Physics - Grade XI or Standard XI
Chapter 7: Thermodynamics
Temperature:
It is a parameter that tells us how hot or cold a given body is.
Ice point or freezing point of water:
At standard pressure, the temperature at which pure water freezes at standard atmospheric pressure is called as ice point or freezing point of water.
Boiling point or steam point of water:
At standard pressure, the temperature at which pure water boils at standard atmospheric pressure is called as boiling point or steam point of water.
Celsius scale:
The temperature scale on which ice point (freezing point) is marked at 0oC (to be read as zero degree Celsius) and the steam point (boiling point) is marked at 100oC (to be read as one hundred degree Celsius) is called celsius scale. The interval between these points is divided into 100 equal parts and each part or division is called as one degree Celsius and is written as 1oC.
Fahrenheit scale:
The temperature scale on which ice point (freezing point) is marked at 32oF (to be read as thirty two degree Fahrenheit) and the steam point (boiling point) is marked at 212oF (to be read as two hundred twelve degree Fahrenheit) is called Fahrenheit scale. The interval between these points is divided into 180 equal parts and each part or division is called as one degree Fahrenheit and is written as 1oF.
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Kelvin scale:
The temperature scale that has its zero at – 273.15oC and temperature intervals are same as that on the Celsius scale is called Kelvin scale or absolute scale. Thus ice point on Kelvin scale is at 273.15 K (to be read as two seventy three point fifteen Kelvin) and boiling point on Kelvin scale is at 373.15 K.
Boyle’s law:
Boyle’s law states that at constant temperature, the volume of a given mass of a gas is inversely proportional to its pressure.
Ideal gas equation:
The relation between three variables of a gas, namely, pressure, volume and temperature, is called an ideal gas equation, and it is expressed as follows:
PV = RT
where P = pressure, V = volume, T = temperature, and R = 8.31 J.K-1mol-1 = univesal gas constant.
Coefficient of linear expansion (α):
Coefficient of linear expansion of a solid (α) is defined as increase in the length per unit original length at 0oC per degree rise in temperature. SIU of α is per degree celcius or per kelvin ( /oC or /K).
Coefficient of areal expansion (β):
Coefficient of areal expansion of a solid (β) is defined as increase in the area per unit original area at 0oC per degree rise in temperature. SIU of β is per degree celcius or per kelvin (/oC or /K).
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Coefficient of cubical expansion (γ):
Coefficient of cubical expansion of a solid (γ) is defined as increase in the volume per unit original volume at 0oC per degree rise in temperature. SIU of γ is per degree celcius or per kelvin ( /oC or /K).
Specific heat or specific heat capacity (c):
Specific heat or specific heat capacity (c) of a substance is defined as the quantity of heat required to raise the temperature of unit mass of a substance through 1oC or 1 K. SIU of specific heat is Jkg-1K-1.
Specific heat of a gas at constant volume (cv):
Specific heat of a gas at constant volume (cv) is defined as the quantity of heat required to raise the temperature of unit mass of a gas through 1oC or 1 K at constant volume.
Specific heat of a gas at constant pressure (cp):
Specific heat of a gas at constant pressure (cp) is defined as the quantity of heat required to raise the temperature of unit mass of a gas through 1oC or 1 K at constant pressure.
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Notice:
The specific heats defined for unit mass of a gas are also called principal specific heats. The specific heats defined for one mole of a gas are called molar specific heats.
Molar specific heat of a gas at constant volume (Cv):
Molar specific heat of a gas at constant volume (Cv) is defined as the quantity of heat required to raise the temperature of one mole of a gas through 1oC or 1 K at constant volume.
Molar specific heat of a gas at constant pressure (Cp):
Specific heat of a gas at constant pressure (Cp) is defined as the quantity of heat required to raise the temperature of one mole of a gas through 1oC or 1 K at constant pressure.
Relation between molar and principal specifc heats of gases:
Relation between molar and principal specific heats of gases is given by the following expression:
molar specific heat = molecular weight × principal specific heat
Calorimetry:
Calorimetry means measurement of heat.
Change of state:
Matter has three states: solid, liquid, and gas. Transition from one of these states to another state is called as change of state.
Melting point of substance:
At standard pressure, the temperature at which a substance changes its state from solid to liquid is called as its melting point.
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Boiling point of substance:
At standard pressure, the temperature at which a substance changes its state from liquid to gas is called as its boiling point.
Triple point of water:
Triple point of water is that temperature and pressure at which water exists in all three states, namely, solid (ice), liquid (water), and gas (steam). The triple point of water is as follows: pressure is 4.58 mm of Hg and temperature is 273.16 K (0.01oC).
Latent heat of a substance:
The quantity of heat required to change the state of unit mass of the substance without changing its temperature is called as latent heat of substance. SIU of latent heat is J/kg.
Latent heat of fusion:
The quantity of heat required to convert unit mass of a substance from its solid state to liquid state, at its melting point, without change in its temperature is called its latent heat of fusion.
Latent heat of vaporisation:
The quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point, without change in its temperature is called its latent heat of vaporisation.
Modes of heat transfer:
Heat always flows from region of higher temperature to region of lower temperature and this flow of heat is called heat transfer. There are three modes of heat transfer, namely, conduction, convection, and radiation.
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Conduction:
Conduction is that mode of heat transfer in which heat is transferred from hot end to cold end of a substance without actual migration of molecules or atoms of substance.
Convection:
Convection is that mode of heat transfer in which molecules or atoms of substance actually migrate from hot end to cold end.
Radiation:
Radiation is that mode of heat transfer in which heat is transferred in the form of electromagnetic waves.
Coefficient of thermal conductivity (K):
The coefficient of thermal conductivity (K) of material is defined as the quantity of heat conducted in the steady state through a material per unit time, per unit cross sectional area, per unit temperature gradient. SIU of coefficient of thermal conductivity (K) is J/(msK).
Temperature gradient:
In steady state there is a uniform fall of temperature along the length of the rod from hot end to cold end. If dx be the length of a small section of the metal rod and dθ be the difference between the steady temperatures at the two ends of section, then dθ/dx is called temperature gradient.
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Newton’s law of cooling:
Newton’s law of cooling states that the rate of fall of temperature of a body is directly proportional to (θ – θ0) where θ is the temperature of the body and θ0 is the temperature of the surrounding. Thus:
dθ/dt α (θ – θ0) OR dθ/dt = k (θ – θ0)
where dθ/dt is rate of cooling of the body, dθ is fall in temperture of the body in time dt, and k is the constant of proportionality. This law holds good only for small temperature difference (about 40oC).
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Temperature:
It is a parameter that tells us how hot or cold a given body is.
Ice point or freezing point of water:
At standard pressure, the temperature at which pure water freezes at standard atmospheric pressure is called as ice point or freezing point of water.
Boiling point or steam point of water:
At standard pressure, the temperature at which pure water boils at standard atmospheric pressure is called as boiling point or steam point of water.
Celsius scale:
The temperature scale on which ice point (freezing point) is marked at 0oC (to be read as zero degree Celsius) and the steam point (boiling point) is marked at 100oC (to be read as one hundred degree Celsius) is called celsius scale. The interval between these points is divided into 100 equal parts and each part or division is called as one degree Celsius and is written as 1oC.
Fahrenheit scale:
The temperature scale on which ice point (freezing point) is marked at 32oF (to be read as thirty two degree Fahrenheit) and the steam point (boiling point) is marked at 212oF (to be read as two hundred twelve degree Fahrenheit) is called Fahrenheit scale. The interval between these points is divided into 180 equal parts and each part or division is called as one degree Fahrenheit and is written as 1oF.
Kelvin scale:
The temperature scale that has its zero at – 273.15oC and temperature intervals are same as that on the Celsius scale is called Kelvin scale or absolute scale. Thus ice point on Kelvin scale is at 273.15 K (to be read as two seventy three point fifteen Kelvin) and boiling point on Kelvin scale is at 373.15 K.
Boyle’s law:
Boyle’s law states that at constant temperature, the volume of a given mass of a gas is inversely proportional to its pressure.
Ideal gas equation:
The relation between three variables of a gas, namely, pressure, volume and temperature, is called an ideal gas equation, and it is expressed as follows:
PV = RT
where P = pressure, V = volume, T = temperature, and R = 8.31 J.K-1mol-1 = univesal gas constant.
Coefficient of linear expansion (α):
Coefficient of linear expansion of a solid (α) is defined as increase in the length per unit original length at 0oC per degree rise in temperature. SIU of α is per degree celcius or per kelvin ( /oC or /K).
Coefficient of areal expansion (β):
Coefficient of areal expansion of a solid (β) is defined as increase in the area per unit original area at 0oC per degree rise in temperature. SIU of β is per degree celcius or per kelvin (/oC or /K).
Coefficient of cubical expansion (γ):
Coefficient of cubical expansion of a solid (γ) is defined as increase in the volume per unit original volume at 0oC per degree rise in temperature. SIU of γ is per degree celcius or per kelvin ( /oC or /K).
Specific heat or specific heat capacity (c):
Specific heat or specific heat capacity (c) of a substance is defined as the quantity of heat required to raise the temperature of unit mass of a substance through 1oC or 1 K. SIU of specific heat is Jkg-1K-1.
Specific heat of a gas at constant volume (cv):
Specific heat of a gas at constant volume (cv) is defined as the quantity of heat required to raise the temperature of unit mass of a gas through 1oC or 1 K at constant volume.
Specific heat of a gas at constant pressure (cp):
Specific heat of a gas at constant pressure (cp) is defined as the quantity of heat required to raise the temperature of unit mass of a gas through 1oC or 1 K at constant pressure.
Notice:
The specific heats defined for unit mass of a gas are also called principal specific heats. The specific heats defined for one mole of a gas are called molar specific heats.
Molar specific heat of a gas at constant volume (Cv):
Molar specific heat of a gas at constant volume (Cv) is defined as the quantity of heat required to raise the temperature of one mole of a gas through 1oC or 1 K at constant volume.
Molar specific heat of a gas at constant pressure (Cp):
Specific heat of a gas at constant pressure (Cp) is defined as the quantity of heat required to raise the temperature of one mole of a gas through 1oC or 1 K at constant pressure.
Relation between molar and principal specifc heats of gases:
Relation between molar and principal specific heats of gases is given by the following expression:
molar specific heat = molecular weight × principal specific heat
Calorimetry:
Calorimetry means measurement of heat.
Change of state:
Matter has three states: solid, liquid, and gas. Transition from one of these states to another state is called as change of state.
Melting point of substance:
At standard pressure, the temperature at which a substance changes its state from solid to liquid is called as its melting point.
Boiling point of substance:
At standard pressure, the temperature at which a substance changes its state from liquid to gas is called as its boiling point.
Triple point of water:
Triple point of water is that temperature and pressure at which water exists in all three states, namely, solid (ice), liquid (water), and gas (steam). The triple point of water is as follows: pressure is 4.58 mm of Hg and temperature is 273.16 K (0.01oC).
Latent heat of a substance:
The quantity of heat required to change the state of unit mass of the substance without changing its temperature is called as latent heat of substance. SIU of latent heat is J/kg.
Latent heat of fusion:
The quantity of heat required to convert unit mass of a substance from its solid state to liquid state, at its melting point, without change in its temperature is called its latent heat of fusion.
Latent heat of vaporisation:
The quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point, without change in its temperature is called its latent heat of vaporisation.
Modes of heat transfer:
Heat always flows from region of higher temperature to region of lower temperature and this flow of heat is called heat transfer. There are three modes of heat transfer, namely, conduction, convection, and radiation.
Conduction:
Conduction is that mode of heat transfer in which heat is transferred from hot end to cold end of a substance without actual migration of molecules or atoms of substance.
Convection:
Convection is that mode of heat transfer in which molecules or atoms of substance actually migrate from hot end to cold end.
Radiation:
Radiation is that mode of heat transfer in which heat is transferred in the form of electromagnetic waves.
Coefficient of thermal conductivity (K):
The coefficient of thermal conductivity (K) of material is defined as the quantity of heat conducted in the steady state through a material per unit time, per unit cross sectional area, per unit temperature gradient. SIU of coefficient of thermal conductivity (K) is J/(msK).
Temperature gradient:
In steady state there is a uniform fall of temperature along the length of the rod from hot end to cold end. If dx be the length of a small section of the metal rod and dθ be the difference between the steady temperatures at the two ends of section, then dθ/dx is called temperature gradient.
Newton’s law of cooling:
Newton’s law of cooling states that the rate of fall of temperature of a body is directly proportional to (θ – θ0) where θ is the temperature of the body and θ0 is the temperature of the surrounding. Thus:
dθ/dt α (θ – θ0) OR dθ/dt = k (θ – θ0)
where dθ/dt is rate of cooling of the body, dθ is fall in temperture of the body in time dt, and k is the constant of proportionality. This law holds good only for small temperature difference (about 40oC).