ICSE Class 8 Physics Notes Chapter 6 Heat Transfer

ICSE Physics Class 8 Notes Chapter 6 Heat Transfer

→ Heat is a form of energy. When two bodies are in contact heat flows from body at higher temperature to body at lower temperature till the lower temperature of both is same.

→ When a body is heated, its molecules move faster about their means position and kinetic energy increases and with fall in temperature their K.E. decreases.

→ When a substance is heated

• It expands i.e. a change in size takes place
• Change in temperature takes place.
• Change in state takes place.

→ CHANGE OF STATE : “The process of change from one state to another at a constant temperature is called change of state.”

→ Solid on heating changes into LIQUID. LIQUID on absorbing heat changes to VAPOURS some SOLIDS on heating DIRECTLY change in vapours called SUBLIMATION. Substance is called SUBLIMATE. SOLIDIFICATION on cooling when a vapours change into SOLID.

→ GAS OR VAPOURS on cooling img changes to LIQUID also called LIQUIFACTION.

→ MELTING : Change of solid into liquid at constant temperature. FUSION FREEZING is change of LIQUID into SOLID at constant temperature and change of solid into liquid at a constant temperature is called FUSION.

→ EVAPORATION : “Change liquid to gas at ALL
TEMPERATURES” It is surface phenomenon.

→ VAPOURIZATION : “Change of liquid into vapours at fixed temperature”.

→ METING POINT : “Is the temperature at which a solid starts melting and remains constant till the whole of solid melts.”

• M.P. is same as freezing point.
• M.P. of ice is 0°C or freezing point, of water is 0°C.

→ BOILING POINT: “Is the temperature of a liquid at which it start, boiling i.e. change into vapours or gaseous state.” B.P. of pure water is 100°C.

→ ABSOLUTE ZERO : “The temperature at which molecular motion completely ceases.”

→ FACTORS EFFECTING THE RATE OF EVAPORATION :

• Temperature: Increases with increase in temperature
• S.A.: Increases with increase in S.A.
• BLOWING AIR – Renewal of air increases evaporation.
• NATURE – Some liquids like spirit, alcohol, petrol evaporate easily.

→ EVAPOURATION → produces coolness, BOILING produces Hotness.

→ LINEAR EXPANSION : When a solid rod (metal) is heated change in length takes place, which depends upon

• original length(L₀)
• Increase in temperature
• Material of rod.

Let L₀ be the original length at 0°C, when heated to T°C final length becomes L_t Increase in length (L_T – L₀) α L₀ (T – 0)
Or
Coefficient of linear expension a which depends upon material of rod.
L_t – L₀ = L₀ α T

→ When a metal plate is heated, change in area takes place and the expansion is called SUPERFICIAL expansion.

→ When a solid of volume V₀ is heated change in volume called cubical expansion takes place.

→ α : ß : γ = 1 : 2 : 3

ICSE Class 8 Physics Notes