Science, Technology and the Environment (SC NATS 1840A) - Tuesday
October 23rd, 2012
READING: CH 3.4, 3.7
Heat and Electromagnetic Radiation: Intimately connected in a manner crucial to understanding climate
and climate change.
Example: A rock is exposed to sunlight. Its surface absorbs E.M. energy continuously. It is going to
absorb some of the energy from the sun and as it does so, but how does this absorption mechanism
work, you have to think about the atoms that make up the rock. Through conduction the temperature
increases due to the E.M. energy being absorbed.
Molecules’ average speed increases.
Thermal energy (temperature) increases.
Why does the rock not eventually melt?
Motion of molecules causes emission of E.M. Radiation.
Rock loses thermal energy in E.M. form at the same time as it absorbs it.
Frequency of emitted ration determine by object’s temperature.
The Object “glows”
Constant temperature (steady state) reached when
(Total energy absorbed)/sec = (Total energy radiated)/sec
No further net energy build-up in the object.
At night no energy is absorbed, but rock continues to “glow” (lose energy) due to thermal content. It
All objects glow in this way. Average frequency of glow increases with object’s temperature.
“Glow” is distributed over a range (spectrum) of frequencies, with the peak determined by
Units of Energy and Power:
Energy: Metric unit is the Joule (J). 1 J = 4.18 calories (cal)
1 Calorie = Energy needed to raise temperature of 1 gram of water 1 degree Celsius. Power: Watt (W) defined as 1 Joule per second.
Often, if power is expressed in kilowatts (kW) then energy is given in units of kilowatt-hours (kWh).
1 kWh = 1000 W x 1 hr = 3.6 x 10^6 J = 3.6 MJ
Everyday energy scales – Some examples:
Example: Energy used by a 100w light bulb
Left on for one hour
100w = 100 J/S
Energy used = 100 J/S x 3600sec = 3.6 x 10^5 J OR 360kj OR 0.1kWh
Example: Clothes dryer running for one Hour = 18MJ
Human body (one day; at rest) 6-8MJ (1600 kcal)
Car travelling at 100 km/h = at least 460kJ
Jumbo jet at take-off speed = at least 1.3GJ
Jumbo jet at cruising altitude (11km) = at least 45 GJ
Energy can be converted form one form to another, Example: E.M. energy converted into
thermal energy, and vice versa
For tire swim, continuous conversion of potential energy (1) into kinetic energy (2) and back again.
At intermediate stages the energy is partly kinetic ad partly potential (3).