Mar 31 2011
Introduction to Energy Math
Four Types Of Problems
E = Energy- basic unit is joules (J)- P x T
P = Power- basic unit is watts (w) = J/sec- E/T
T = Time- E/P
Example: How much energy (in J) does a 30 w bulb if on for 30 min?
E = P x T = 30 J/sec x 60 sec/min x 30 min = 54,000 J
2. Heat Transfer:
Q = mc x change in temp.
Q = heat energy in calories m = mass c = specific heat of water = always equals 1
English version of this equation is in BTU’s- British Thermal Unit
BTU = lb x 1 btu/lbxF x change in temp.
3. Cost / Price:
Units used x Price/Units = Price = cost or bill
4. Efficiency of Devices:
Input = output/efficiency
example: 60 w = output/ .05 = 3 w to light
Commonly asked Efficiency
- Incandescent bulb = 5%
- CFL = 20-25%
- Coal Plant = 30%
- Nuclear Plant = 25%
- Car Engine = 20-25%
- Photosynthesis 1%
Energy not used for energy is lost to heat.
