1. MSTC Physics B
Chapter 11 Section 4

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

3. 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

4. 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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. 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.

12. 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.