Thermal physics revision questions

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Sherilyn Brooks
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Thermal physics revision questions ONE SECTION OF QUESTIONS TO BE COMPLETED AND MARKED EVERY WEEK AFTER HALF TERM. Section 1: Energy 1. Define the law of conservation of energy.

ΔQ = ΔW + ΔU All energies so are all measured in Joules. 3. What are the equations for kinetic energy and gravitational potential energy.

1 Thermal physics revision questions ONE SECTION OF QUESTIONS TO BE COMPLETED AND MARKED EVERY WEEK AFTER HALF TERM. Section 1: Energy 1. Define the law of conservation of energy. Energy is neither created nor destroyed, only transferred from one for to another. 2. State the first law of thermodynamics along with all the units. ΔQ = ΔW + ΔU All energies so are all measured in Joules. 3. What are the equations for kinetic energy and gravitational potential energy. Kinetic energy: KE = 1 2 mv2 Gravitational potential energy: GPE = mgh 4. What is the ideal gas equation, and what does each letter stand for? PV = NkT P pressure, V volume, N number of atoms, k Boltzmann s constant, T temperature. 5. Convert 20 cm 3 into m 3. 2 x 10-5 m 3 6. Convert 4 m 3 into cm 3. 4 x 10 6 cm 3 7. What are the four states of matter? Solid, liquid, gas, plasma. 8. What is the force required to move an object by 7 m if the work done is measured to be 56 J? F = W s = 56 7 = 8N 9. State the second law of thermodynamics and explain it. ΔS > 0 Entropy is always increasing. This is because the universe is always becoming MORE disordered. 10. Define work done. Force applied over a distance. 11. Give the equation for the work done by a force. W = F s Note: this Δs is change in distance, we use lower case s for distance (displacement) and upper case S for entropy.

2 12. Define efficiency. The ratio of the useful output energy to the total input energy. Normally given as a percentage. 13. What are the energy changes for a light bulb? Electrical -> Heat + light 14. *What is meant by the symbol ΔU from the first law of thermodynamics, and explain how we can calculate it given that we are looking at a beaker full of air which is contained with a movable piston. Include a diagram in your answer. ΔU is the CHANGE in internal energy. Diagram should show a beaker with a moveable piston stopping the air leaving the container. If we heat up the container (add heat energy, Q) we get a change in heat energy in the container ΔQ. This increase in heat energy results in the gas particles inside the container gaining kinetic energy (moving faster) and the pressure in the container increases. These fast-moving particles collide with the piston and transfer some of their energy into the piston by doing work on it (ΔW). The change in internal energy is tiny so is calculated by ΔQ ΔW. 15. *A child is throwing their rucksack up into the air. The rucksack has a mass of 2 kg and is thrown to a height of 2.5 m. a) Calculate the GPE when the bag reaches the top. GPE = mgh = 2 x 9.81 x 2.5 = 49 J b) On the way down, the bag loses 20% of its energy and the other 80% of its energy is converted into kinetic energy. How has the bag lost energy if it has not yet reached the floor? The bag loses energy due to resistive frictional forces between the bag and the air molecules. c) Calculate the speed of the bag just before the child catches it. 80% of the GPE is transferred into KE: 49 x 0.8 = 39.2 J The we rearrange the KE equation to get speed as the subject: v = 2KE m = 2(39.2) = 6.3 m/s 2 Section 2: Entropy 16. State the second law of thermodynamics. 17. What does ΔS stand for? 18. What is meant by ΔS > 0? 19. *Give an example of how you know entropy is increasing. Section 3: Isothermal and Adiabatic 20. Define isothermal. 21. Define adiabatic.

3 22. *We never get a system that is perfectly adiabatic, explain why. Section 4: Engines 23. What is a Carnot engine? 24. Draw a heat pump and a refrigerator. What is the difference between them? 25. What is the difference between heat engines, heat pumps and refrigerators? 26. What is the equation for the maximum theoretical efficiency and state the units. 27. What is an idealised engine cycle? 28. What does the coefficient of performance tell you? 29. *Why is the coefficient of performance greater for a heat pump compared with a refrigerator? You may want compare equations when answering. 30. *With reference to how a heat engine works, explain how a power station works. 31. *With reference to how a heat engine works, explain how a car engine works. 32. **How do air conditioning systems work. Section 5: Specific heat capacity and specific latent heat 33. State the equation for specific heat capacity and give the units. 34. Rearrange the equation in question 33 to find the change in temperature. 35. Define specific heat capacity. 36. A 20 kg concrete block is at 20 C. Given that the specific heat capacity of concrete is 3400 J/kg C, find the energy required to heat this block to 60 C. 37. **A 150 g piece of cotton is initially at 15 C. 0.2kJ of heat energy is given to the cotton. Given that the specific heat capacity is 1400 J/kg C, calculate the final temperature of the cotton. 38. Define the latent heat of vaporisation.

4 39. Define the latent heat of fusion. 40. *Why does a scald from steam at 100 C hurt more than a scald from water at 100 C? Section 6: Materials 41. State the equation for stress along with all the units. 42. A strip of rubber is originally 75 mm long. It is stretched until it is 100 mm long. Calculate the tensile strain of the rubber. 43. What does the gradient of a stress vs strain graph tell you? 44. Give 2 properties of copper. 45. Define brittleness. 46. Describe the difference between tensile and compressive stress. 47. What is meant by fatigue. 48. Name two material properties that have the unit of pascals, Pa. 49. Draw a typical stress vs strain graph and label the 4 key stages. 50. A force of 4 N is applied over an area of mm, calculate the stress. 51. Define Young s Modulus. 52. What has a higher Young s Modulus steel or rubber? And why? 53. Define yield point. 54. Define strain and explain why it does not have any units? 55. *When would you observe creep? And why does this happen? 56. *What does the elastic hysteresis curve show? Why do we observe this shape when applying a load and then unloading a piece of rubber? 57. *Draw a stress strain graph for a ductile and brittle material and explain how the materials different in terms of applied loads and extensions. 58. **A 3.0m length of steel rod is going to be used in the construction of a bridge. The tension in the rod will be 10kN and the rod must extend by no more than 0.001m. Calculate the minimum cross-sectional area required for the rod. (Young s modulus of steel = 1.9 x Pa).

5 Section 7: Hooke s Law 59. Define Hooke s law and give the equation. 60. Sketch a plot of a material that obeys Hooke s law. 61. What does the gradient of a force vs extension graph give you? 62. *What does the area beneath a force vs extension graph give you? 63. *Describe the behaviour of a wire which obeys Hooke s law. Section 8: Fluid mechanics 64. Define viscosity. 65. What is a fluid? 66. What is meant if a fluid has a high viscosity, and give an example. 67. What is the difference between streamline and turbulent flow? 68. Define viscous drag. 69. What is the difference between viscous flow and non-viscous flow in terms of viscous drag? 70. *What is meant by an incompressible fluid? 71. *What does the mass flow rate continuity equation state? 72. **How does the design of an aeroplane wing result in lift? Use Bernoulli s principle to explain your answer.

Thermal physics revision questions

Thermal physics revision questions ONE SECTION OF QUESTIONS TO BE COMPLETED AND MARKED EVERY WEEK AFTER HALF TERM. Section 1: Energy 1. Define the law of conservation of energy. 2. State the first law