rollers are used at one end of the bridge
• Loops are provided in metal pipelines carrying hot gases
• Concrete blocks for pavements and road surfaces are laid with gaps or joints between them
• The iron rim is made slightly smaller than the wheel. The rim is heated and slipped in the wheel. Immediately cold water is sprinkled on it

Thermal Expansion in liquids

Most liquids expand when heated and contract when cooled. Water, however, is an exception. Between 0°C to 4°C water contracts when heated and above 4°C it expands, unlike any other liquids. This is known as the anomalous expansion of water. The expansion of liquid depends mainly on the nature of the liquid. Different liquids expand by different amounts. The liquid thermometer uses the property of the expansion of liquids.

Thermal Expansion in Gases

Gases expand on heating and contract on cooling. E.g. Automobile tire tightly filled with air can burst during summer days. This is due to the expansion of air as they get heated up when the car runs. The gas thermometer uses the principle of expansion of gases on heating.