Following notes is of Heat and Temperature chapter of the second unit of Physics 'Heat and Thermodynamics' of Grade 11.

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Grade 11 NEB Physics Chapter List | Notes and Solution Guide

Heat and Temperature

Important Point

➢ A body contains internal energy but it does not contain heat energy because heat energy is the energy in flow.

Internal Energy

➢ Internal energy is the total energy contained by a body. It is mainly the sum of Kinetic Energy and Potential Energy of a body. i.e. U = KE + PE

Heat Energy

➢ Heat energy is the energy in flow between two or more bodies due to the temperature differences.

Flow of Heat Energy

➢ When two bodies can conduct heat, it always flows from a body at higher temperature to another body at lower temperature, regardless of their internal energy.
➢ After the transfer of heat, internal energy of aa hot body decreases while that of a cold body increases.
➢ The SI unit of Heat is Joule and CGS unit is Calorie.
➢ 1 Calorie = 4.2 Joule

Temperature

➢ Temperature is a fundamental physical quantity that is a measure of how hot or cold a body is.
➢ The SI unit of temperature is Kelvin and CGS unit is Degree Celsius.

Thermal Equilibrium

➢ When two bodies are in thermal contact, the state at which there is no net flow of heat energy is said to be thermal equilibrium.
➢ If two bodies are connected to each other by a conducting surface, then the temperature of both the bodies must be same to attain thermal equilibrium.
➢ If two bodies are completely insulated or the system are isolated, bodies can be in thermal equilibrium without having the same temperature.
➢ Thermal Equilibrium = No net flow of Heat Energy
➢ Even when two bodies are in thermal equilibrium there can be flow of energies or mutual exchange of energies but the energies in flow are equal and cancel each other. Therefore, there is no net flow of energy.

Zeroth Law of Thermodynamics

➢ It is the most fundamental law of thermodynamics.
➢ Zeroth law of thermodynamics states, "If two bodies A and B separated by an insulating wall are separately in thermal equilibrium with another body C, bodies A and B are also in thermal equilibrium with each other."
➢ This is to say: if $A = C$ and $B = C$ $\implies A = B$

Q. Why is Zeroth law named so?

A. Zeroth law was introduced much lateral after the introduction of other laws, namely first law and second law of thermodynamics. But this law was fundamental to all of them. So, it was named as the Zeroth law of thermodynamics.

Q. How does the Zeroth law of thermodynamics give the concept of temperature?

A. For bodies to satisfy zeroth law, they must be at thermal equilibrium. And, in thermal equilibrium, bodies have the same temperature. So, zeroth law of thermodynamics gives the concept of temperature in the above-mentioned way.

Thermometer

➢ Thermometer is an instrument used to measure the temperature of a substance using principle of thermal expansivity.
➢ It works on the principle that the physical properties of substance such as volume, pressure, resistance, etc. show changes with increase or decrease in temperature.
➢ It always measures its own temperature and shows the accurate reading of a body's temperature once it attains thermal equilibrium with that body.

➢ Following are the types of thermometer:

  • Gas Thermometer: It uses gas as a thermometric substance and works on the basis of either change in volume or change in pressure with temperature when one of the two properties is kept constant. They are the most sensitive thermometers. Eg: Constant Volume Hydrogen Thermometer.
  • Vapour Pressure Thermometer: These thermometers are based on the principle of change of vapour pressure with change in temperature. They are also called as Cryometers. Eg: Helium vapour pressure thermometer.
  • Radiation Thermometer: These thermometers are based on the quantity of heat radiations emitted by a body. They are also known as pyrometers.
  • Resistance Thermometer: It is based on the principle of change in resistance with change in temperature. Eg: Platinum resistance thermometer.
  • Thermo-electric Thermometer: It is based on the principle of thermo-electricity.
  • Liquid Thermometer: It is based on the principal of change in volume of liquid with change in temperature.

Types of Liquid Thermometer

There are mainly two types of liquid thermometer which are explained below:

a) Mercury Thermometer

➢ It is a liquid thermometer that uses mercury as a thermometric liquid.➢ It has a bulb with a kink or constriction to prevent the backflow of mercury.

Advantages:

  1. Mercury's boiling point is 357 degree C and melting point is -39 degree C. So, it can be used to measure a large range of temperature.
  2. It has low specific heat capacity and high thermal conductivity.
  3. It has uniform expansion over a wide range of temperature.
  4. it does not wet the glass of the tube.
  5. It is opaque and shining in color.

Disadvantages:

  1. Its melting point is -39 degree C and boiling point is 357 degree C. So, it cannot measure temperature below or above this range.
  2. Mercury is a poisonous liquid.

b) Alcohol Thermometer

➢ It is a liquid thermometer that uses alcohol (ethanol) as a thermometric substance.➢ It does not have any kink.

Advantages:

  1. Its freezing point is -117 degree C so, it can measure temperature of very cold places.
  2. It is more sensitive thermometer as its rate of expansion is seven times more than that of mercury.

Disadvantages:

  1. Its expansion is not uniform.
  2. It has limitation in measuring hot temperature since its boiling point is just 78 degree C.
  3. It is colorless and bad conductor of heat.
  4. It sticks to the wall of the glass tube.

Temperature Scales

Tempearture ScaleLFPUFPNo. of divisions
Centigrade or Celsius Scale ($^O$ C)0100100
Fahrenheit Scale ($^O$ F)32212180
Kelvin Scale (K)273373100
Reaumer Scale ($^O$ Re)08080
Rankine Scale ($^O$ Rn)492672180

Conversion of Temperature from one scale to another

➢ Let one scale be represented by x and the other by y.

$\dfrac{x - L.F.P._x}{U.F.P._x - L.F.P._x} = \dfrac{y - L.F.P._y}{U.F.P._y - L.F.P._x}$

➢ From the above-mentioned formula, we can obtain the following relation:

$\dfrac{C-0}{100-0} = \dfrac{F- 32}{212-32} = \dfrac{K-273}{373-273}$

$or, \dfrac{C}{100} = \dfrac{F-32}{180} = \dfrac{K-273}{100}$

➢ $\therefore, \dfrac{C}{5} = \dfrac{F-32}{5} = \dfrac{K-273}{5}$

Difference in Temperature

$\triangle C = \triangle K = \dfrac{5}{9} \triangle F = \dfrac{5}{4} \triangle Re$

Faulty and Accurate Temperature Scale

$\dfrac{Accurate - L.F.P._{accurate}}{U.F.P._{accurate} - L.F.P._{accurate}} = \dfrac{Faulty - L.F.P._{faulty}}{U.F.P._{faulty} - L.F.P._{faulty}}$

Absolute Zero Temperature

➢ Absolute Temperature is the temperature measured in Kelvin scale.

➢ 0 kelvin temperature is regarded as the absolute zero temperature.

➢ The lowest possible temperature, theoretically, in which the molecular motion of substances ceases is called absolute zero temperature.

➢ In absolute zero temperature, either the volume or pressure becomes zero.

➢ It has not been achieved practically, so far.

➢ The molecular motion of a substance ceases (or stops).

➢ Kinetic Energy of a substance becomes zero. However, there is some energy in the body in the form of potential energy.

Mercury Thermometer

Construction of Mercury Thermometer

➢ A thin capillary tube is taken which is attached with a cylindrical bulb at one end and the other end is kept open for filling mercury inside.

➢ All these objects are placed within a cylindrical vessel or tube.

➢ Then mercury is filled by alternate heating and cooling until it reaches the full height of the capillary tube. Then, the thermometer is completely covered.

Calibration of Mercury Thermometer

➢ To calibrate the Lower Fixed Point, the thermometer is dipped in a container of ice at its melting point. The system should be made air-tight and at normal room temperature and pressure.

➢ The ice that melts is collected in a beaker.

➢ To calibrate Upper Fixed Point, the thermometer is placed inside a beaker (flask) which has boiling water inside it.

➢ The thermometer must not be dipped inside the boiling water because:

  • Water is heated by convection process
  • There is chances of thermal expansion of bulb of the thermometer.

➢ There must be an outlet for the steam to pass away to prevent crack in the beaker (flask).

➢ Thermometer shall be calibrated once it maintains a stable height of mercury in the capillary tube.

➢ Once the LFP and UFP are calibrated, the thermometer is divided into sub-divisions according to its temperature scale.