Class 7 Heat Notes for exams

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HEAT

Smart notes

This Class 7 Science Chapter 4 Heat notes page for exams explains heat as a form of energy, the units of heat and temperature, the clinical and laboratory thermometers, the conversion of calories to joules and of Celsius to Kelvin and Fahrenheit, and the three modes of heat transfer: conduction, convection and radiation, with their everyday applications.

HEAT

• Heat is a form of energy.
• It is the energy that is transferred from one body to another as a result of a temperature difference.
• The S.I. unit of heat energy is joule (J).
• The CGS unit of heat energy is calorie (cal).

Relationship between joule and calorie:
1 calorie = 4.182 joules = 4.2 joules (approx.)
1 cal = 4.2 J

TEMPERATURE

• Temperature is the degree of hotness or coldness of a body.
• It is the thermal condition of a body which determines 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 degree Celsius (°C) and degree Fahrenheit (°F).
• Temperature is measured by a device called a thermometer.

There are two types of thermometer:
• Clinical thermometer
• Laboratory thermometer

Difference between clinical and laboratory thermometer

NUMERICALS

Numericals: 1 cal = 4.186 or 4.2 J; K = °C + 273; °F = \( \frac{9}{5} \) °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
e. 0.5 cal to J
f. 10 cal to J
g. 2.1 J to cal
h. 8.372 J to cal
(Least count to be discussed)
i. 45 °C to K
j. Boiling point of water (°C) to K
k. Freezing point of water (°C) to °F
l. 30 °C to °F
m. -40 °F to °C
n. 90 °C to °F

Answer:
a. 5 cal = 5 × 4.186 = 20.93 J
b. 4.2 cal = 4.2 × 4.186 = 17.58 J
c. 41.86 J = 41.86 / 4.186 = 10 cal
d. 29.4 J = 29.4 / 4.186 = 7.02 cal
e. 0.5 cal = 0.5 × 4.186 = 2.09 J
f. 10 cal = 10 × 4.186 = 41.86 J
g. 2.1 J = 2.1 / 4.186 = 0.50 cal
h. 8.372 J = 8.372 / 4.186 = 2 cal
i. 45 °C = 45 + 273 = 318 K
j. Boiling point of water = 100 °C = 100 + 273 = 373 K
k. Freezing point of water = 0 °C, so °F = \( \frac{9}{5} \) × 0 + 32 = 32 °F
l. 30 °C, so °F = \( \frac{9}{5} \) × 30 + 32 = 86 °F
m. -40 °F, so °C = \( \frac{5}{9} \) × (-40 - 32) = -40 °C
n. 90 °C, so °F = \( \frac{9}{5} \) × 90 + 32 = 194 °F

Teacher's Note:
a) Use the same value of the calorie in every conversion, either 4.186 or 4.2 J.
b) For Fahrenheit, multiply by 9/5 first and then add 32.
c) Kelvin is found by adding 273 to the Celsius value, never by subtracting.

TRANSFER OF HEAT

a. Conduction is the process of transfer of heat from the heated part of a body to the colder part through the 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.

Applications of conduction:
• 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 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 become hot, expand, become less dense and rise. Their place is taken by colder portions, thus setting up a convection current.

Applications of convection:
• Exhaust fans are installed near the roof.
• Chimneys are installed over the gas burners in the kitchen.
• Occurrence of land and sea breezes.
• Wind and ocean currents.
• Use of chimneys in factories.

c. Radiation 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 the temperature of the body and the 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 surfaces 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 the radiating body
• Temperature of the surroundings
• Mass of the radiating body
• Area of the radiating body
• Nature of the radiating body
• Specific heat of the radiating body

Applications of radiation:
• 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 are painted black.
• Shiny, polished reflectors are used in electric heaters.
• Construction of a thermos flask.
• Fire fighters wear shining suits and helmets.

❖ temperature of radiating body

❖ temperature of surroundings

❖ mass of radiating body

❖ area of radiating body

❖ nature of radiating body