Heat Notes

Everything is made up of extremely small particles called molecules which show movement.
The molecules of a substance move because they possess energy. The sum of the energy possessed by all the moving molecules of the substance is called its thermal energy.


When a hot object and a cold object come in contact with each other, a type of energy flows from the hot object to the colder one. This energy is called heat energy. The SI unit of heat is Joule (J).
Calorie is the older metric unit.


The degree of hotness or coldness of an object is called its temperature. The SI unit of temperature is Kelvin (K).

Measurement of Temperature

The instrument used to measure temperature is called a thermometer.

Clinical Thermometer:

  • The commonly used clinical thermometer is made up of a narrow tube (capillary) of thick glass.
  • At the end of the capillary tube is a thin glass bulb filled with mercury.
  • Mercury is preferred in thermometers because it is present in liquid state over a wide range of temperatures. It is a silvery grey in colour, therefore it is relatively easy to observe. It does not stick to the glass in which it is enclosed.
  • The scale in a clinical thermometer extends over a few degrees on either side of the normal body temperature of 37 degree Celsius.
  • It can measure temperature from 35 degree Celsius to 42 degree Celsius.
  • The clinical thermometer is marked to indicate a small change of 0.2 degree Celsius.
  • The capillary tube of a clinical thermometer has a kink or bend that does not allow the mercury to flow back into the bulb before the temperature has been read.
  • To lower the level of mercury, the thermometer has to be given a firm jerk.
While using a clinical thermometer, the following precautions should be taken:
  1. The thermometer should be washed before and after use preferably with an antiseptic solution.
  2. The thermometer should be read keeping the level of Mercury along the line of sight.
  3. It should be handled with care.
  4. The bulb of the thermometer should not be touched while taking a reading. Hold the thermometer by its glass body.
  5. It should be ensured that before use, the mercury level is below 35 degree Celsius or 95-degree Fahrenheit.

Digital thermometer:

  • Nowadays, digital thermometers are used to measure body temperature.
  • They work on tiny dry cells.
  • It is a safer device because it does not contain mercury which is a toxic substance.

Laboratory thermometer:

  • A laboratory thermometer is made up of a glass tube.
  • It has a glass work with mercury which on gaining heat rises in the glass capillary of the tube.
  • The range of temperature that laboratory thermometers can measure is between -10 degree Celsius to 110 degree Celsius.
The following are some precautions to be taken while using a laboratory thermometer:
  1. Handle the thermometer with care.
  2. Do not hold the thermometer by the bulb while taking a reading.
  3. The thermometer should be kept upright, not tilted. The bulb should not touch the surface of the container.
  4. Removing the thermometer from a boiling water bath and immediately immersing it in an ice bath may cause the thermometer to break.

Temperature Scales

Temperature is measured using different temperature scales. In a temperature scale two reference temperatures are chosen (most commonly the freezing and boiling point of water).
The commonly used scales are the Celsius scale the Fahrenheit scale and the Kelvin scale.

Celsius scale:
Around 1743, Anders Celsius invented the Celsius scale. He determined the freezing temperature for water to be at 0 degree and the boiling temperature at 100 degrees. There are 100 divisions between these two points.

Fahrenheit scale:
This scale was developed by Daniel Gabriel Fahrenheit. The unit of this scale is degree F.
On this scale, the freezing point of water is taken as 320 Fahrenheit and the boiling point of water is taken as 2120 Fahrenheit. There are 180 divisions between these two points.

Kelvin scale:
This scale was invented by Lord kelvin. The unit of the scale is K. On this scale, the freezing point of water is taken as 273 K and the boiling point of water is taken as 373 K.

Temperature scale
Boiling point of water
Freezing point of water
Degree Fahrenheit
Degree Celsius

Conversion of Scales:
The three temperature scales (Celsius, Fahrenheit and Kelvin) can be converted from one to the other, using the following formula:
$T_F=\frac {9}{5} T_C+ 32^0$

Transfer of Heat

Heat energy always flows from a region of higher temperature to a region of lower temperature, till both attain the same temperature. This phenomenon is called transfer of heat.
The main ways by which transfer of heat occurs are Conduction, Convection and radiation.


Conduction is the transfer of heat from a hotter part to a colder part of an object due to collision of its particle without actual movement of the particles from their position. Heat transfer through conduction occurs within solids and between solid substances that are in direct contact with each other.
How does conduction of heat occur?
  • In a solid, the molecules are closed arranged. When a solid is heated, the molecules near the source of heat gain thermal energy and begin to vibrate faster.
  • These molecules then collide with the colder slower molecules and transfer some thermal energy, making them vibrate faster, become warm and then in turn collide and transfer the heat to their neighboring particles and so on
  • In this way, heat is transferred from one particle to the next through collision of vibrating particles within substances.

Good and Poor conductors of heat:
The rate of conduction of heat is different in different solids.
Materials that conduct heat easily are called conductors. Metals are usually good conductors of heat.
Materials that do not allow the conduction of heat are called insulators. For example, wood, cotton rubber, clay and cock.
Air and water are also insulators of heat.
Materials such as wool, feather and fur act as insulators because air is trapped inside their fibres.

Practical application of conduction:
  1. Pans used for cooking are usually made of metals such as steel, copper or aluminium (good conductors), while their handles are made of plastic (insulator).
  2. Clothes made of fur trap air inside them and this trapped air then acts as an insulator. It prevents heat from escaping, keeping our body warm.


Convection is the transfer of heat by the movement of particles. Convection is the primary mode of heat transfer in fluids (liquid and gases).

How does convection take place?
Convection currents:
  • When a liquid or gas is heated it expands. As a result, it becomes lighter and moves upwards.
  • The cooler, heavier fluid from the surroundings rushes to take its place and is heated in turn.
  • This process continues and gives rise to a convection current through which heat energy flows and the entire liquid or gas gets heated.
Convection currents give rise to Breeze in coastal regions.
During the day, the land heats up faster than the sea. So the air above the land becomes warmer and rises upwards. The cooler air from above the sea rushes towards the land and takes its place, resulting in a sea breeze during the day.
At night, the land cools faster. Therefore, the air above the sea is warmer than the air over the land. At night, cooler air from the land replaces the warmer air above the sea resulting in a land breeze.

Practical application of conduction:
  1. Exhaust fans in kitchens are always fitted high, close to the ceiling so that they can remove the hot air that rises up.
  2. Room heaters are placed on the floor of a room. When the cool air is heated, it rises up and the cold air flows down to the floor. This results in effective heating of the entire room.
  3. The ventilators of a room at the top of a window or the door. This is because when the air in the room gets heated, it rises and passes out from the ventilators at the top.


The transfer of heat in the form of rays, for which a medium is not required is called radiation.
These waves can move through vacuum.

Working of a thermos flask:
  • A thermos flask is used to maintain the temperature of liquids kept in it. It keeps hot liquids hot or cold liquids cold for a longer time.
  • The outer casing of a thermos flask is made up of plastic, which acts as an insulator preventing the heat loss through conduction
  • Inside it, is a double walled container made of glass or stainless steel.
  • Both the walls are polished, so they are shiny.
  • The space between the two walls is a vacuum; this prevents heat loss through convection.
  • The shiny surface of the wall also prevents heat loss through radiation.

Practical application of radiation:
  1. Solar heaters are designed to use the heat energy from the sun for cooking food.
  2. We prefer wearing light colored clothes in summer because light colors are poor absorbers of heat. Conversely, we prefer wearing dark- colored clothes in winter because they are good absorbers of heat and thus, keep us warm.

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