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Heat and temperature.pdf

  1. Physics Xavier International College Shubheksha Dhakal Lecturer, Physics GRADE: 11
  2. 9.1 Molecular concept of thermal energy, heat and temperature and cause and direction of heat flow 9.2. Meaning of thermal equilibrium and zeroth law of thermodynamics 9.3 Thermal equilibrium as a working principle of mercury thermometer Key Points Teaching period= 3 Chapter- 9 Heat and Temperature
  3. NO Yes
  4. Molecular Concept of Thermal Energy: Internal energy : Kinetic energy + potential energy ➢ Total internal kinetic energy possessed by the molecules present in the system is called thermal energy. ➢ It is also known as random or internal kinetic energy, due random motion of molecules. ➢ In solid and liquid case, atoms, molecules vibrate at their mean position. ➢ In gases, molecules move freely collide with each other and in walls of vessels, thus posses kinetic energy. ➢ Kinetic energy is seen in three forms: Translational , Rotational and Vibrational. Figure: A system at certain temperature 3 Heat of thermodynamics
  5. Thermal Energy: ➢ Faster the molecules move higher will be the temperature. ➢ Warmth of sun, fire
  6. ➢ The thermal energy of the system is dependent on the temperature of the system which further depends on the motion of molecules of the system. ➢ If a cup of boiling tea is placed next to the freezing pool, the cup of tea will freeze first because it has less thermal energy. ➢ The relatively small sample at a somewhat high temperature such as cup of tea at its boiling condition could have less thermal energy than the large sample such as pool that is at low temperature. ➢ Even though two materials might have the same temperature, their thermal energies might be different due to how many particles are present in the object. ➢ For example, a glass of water can have the same temperature as a ocean , but the ocean has much more thermal energy because the ocean contains many more water molecules.
  7. Cause and direction of heat flow: ➢ Heat flow occur due to the more energetic molecular activity in the hot material transferring some of its kinetic energy to adjacent cooler materials. ➢ Heat transfer occur from high temperature to low temperature. 50°C 10°C 50°C 10°C Figure: A heat transfer by conduction method Heat and Temperature
  8. Thermal Equilibrium ➢ Thermal contact ➢ When two bodies of different temperature are brought in contact with each other, then the heat flow from hot body to cold body and finally attains the condition where heat flow ceases and both attain same temperature. This is the condition of thermal equilibrium. ➢ When two thermometer read same temperature of different objects then these objects are in condition of thermal equilibrium. Iron ball T1 Cold water T2 Heat and Temperature
  9. ZEROTH LAW OF THERMODYNAMICS ➢ If two system are in thermal equilibrium with the third system then they must be in thermal equilibrium with each other. B C A Conducting wall Figure: zeroth law of Thermodynamics B C A Conducting wall ➢ An example : Consider two glasses of water in which One glass will have hot water and the other will contain cold water. Now if we leave them in the table for a few hours they will attain thermal equilibrium with the temperature of the room. Heat and Temperature
  10. Explanation: ➢ Let us consider two systems B and C separated by an insulating wall, each being in contact with the third system A as shown in figure in previous slide. After sometime, the system B will be in thermal equilibrium with the system A and the system C will be in thermal equilibrium with the system A. i.e. ➢ the temperature of system A and B will be equal, similarly the temperature of system A and C will be equal. Now, the insulating wall between system B and C will be replaced by conducting wall, then experiment showed that no further change in temperature takes place in system B and C indicating thermal equilibrium in between system B and C. ➢ Therefore, when two system are in thermal equilibrium with the third system, the first two system themselves are also in thermal equilibrium which is called zeroth law of thermodynamics. Heat and Temperature
  11. Thermometer: An instrument which is used to measure the temperature is called thermometer. The study of temperature and its measurement is known as thermometry. Types of thermometer: 1. Liquid thermometer 2. Gas thermometer 3. Resistance thermometer 4. Thermo-electric thermometer 5. Radiation thermometer 6. Vapour pressure thermometer Heat and Temperature
  12. Temperature Scales Lower Fixed points: ➢ The temperature at which ice melts under atmospheric pressure i.e 760mmHg is L.F.P ➢ It is also called ice point. Upper Fixed Points: ➢ The temperature at which pure water boils under a standard atmospheric pressure i.e. 760mmHg is U.F.P ➢ It is also called steam point. ➢ Write about different temperature scales in detail for making notes.
  13. Relation Between Different Temperature Scales: If C, F,R, and K be the temperature of a body in celsius, Fahrenheit, Reameur, kelvin scale respectively, then we have, Reading point –ice point =Constant Steam point – ice point = = = 𝐶 − 0 𝐹 − 32 𝑅 − 0 𝐾 − 273 100 − 0 212 − 32 80 − 0 373 − 273 𝐶 𝐹 − 32 100 180 = = 80 = 𝑅 𝐾 − 273 100 = = = 𝐶 𝐹 − 32 𝑅 𝐾 − 273 5 9 4 5 Heat and Temperature
  14. °F °C (C,F) 0°C 32 °F Since similar triangles i.e ABC and ADF ratio of sides are equal. 𝐶 − 0 𝐹 − 32 100 − 0 = 212 − 32 𝐶 𝐹 − 32 100 = 180 100 °C A D C F 1B 00,212 Heat and Temperature 𝐴 𝐹 𝐴 𝐶 = 𝐷 𝐹 𝐷 𝐶
  15. Numerical questions 1. Calculate the value of temperature at which Fahrenheit scale and centigrade scales give same reading. Solution: Let value of temperature at Fahrenheit scalebe F and that of centigrade scale be C. Let us assume, F=C= 𝑥 Heat and Temperature X = -40 degree or -40 F 𝐹 − 32 9 = 𝐶 5 𝑥 − 32 9 = 𝑥 5 5 (x-32) = 9 𝑥 5 x-160 -160 = 9x-5x -160 =4x X = -40 = 9x Then, Using Relation of temperature scale
  16. 2. A faulty thermometer has its fixed point is marked 2°C and 98°C. Temperature of a body as measured by the faulty thermometer is 64°C. Find the correct temperature of the body on correct Celsius scale. Soln, The lower fixed point( 𝐿 . 𝐹 . 𝑃 ) :2°C The upper fixed point ( 𝑈 . 𝐹 . 𝑃 ) : 98°C Temperature of faulty thermometer: 64°C The correct temperature of thermometer= ? Then, Thus, the correct temperature of the body on correct Celsius scale is 64.58°C . Heat and Temperature C on correct scale − (L . F . P)correct (U . F . P)correct − (L . F . P)correct = Faculty on scale − (L . F . P)faculty (U . F . P)faculty − (L . F . P)faculty c= =64.58 𝐶 − 0 100 − 0 = 64 − 2 98 − 2 𝐶 100 = 62 96 6200
  17. Classwork Q. At what temperature does the kelvin scale coincide with Fahrenheit scale ?[HSEB2061]
  18. Mercury Thermometer and its construction ➢ It is one of the most familiar used thermometer as shown in figure aside. Working Principle: ➢ Based on the principle of Thermal equilibrium ➢ A glass tube is filled with mercury and standard scale is marked on the tube. ➢ We can read the temperature from the scale when the thermometer and contact body are in thermal equilibrium with each other. ➢ used to determine body, liquid, and vapor temperature. ➢ Householdslaboratory experiments and i n d u s t r i a l applications. Figure :Mercury thermometer Heat and Temperature
  19. Advantages of Mercury thermometer: ➢ It is visible, because its color is silvery shining. ➢ Its expansion is regular and uniform. ➢ It does not wet the wall of the glass and can be easily obtained in pure state. ➢ It’s boiling point is 357°C and freezing point is -39°C. Therefore, it can be used over a wide range of temperature. ➢ It has low specific heat, so it doesn’t take much heat from the body whose temperature is to be measured. Disadvantages of Mercury Thermometer: ➢ It cannot measure the temperature below -39°C and above 357°C.Therefore this thermometer cannot be used in Arctic and Antarctic region. ➢ Mercury is poisonous, so it is harmful if its tube is broken. Heat and Temperature
  20. Construction of mercury Thermometer : ➢ A glass containing fine capillary tube having one bulb at one end is taken. ➢ The bulb is filled with pure and dry mercury by alternate heating and cooling to remove air bubbles present in it. ➢ The glass is kept in hot water bath having temperature higher than the temperature it has to measure. ➢ After all the air exhausted from the glass, open end of the tube is sealed. Then, the tube of mercury is kept for cooling before marking fixed points.
  21. Determination of fixed points: ➢ Tomark a lower fixed point, the bulb of a thermometer is kept in melting ice as shown in figure. ➢ Tomark upper fixed point, the bulb of a thermometer is kept inside hypsometer containing water inside as shown in figure. Note: Write about construction of mercury in detail from book
  22. Absolute zero Temperature: At lowest possible value of temperature, all molecular motion does not cease but it vibrates with lowest possible energy and this energy from molecular motion cannot be transferred to other system. Thus, this lowest possible value of temperature where molecules are at lowest energy value is called absolute zero temperature. Value = -273.15°C
  23. Absolute scale of temperature: ➢ Kelvin scale is known as absolute scale. ➢ Scale of temperature whose zero corresponds to absolute zero of the temperature(i.e. -273.15°C) i.e 0K=-273.15°C Adding 273.15 on both sides, 273.15 K= 0°C Similarly adding T on both sides, (T+273.15) K= T°C Thus , T absolute is (T +273.15) K
  24. Assignments Complete exercise from principle of physics Textbook