Heat

Concept of Heat

• Heat is a form of energy.
• We can feel heat by the sense of touch.
• This form of energy can be transferred between two objects with varying temperatures.
• The S.I. unit of heat is Joule (J).

Temperature

• Heat cannot be measured with any instrument directly, but its effect on a substance can be detected. This measurement is the temperature of the body.
• Temperature is defined as the degree of hotness or coldness of a body.

Thermometer

• A thermometer is a device which measures the temperature in a reliable manner.
• It is made of a long, narrow, uniform glass tube with a bulb at one end.
• It is filled with mercury as it is in liquid state at room temperature. It expands or contracts with an increase or decrease in the temperature.
• There are different types of thermometers:

Clinical Thermometer

• A clinical thermometer is used to measure the temperature of the human body.
• It is very accurate because of its narrow tube in which the liquid rises fast.

• The scale on the clinical thermometer is called the Celsius Scale, indicated by °C.
• It reads the temperature from 35°C to 42°C.
• It is used in clinics by doctors.

Laboratory Thermometer

• A laboratory thermometer is used to measure temperatures or the temperature changes of other objects.

• These thermometers are usually filled with red spirit rather than mercury.
• The range of a laboratory thermometer is generally from −10°C to 110°C.

Maximum-Minimum Thermometer

• A maximum-minimum thermometer is used to measure the daily temperature in order to prepare weather reports.

Scales of Temperature

• A reference scale with respect to which the temperatures can be measured is known as the scale of temperature.

• There are different scales of temperature.

Transfer of Heat

• Heat is transferred from a body at a higher temperature to a body at a  lower temperature, i.e. heat flows from a hot object to a cold object.

• The fundamental modes of heat transfer are conduction, convection and radiation.

Conduction

• Conduction is the process by which heat is transferred from the hotter end of an object to the colder end of the object.

• Conductors of heat:
• Materials which allow heat to pass through them easily are called conductors.
• Metals are good conductors of heat.
• Examples: Aluminium, iron, copper, silver etc.
• Insulators or bad conductors of heat:
  • Materials which do not allow heat to pass through them easily are called insulators.
  • They are also known as poor or bad conductors of heat.
  • Examples: Plastic, wood, glass, rubber etc.
• Applications of good and bad conductors of heat in our day-to-day activities:
• Boilers in industries and cooking utensils are usually made of metals which allow heat to flow quickly through the vessel for faster boiling and cooking.
• Mercury is used as a thermometric liquid as it is a good conductor of heat.
• Wood and plastic are bad conductors of heat; so, they are used as handles of cookware.

Convection

• Convection is the transfer of heat by the movement of particles of a medium from one place to another.
• It occurs only in liquids and gases.
• Examples: Wind currents, the lower floor of a building is cooler than the upper floor, and the atmosphere at the seashore is always pleasant because of convection.

Radiation

• Radiation is the transfer of heat which does not require a medium.
• All hot bodies emit heat by the process of radiation.
• We get heat from the Sun because of radiation as there is no medium present between the atmosphere of the Earth and the Sun.

Examples:

• When we sit near a room heater, we get warmth by the process of radiation.
• A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation.
• Our body produces heat and receives heat from the surroundings by radiation.

SUMMER AND WINTER CLOTHING:

• Light colors reflect most of the heat falling on it, hence it is advised to were light colors in summer
• On the other hand, darker colors absorb most heat and helps us keep warm in the winter season.
• In recent times, hollow bricks are used for construction as they trap layers of air to keep the houses cool.

Mind Map