1. Heat and Temperature
Related to energy
Chapter 12

2. Heat Q Energy transferred because of a temperature difference high T
low T
heat Q
Zero heat flow at thermal equilibrium

3. “0th Law of Thermodynamics”
Thermal Equilibrium
Insulation A B
At equilibrium TA = TB
“0th Law of Thermodynamics”

4. We could measure it before we knew what it was
Temperature Scales
We could measure it before we knew what it was
§ 12.1

5. Fahrenheit Scale
0 °F: salt-ice slush
100 °F: body temperature

6. Celsius Scale
0 °C: water freezes
100 °C: water boils

7. Converting
Linear relation, 2 points known (0 °C, 32 °F), (100 °C, 212 °F)
°F°C: x =
9 °F
5 °C
(y – 32 °F)
°C°F: y =
9 °F
5 °C
x + 32 °F

8. Poll Question
There is a temperature at which the readings in °C and °F are the same.
True.
False.

9. Kelvin Scale
Absolute temperature
§ 12.2

10. Temperature and Gas Pressure
(constant volume)
different samples
Absolute zero
Source: Griffith, Physics of Everyday Phenomena, Fig. 10.6

11. Absolute Zero Gas p decreases with T Absolute zero  T at which p = 0
Minimum total molecular kinetic energy

12. Kelvin scale Unit: K K = same size as °C but 0 K = absolute zero
(T in K) = (T in °C)

13. Poll Question How do the temperatures 273.15 °C and 273.15 K compare?
°C is the same as K.
°C is half K.
°C is twice K.
°C is absolute zero.

14. Thermal Expansion
§ 12.4–12.5

15. Linear Expansion L0 T0 L0 + DL T0 + DT DL = a DT L0
a = coefficient of linear expansion
valid for small DT

16. CPS Question
A metal ring is heated. What happens to its outer diameter?
It increases.
It stays the same.
It decreases.
Impossible to determine.

17. CPS Question
A metal ring is heated. What happens to its inner diameter?
It increases.
It stays the same.
It decreases.
Impossible to determine.

18. Volume Expansion T0 + DT V0 + DV V0 T0 V0 DV = b DT
If expansion is isotropic,
and DT is small,
b = 3a

19. Water is Weird Density maximum disappears above ~2.5% salt
Source: Young and Freedman, Figure 17.12
Density maximum disappears above ~2.5% salt

20. Poll Question Between 0°C and 4°C, warmer fresh water
Rises above colder water.
Sinks beneath colder water.
Has neutral buoyancy.
It is impossible to determine.

21. Thermal Stress
Results from:
Uneven composition
Uneven temperature

22. Mechanical Equivalent of Heat
energy is energy
§ 12.6

23. Mechanical Equivalent of Heat
James Joule’s life-long obsession
Identical effects of adding heat to a system and doing work on it.
Source: Griffith, The Physics of Everyday Phenomena

24. First Law of Thermodynamics
DE = Q – W
E = internal energy
Total molecular kinetic + potential energy
Q = heat added to the system
W = work done by the system

25. Heat Units
Joule
Calorie (cal): heat needed to raise 1 gram of water 1 degree C (or K) = J.
British Thermal Unit (BTU): heat needed to raise 1 pound of water 1 degree F = J

26. Another Heat Unit U.S. Food Calorie: Cal = 1000 cal
Food energy values are often presented in kJ in other countries

27. Board Work Human food intake is about 2000 Cal /day.
What is this in joules / day?
What is this in watts?