Physics 100 Lecture 5. Laws of Thermodynamics February 5, 2018

Transcription

1 3 Physics 100 Lecture 5 Laws of Thermodynamics February 5, 2018

2 4 Class Quiz 2-3: A block of wood loses 100 J of gravitational potential energy as it slides down a ramp. If it has 90 J of kinetic energy at the bottom of the ramp, then A. mechanical energy must have been conserved. B. kinetic energy must have been conserved. C. 190 J of energy must have been lost. D. 10 J of energy must have been converted to other forms.

3 5 Energy Units (from lecture #1) 1.00 Joule = 1.00 N m 1.00 calorie = J 1.00 BTU = 1055 J 1.00 food calorie = 1000 cal = 1.00 kcal Converting units example: Convert 5000 ft lb to Joules

4 6 Another energy unit (from lecture #3) Electricity is sold in units of kwh W P t W Pt kw h 1000 W 1 J/s 3600 s 1 kw W h 6 1 kwh J

5 7 How many kwh of energy are in a 287 (food) Calorie doughnut? A kwh B kwh C. 300 kwh D kwh

6 8 How many kwh of energy are in a 287 (food) Calorie doughnut? A kwh B kwh C. 105 kwh D kwh 4186 J 1 kwh 287 Cal kwh 6 Cal J

7 9 Chapter 4 Temperature Sensing temperature Three scales: F, C, K Absolute temperature Thermometers Linear expansion Bimetallic strip Thermocouple Infrared

8 10 Liquid nitrogen boils at 196 C (which is 321 F). What is that temperature in Kelvins? A. 469 K B. 48 K C. 77 K D. 469 K

9 11 Liquid nitrogen boils at 196 C (which is 321 F). What is that temperature in Kelvins? A. 469 K B. 48 K T K T C C 273 C. 77 K T K 77 K D. 469 K

10 12 Thermal Energy Temperature is a measure of the average kinetic energy of molecules The energy stored in the motions and interactions of molecules in a system is called thermal energy. There are four types of thermal energy: Translational KE Potential energy (phase changes, latent heat) Rotational KE Vibrational KE

11 13 Heat Heat is energy added to (or subtracted from) a system that changes its thermal energy Compare that with Work is energy added to (or subtracted from) a system that changes its mechanical energy

12 14 Specific Heat The heat required to change the temperature of an object is: Q = heat (J or cal) m = mass (kg) Q c = specific heat capacity (given in the question or look it up p. 98) DT = change in temperature ( C) mcdt

13 15 The specific heat of water The specific heat of water is quite large (see page 98) J c 4186 water Joule s experiment: kg C 1.00 cal = J c c water water 1.00 cal g C 1.00 BTU lb F

14 16 Thermal energy The larger the specific heat c, the more heat energy Q is required to change the temperature The larger the mass m, the more heat energy Q is required to change the temp. The larger the temperature change DT, the larger the required amount of heat Q. Cooling soda example

15 17 Thermal Energy The more ways to store the energy, the larger the specific heat During a phase change the energy is stored as latent heat (potential energy) Translational KE Rotational KE Potential energy (phase changes, latent heat) Vibrational KE

16 18 Which of the following does NOT determine the amount of thermal energy an object has? A. The altitude of the object. B. The amount of material it contains. C. The type of material from which it is made. D. The temperature of the object

17 19 You stir some juice with a force of 20 N for a total distance of 30 m (in the direction of the force). How much heat have you transferred? A. 143 cal B. 300 cal C. 600 cal D cal

18 20 You stir some juice with a force of 20 N for a total distance of 30 m (in the direction of the force). How much heat have you transferred? A. 143 cal B. 300 cal C. 600 cal D cal W F d W 20 N 30 m 1 cal 600 J 143 cal J

19 21 If the mass of the juice is 2.0 kg, what is the temperature change of the juice? A C B. 14 C C C D C

20 22 If the mass of the juice is 2.0 kg, what is the temperature change of the juice? A C B. 14 C C C D C DT Q mc 600 J 2.0 kg 4186 J/kg/ C DT C

21 23 Zeroth Law of Thermodynamics If two objects are in thermal contact with each other and are at the same temperature, they are in thermal equilibrium This law is useful only for defining how a temperature should be measured.

22 24 First Law of Thermodynamics The thermal energy of a system can be increased either by adding heat or doing certain kinds of work on the system. DTE W Q certain kinds to the system If we consider all work, including mechanical work, DE D KE + PE + TE W Q on system to system This law is essentially a statement of the conservation of energy

23 25 While a gas is being compressed, 8.0 J of heat are removed and 20 J of work are done on the gas. What is the change in thermal energy of the gas? A. 28 J B. 12 J C. +12 J D. +28 J

24 26 While a gas is being compressed, 8.0 J of heat are removed and 20 J of work are done on the gas. What is the change in thermal energy of the gas? A. 28 J B. 12 J C. +12 J D. +28 J DTE W Q on to 20 J 8.0 J 12 J

25 27 Second Law of Thermodynamics Every spontaneous process increases the entropy of the universe We ll examine this law and its consequences in more detail in the future.

26 28 The Third Law of Thermodynamics The temperature of any collection of molecules is greater than absolute zero. It is impossible to make the kinetic energy of an entire collection of molecules exactly equal to zero. This law will later be used to explain why certain processes are impossible and why it is impossible to build a 100% efficient heat engine.

27 29 Class Quiz 4a: Is it possible for the temperature of a system to remain constant even though heat flows into it? A. Yes, if the system also does some work. B. Yes, if some work is also done on the system. C. No, by definition heat flow must change the temperature. D. No, because the specific heat of a system cannot change.

28 30 A Revolutionary Machine? Figure 14-7 Kirkpatrick & Francis, Physics: A World View, 6 th ed. (c) 2007 Thomson Brooks/Cole

29 31 A Revolutionary Machine? Figure 14-8 Kirkpatrick & Francis, Physics: A World View, 6 th ed. (c) 2007 Thomson Brooks/Cole

30 32 A Revolutionary Machine? Figure 14-6 Kirkpatrick & Francis, Physics: A World View, 6 th ed. (c) 2007 Thomson Brooks/Cole