MSTC Physics B Chapter 11 Section 4.

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Presentation transcript:

MSTC Physics B Chapter 11 Section 4

Radiation Process by which energy is transferred by means of electromagnetic waves

Electromagnetc Wave Wave formed by electric and magnetic fields fluctuating together ex – visible light, radio, microwaves Doesn’t require a medium ex – sunlight travels through the vacuum of space

Radiation cont’d. All bodies continuously radiate energy in the form of EM waves Objects absorb and emit EM waves simultaneously If object is the same temp as surroundings then amount of radiant energy absorbed = amount of radiant energy emitted Material that is a good emitter of energy is also a good absorber of energy ex – black shirt on a hot day

Stefan Boltzmann Law of Radiation Q = еσT4At e – emissivity σ – Stefan Boltzmann constant = 5.67 x 10-8 J/sm2K4 T – temp in Kelvin A – surface area t – time in seconds

emissivity Number between 0 and 1 that indicates how close to being a perfect emitter of energy a material is Black e = 1 Shiny e = o

Convection Process in which heat energy is carried from place to place by the bulk movement of fluid When part of a fluid is warmed that part expands, decreasing density, causing it to rise ------ cooler fluid replaces it ----- repeats ex – smoke rises from a campfire - baseboard heating - cooling coils are at top of refrigerator

Convection H = hAΔT H – rate of convection heating h – convection coefficient A – surface area of body ΔT – temperature difference

Conduction Process where heat energy is transferred directly through a material Energy is transferred when atoms in hotter part collide and pass on energy to atoms in cooler part Metals are good conductors due to free electrons ex – metal spoon in hot water

Conduction Q = (kAΔT t)/L Q – heat transferred k – thermal conductivity [k] = J/smCª metals have high k values A – surface area ΔT – temperature difference t – time in seconds L - length

Sample Problem One way the body disperses of excess heat energy is via conduction through body fat. Suppose that heat energy travels through 3 cm of fat in reaching the skin, which has a total surface area of 1.7 m2 and a temperature of 34ªC. If the body interior is maintained at 37ªC, find the amount of energy that reaches the skin in half an hour. The thermal conductivity is 0.2 W/mCª.

Sample Problem A portable ice chest has walls 2 cm thick. The area of the walls is 0.66 m2. On a day when the temperature is 35ªC, the chest is partially filled with 3 g of ice at 0ªC. Find the time required to melt the ice when the chest is made from Styrofoam. The thermal conductivity for Styrofoam is 0.01 J/smCª.