Chapter 4
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
Also Read: – Class 6th Notes
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