Class 7 Heat Notes for exams

Class 7 Heat Notes for exams. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

HEAT

Smart notes

Heat 

● It is a form of energy.

● It is the energy that is transferred from one body to another as a result of temperature difference.

● The S.I. unit of heat energy is Joules (J).

● The CGS unit heat energy is calorie (cal).

Relationship between Joule and calorie:

1 calorie = 4.182 Joules =4.2 Joules (approx)

1cal = 4.2 J

Temperature

● It is the degree of hotness or coldness of the body.

● it is the thermal condition of the body which would determine the direction of flow of heat when this body is placed in contact with another body.

● or, it is the property of a body or region of space determining the rate at which heat will be transferred to or from it.

● The S.I. unit of temperature is Kelvin (K).

● The other units of temperature are ________________________________.

● Temperature is measured by a device called ________________________.

There are two types of thermometer:

● Clinical thermometer

● Laboratory thermometer

Numericals:

1cal = 4.186 or 4.2 J;        K = °C + 273;       °F = 9/5 °C+32

Convert:

a. 5 cal to Joules 

b. 4.2 cal to Joules 

c. 41.86 J to cal 

d. 29.4 J to cal l. 30 °C to °F

e. 0.5 cal to J m. -40 °F to °C

f. 10 cal to J n. 90 °C to °F

g. 2.1 J to cal

h. 8.372 J to cal

i. 45 °C to K

j. Boiling point of water ( °C) to K

k. Freezing point of water ( °C) to °F

● (Least count to be discussed)

Transfer of heat

a. Conduction - it is the process of transfer of heat from the heated part of a body to the colder part through passage of energy from particle to particle without actual movement of the particles.

Conditions necessary for conduction to take place:

● two objects should be in physical contact.

● two objects should be at different temperatures.

Application:

● cooking utensils are provided with wooden handles but metallic bases

● in winters, metallic handles of wooden doors are colder

● a new quilt is warmer than an old one

● in winters, birds often swell their feathers

● eskimos make double walled houses of blocks of ice

b. Convection - It is the process of transfer of heat through a liquid or gas by the actual movement of the particles of the fluid. Portions in contact with the source of heat becomes hot, expands, becomes less dense and rise; their place is taken by colder portions, thus setting up convection current.

Application:

● exhaust fans are installed near the roof

● chimneys are installed over the gas burners in kitchen

● occurrence of land and sea breeze

● wind and ocean currents

● use of chimneys in factories

c. Radiation - It is the phenomenon of transfer of heat from the source to the receiver without any actual movement of the source or receiver. It does not need a medium to transfer heat.

⇒ Every object whose temperature is above zero Kelvin, emits radiations ( heat). The heat radiations emitted depend upon -

❖ temperature of the body

❖ nature of the radiating surface

⇒A hot object radiates heat. When it falls on other objects, a part is reflected, a part is absorbed and a part is transmitted. The temperature of an object depends upon the absorbed part of heat.

⇒ Good absorbers of heat are also good radiators

⇒ Black absorbers are better absorbers of heat and hence better radiators of heat than white and polished surfaces.

When a body cools by radiation, the rate of cooling depends on:

❖ temperature of radiating body

❖ temperature of surroundings

❖ mass of radiating body

❖ area of radiating body

❖ nature of radiating body

❖ specific heat of radiating body

Application:

● in summers, light coloured clothes are preferred

● cooking utensils are blackened at the bottom and polished at the upper surface.

● hot water pipes and radiators used in rooms painted black

● shiny, polished reflectors are used in electric heaters

● construction of a thermos flask

● fire fighters wear shining suits and helmets

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