New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory Page 1 CHAPTER 5GAS LAWS AND KINETIC THEORY 1 Pressure Definition of pressure Microscopic View of Gas Pressure Measuring Gas Pressure
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New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
Page 1
CHAPTER 5GAS LAWS AND KINETIC THEORY
1 Pressure
Definition of pressure
Microscopic View of Gas Pressure
Measuring Gas Pressure
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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Atmospheric Pressure
2 Gas Laws Physical Quantities of a gas Number of moles: Volume: Pressure: Temperature
Boyle’s Law
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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Pressure Law
Charles’ Law
General Gas Law
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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For fixed amount of gas
Other common symbols Number of molecules: Number of molecules per mole: Mass of gas: Mass of a gas molecule:
Other forms of general gas law
[HKCEE] A fixed mass of gas at 120°C is heated at constant volume so that its pressure is tripled. Find the new temperature of the gas.
A. 40°C B. 360°C C. 633°C D. 906°C E. 1179°C
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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[HKALE] An inexpansible vessel contains 1.2 kg of gas at 300 K.What is the mass of gas expelled from the vessel if it isheated from 300 K to 400 K under constant pressure? A. 0.25 kg B. 0.3 kg C. 0.6 kg D. 0.75 kg E. 0.9 kg
[HKALE] An ideal gas is contained in two metal cylinders A andB connected by a tap, which is initially closed. The volume and pressure of the gas in the cylinders are asfollows : pressure / Pa volume / m3 A 5 x 105 11 x 10-3 B 2 x 105 4 x 10-3 When the tap connecting the two cylinders is opened,what will be the final pressure in the vessel? You mayassume that the temperature remains constant. A. 2.4 x 105 Pa B. 3.5 x 105 Pa C. 4.2 x 105 Pa D. 5.0 x 105 Pa E. 6.9 x 105 Pa
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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3 Kinetic Theory
Objective of Kinetic Theory
Explanation of physical quantities by kinetic theory: Picture of kinetic theory: Temperature Volume Pressure
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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Model for Kinetic Theory
Correspondence of physical quantities and model quantities Pressure: __________________________________________________________________ Volume: __________________________________________________________________ Temperature: __________________________________________________________________
Explanation of Boyle’s Law Statement of Boyle’s Law: _______________________________ Explanation: Fixed parameter: ________________________________________ Pressure ↑ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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Explanation of Pressure Law Statement of Pressure Law: _______________________________ Explanation: Fixed parameter: ________________________________________ Temp ↑ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________ Explanation of Charles’ Law Statement of Charles’ Law: _______________________________ Explanation: Fixed parameter: ________________________________________ Temp ↑ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________ ⇒ ________________________________________
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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[HKCEE]
The above diagram shows a mechanical model of a gas. The ball bearings are set into motion by a motor-driven vibrating platform. Which of the following statements is/are correct if the operating voltage of the motor is increased ? (1) The disc rises to a higher level. (2) The average speed of the ball bearings increases. (3) The average spacing between the ball bearings increases. A. (2) only B. (3) only C. (1) and (2) only D. (1) and (3) only E. (1), (2) and (3)
[HKCEE] A fixed mass of gas is heated at a constant pressure. Which of the following statements is/are correct ? (1) The average speed of the gas molecules increases. (2) The average spacing between the gas molecules increases. (3) The number of gas molecules remains unchanged. A. (2) only B. (3) only C. (1) and (2) only D. (1) and (3) only E. (1), (2) and (3)
[HKCEE]
The above apparatus is used to study the relationship between the pressure and volume of a fixed mass of gas at constant temperature. Which of the following can improve the accuracy of the experiment ? (1) Pressing the piston quickly (2) Using a large syringe (3) Using a short length of rubber tubing A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3)
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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[HKCEE]
The figure shows a mechanical model of a gas. A large number of ball bearings are set in motion by a
vibrating platform. The ball bearings represent gas molecules.
(a) Which property of the gas (pressure, volume or temperature) would
(i) the weight of the disc
(ii) the voltage of the d.c. supply
represent in this model. (3 marks)
(b) For a fixed amount of gas, when temperature is kept constant, its
pressure increases as the volume decreases. Describe how this
behavior of gas can be demonstrated using the model.
Describe the change in the average speed of the ball bearings and the frequency of bombardment on
the walls in this demonstration. (6 marks)
(c) In a real situation, gas molecules could keep on moving by themselves without an external energy
supply but in this model energy has to be supplied to the ball bearings continuously by the vibrating
platform. Briefly explain this difference. (3 marks)
(d) A large polystyrene ball is now placed into the cylinder. Briefly describe and explain the motion of
the polystyrene ball. (3 marks)
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4 Deriving Microscopic Equations Using Kinetic Theory
Assumption of Kinetic Theory
1. (Randomness): __________________________________________________________________ 2. (Molecular size): __________________________________________________________________ 3. (Interaction): __________________________________________________________________ 4. (Collision): __________________________________________________________________ 5. (Collision time): __________________________________________________________________ Pressure Equation
Time between two collisions: Average force by each molecule: Average force by all molecules:
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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Average pressure: Root mean square speed: Pressure Equation: Kinetic Energy and Temperature For monatomic gas, average KE:
Maxwell Distribution of Molecular Speed
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[HKALE] The r.m.s. speed of the molecule of a certain gas X is341 m s-1 at 298 K. Find the molar mass of the gas X. (Given : Universal gas constant R = 8.31 J mol-1K-1) A. 5.4 g B. 21.8 g C. 33.8 g D. 42.6 g E. 63.9 g
[HKALE] 1 mole of an ideal mono-atomic gas expands at aconstant pressure of 105 Pa from a volume of 0.02 m3to a volume of 0.03 m3. The increase in internalenergy of the gas is A. 500 J. B. 667 J. C. 1000 J. D. 1500 J. E. 2500 J.
New Senior Secondary Physics Compulsory: Heat Chapter 5 Gas Laws and Kinetic Theory
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[HKALE] Which of the following statements about anexperiment showing the Brownian motion with smokeparticles in air is/are correct? 1) The motion is caused by collisions between airmolecules and smoke particles. 2) The experiment makes it possible to see the motionof the air molecules. 3) The motion is irregular because air is a mixture ofgases, and the molecules have different masses. A. (1), (2) and (3) B. (1) and (2) only C. (2) and (3) only D. (1) only E. (3) only
[HKALE] Which of the graphs bestrepresents the distributionof molecular speeds in agas at 500 K if the dottedcurve represents thisdistribution for the samegas at 300 K?
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[HKALE] Identical containers A and B contain oxygen (O2) andhydrogen (H2) respectively. Both gases are at roomtemperature and atmospheric pressure. Which of thefollowing statements is/are true? In both containers, 1) the number of gas molecules is the same; 2) the r.m.s. speed of gas molecules is the same; 3) the frequency of collision of gas molecules with thewalls of container is the same. A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1) , (2) and (3)
[HKCEE] Which of the following can increase the average kinetic energy of the molecules of a fixed mass of gas ?
(1) increasing the volume of the gas at constant pressure (2) increasing the pressure of the gas at constant volume (3) increasing the pressure of the gas at constant temperature
A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only E. (1), (2) and (3)
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Real gas and Ideal Gas Ideal gas: Comparison: Ideal Gas Real Gas
Macroscopic
Microscopic
FINAL REMARKS
The first part of this chapter introduces how we describe and predict the macroscopic behavior of gases. The general gas law allows us to calculate the change of physical properties when a gas undergoes certain process. The second part of the chapter links the macroscopic behaviors of gases to our microscopic understanding using atomic model. The three main physical properties: gas pressure, volume and temperature are explained by kinetic model.
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