1. Heat Transfer: The Physics of Computer Cooling By Kenneth Yu
Department of Physics and Astronomy
PHYS420: Physics Demonstration
Heat Transfer:
The Physics of Computer Cooling
By Kenneth Yu

2. What is Heat? Definition:
Heat is a form of energy that is transferred by a difference in temperature
Examples:
The heater transfers heat to the air
Our skin transfers heat to water
The sun transfers heat to our skin*

3. What is Heat?
Heater 500 Celsius
18 Celsius

4. What is Heat?
You 30 Celsius
20 Celsius

5. What is Heat?
The 5,500 Celsius
Your 30 Celsius

6. How Does Heat Transfer?
Three Methods:
Conduction
Radiation
Convection

7. How Does Heat Transfer? Conduction
The transfer of heat through a substance by molecular action or from one substance by being in contact with another.
Hot
Cold
Warm

8. How Does Heat Transfer? Q = kA (Thot-Tcold) t d Conduction Formula
= heat transfer [J] Q
= time [s] t
= thermal conductivity [J/msK] k
= temperature [K] T
K = C
= area [m2] A
= thickness of barrier [m] d

9. How Does Heat Transfer?
Q = kA (Thot-Tcold) d t
Conduction Formula

10. How Does Heat Transfer? Thermal Conductivity k Material
Thermal Conductivity (W/mK)
Silver
406.0
Copper
385.0
Aluminum
109.0
Steel
50.2
Ice
1.6
Styrofoam
0.027
0 C
0.024
Wood
0.10

11. Copper vs. Wood How Does Heat Transfer? Conduction Demo Material
Thermal Conductivity (W/mK)
Copper
385.0
Wood
0.10

12. How Does Heat Transfer?
Conduction Example

13. How Does Heat Transfer? Radiation
Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object.

14. What is Heat? Definition:
Heat is a form of energy that is transferred by a difference in temperature

15. How Does Heat Transfer? P = eσ A (Thot4-Tcold4) σ
Radiation: Stefan-Boltzmann Law
P = eσ A (Thot4-Tcold4)
= radiating power = Q/t [J] P
= emmissivity e
= Stefan’s Constant σ
= x 10-8 [W/m2k4]
= radiating area [m2] A

16. How Does Heat Transfer?
Radiation Example: Light Bulb

17. How Does Heat Transfer?
Planck’s Radiation 3000K

18. How Does Heat Transfer?
The 5,500 Celsius
Your 30 Celsius

19. How Does Heat Transfer?
Planck’s Radiation 6000K

20. How Does Heat Transfer?
Planck’s Radiation 1000K

21. How Does Heat Transfer?

22. How Does Heat Transfer?
Planck’s Radiation 373K

23. How Does Heat Transfer? P = eσ A (Thot4-Tcold4) σ Thot Tcold
Radiation: Does it Matter at Low Temp?
P = eσ A (Thot4-Tcold4)
5cm
60 C
= unknown P
= 1 e
= Stefan’s Constant σ
= x 10-8 [W/m2k4]
= [m2] A
= [K]
Thot
= [K]
Tcold
45 W CPU

24. How Does Heat Transfer? Radiation: Does it Matter at Low Temp?
P = 1 x x 10-8 W/m2K4 x m2
x (333K4-296K4)
P = 0.66 Watts = 0.66 J/s
Not a whole lot….

25. How Does Heat Transfer? Convection
Heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it

26. How Does Heat Transfer? m ρ= Heater V Convection: Air
2. As the volume increases, density goes down ρ= m V
Heater
1. As air heats up, it expands
3. Less dense material floats up.
4. Cold, denser air drops down to heater.

27. How Does Heat Transfer?
Convection: Convection Airflow Demo

28. How Does Heat Transfer?
Natural Convection: Air
Heater

29. How Does Heat Transfer?
Natural Convection: Water
Heater

30. How Does Heat Transfer?
Forced Convection: Air
Fan
Heater

31. How Does Heat Transfer? Thermal Conductivity k Material
Thermal Conductivity (W/mK)
Silver
406.0
Copper
385.0
Aluminum
109.0
Steel
50.2
Ice
1.6
Styrofoam
0.027
0 C
0.024
Wood
0.10

32. How Does Heat Transfer? Specific Heat Capacity Material
Specific Heat (J/kg K)
Water
4184
Ice
2050
Wood
1400
Aluminum
900
0 C
716
Steel
502
Iron
447
Copper
386

33. How Does Heat Transfer? Thermal Conductivity k
- how fast can a material moves heat
Specific Heat Capacity
how much heat can a material take
without getting too hot.

34. How Does Heat Transfer? Heat Capacity of Different Materials Material
Specific Heat (J/kg K)
Water
4184
Air
716

35. How Does Heat Transfer? Heat Capacity of Different Materials Material
Specific Heat (J/ L K)
Water
4184
Air
0.92

36. Heat Property of Water Specific Heat Capacity Q: What is a calorie?
A calorie is the energy (J) required to raise the temperature of one gram of water by 1 C (1 K). One calorie = 4.184J
Material
Specific Heat (J/kg K)
Water
4184

37. Heat Property of Water Specific Heat Capacity
On a 45 W heater, how long does it increase the temperature of 100g of water by 5 K?
P = 45 J/s
△T = 5 K
m = 0.1 kg
t =? s
Material
Specific Heat (J/kg K)
Water
4184

38. Heat Property of Water Specific Heat Capacity Hint:
It takes 4184J to heat up 1kg of water by 1K
It takes 5 x 4184J to heat up 1kg of water by 5 K
It takes 0.1 x 5 x 4184J to heat up 0.1kg of water by 5K
It takes 0.1 x 5 x 4184J = 2092 J !

39. Heat Property of Water Specific Heat Capacity Hint:
It takes 0.1 x 5 x 4184J = 2092 J.
The heater is pumping 45 J /s.
It takes the heater 2092 / 45 seconds.
It takes 46 seconds!
Why did it take longer!?
We needed to heat up the beaker
Convection is constantly occurring

41. How are Computers Cooled?
Heat Source
CPU – 50 Watts of Heat
Power Supply – 30 Watts of Heat
Objective: Move the heat outside

42. How are Computers Cooled?
What we have learned so far…
Choice of computer case material
- Steel, Copper, Aluminum, Silver
Convection airflow
- Direction of Convection current
Cooling medium material
- Air, Water, Mercury

43. How are Computers Cooled?
What we don’t know yet… HEATPIPES!!
Heatpipes are highly heat conducting pipes using the cycle evaporation and condensation of low boiling-point fluid.

45. How are Computers Cooled?
Heatpipes Demo
Heatpipes
vs.
Heat Conduction

46. Putting It All Together

47. Putting It All Together

48. Putting It All Together

49. Putting It All Together

50. Putting It All Together