Radiation C ontents: Basic Concept Example Whiteboards.

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Presentation transcript:

Radiation C ontents: Basic Concept Example Whiteboards

Radiation TOC Hot objects radiate photons (black body radiation) Heating up a poker Demo - Show red hot paperclip Demo – Start silver/black demo Bonfire/barn fire/stoves The sun Frost Preheat the oven/drying tennis balls

Radiation ΔQ / t = Heat flow rate in J/s or Watts e = emissivity (0 < e < 1) (Temp?) σ = 5.67x10 -8 Wm -2 K -4 (Stefan Boltzmann constant) A = Area (m 2 ) T = High temperature (K)

Net heat transfer (emitted) - (absorbed) T 1 = object T 2 = surroundings e = e?

Q/t = (0.75)(1.72 m 2 )(5.67E-8 Wm -2 K -4 )(( K) 4 -( K) 4 ) = W 120 W Example: A person has a surface area of 1.72 m 2 and a skin temperature of 33 o C. They are standing in a room whose walls are at 18 o C, and their skin has an emissivity of What is the net rate of radiative heat flow out of their body?

Demo – check temperatures on radiation demo Silver lined blankets/bags reflect radiation back to you

Radiation 1-3

Q/t = (0.92)( m 2 )(5.67E-8 Wm -2 K -4 )(( ) 4 -( K) 4 ) = … W W, 25 mW A power converter is a cube 4.0 cm on a side, and is made of black plastic with an emissivity of It maintains a temperature of o C in a room whose walls are at o C. What is its net rate of radiative heat transfer?

34 = e(4  (0.03 m) 2 )(5.67E-8 Wm -2 K -4 )(( ) 4 -( K) 4 ) 0.78 A 3.0 cm radius (sperical) light bulb radiates 34 Watts of energy. If it has a surface temperature of o C in a room whose walls are at 19.0 o C, what is its emissivity?

1000 = (0.95)(6.00 m 2 )(5.67E-8 Wm -2 K -4 )(T 4 -( K) 4 ), T = T = 45 o C 318 K, 45 o C If you put a Watt hair dryer into a 1.00 m cube box with an emissivity of 0.95, it would heat up the box’s surface until energy flowed out (assume radiatively) as fast as it flowed in. What would be the surface temperature of the box in Celsius if the room it was in had walls at 18.0 o C?