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

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*

What is Heat? Heater Core @ 500 Celsius Air @ 18 Celsius

What is Heat? You Skin @ 30 Celsius Water @ 20 Celsius

What is Heat? The Sun @ 5,500 Celsius Your Skin @ 30 Celsius

How Does Heat Transfer? Three Methods: Conduction Radiation Convection

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

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 = 273 + C = area [m2] A = thickness of barrier [m] d

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

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 Air @ 0 C 0.024 Wood 0.10

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

How Does Heat Transfer? Conduction Example

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

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

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 = 5.6708 x 10-8 [W/m2k4] = radiating area [m2] A

How Does Heat Transfer? Radiation Example: Light Bulb

How Does Heat Transfer? Planck’s Radiation Curves @ 3000K

How Does Heat Transfer? The Sun @ 5,500 Celsius Your Skin @ 30 Celsius

How Does Heat Transfer? Planck’s Radiation Curves @ 6000K

How Does Heat Transfer? Planck’s Radiation Curves @ 1000K

How Does Heat Transfer?

How Does Heat Transfer? Planck’s Radiation Curves @ 373K

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 = 5.6708 x 10-8 [W/m2k4] = 0.0252 [m2] A = 60+273 [K] Thot = 23+273 [K] Tcold 45 W CPU

How Does Heat Transfer? Radiation: Does it Matter at Low Temp? P = 1 x 5.6708 x 10-8 W/m2K4 x 0.025 m2 x (333K4-296K4) P = 0.66 Watts = 0.66 J/s Not a whole lot….

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

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.

How Does Heat Transfer? Convection: Convection Airflow Demo

How Does Heat Transfer? Natural Convection: Air Heater

How Does Heat Transfer? Natural Convection: Water Heater

How Does Heat Transfer? Forced Convection: Air Fan Heater

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 Air @ 0 C 0.024 Wood 0.10

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

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.

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

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

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

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

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 !

Heat Property of Water Specific Heat Capacity Hint: It takes 0.1 x 5 x 4184J = 2092 J. The heater is pumping heat @ 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

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

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

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.

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

Putting It All Together

Putting It All Together

Putting It All Together

Putting It All Together

Putting It All Together http://www.physics.ubc.ca/outreach/ http://www.streaming-river.com