Energy
Energy sources
• Electrical energy
• Magnetic energy
– Today we know it’s “electromagnetic” combined.
• Solar power (energy from light)
• Energy from chemical bonds
– Food, coal, natural gas, biomass, incineration, etc.
• Energy stored in nucleus of atoms (Nuclear)
And…
Los Angeles Dept. of Water & Power
sources of energy (Power Content Label)
Source 2007
Biomass/
1%
waste
2006
1%
2005
1%
1%
13%
<1%
<1%
47%
30%
8%
<1%
11%
<1%
1%
48%
30%
10%
<1%
9%
<1%
1%
53%
26%
10%
Geothermal
Hydroelectric
Solar
Wind
Coal
Natural gas
Nuclear
2004
2003
2002
1%
1%
11%
0%
0%
50%
24%
10%
Southern California Edison
Sources of energy (Power Content Label)
Source 2007
Biomass/
2%
waste
2006
2%
9%
7%
1%
3%
7%
51%
20%
9%
6%
1%
3%
7%
54%
17%
Geothermal
Hydroelectric
Solar
Wind
Coal
Natural gas
Nuclear
2005
2004
2%
11%
4%
1%
4%
14%
44%
20%
2003
2002
Energy quantities we’ll discuss in
Physics 2A
• Energy from forcing things to move
– WORK = ….
• Energy in moving things (Hydroelectric, wind)
– KINETIC ENERGY: KE = …
• Energy stored by gravity (Hydroelectric, water towers)
– GRAVITATIONAL POTENTIAL ENERGY. PEg = …
• Energy stored in stretched things
– ELASTIC POTENTIAL ENERGY (SPRING P.E.) PEs = …
• Heat (geothermal) – CHAPTER 9… Q = …
Units of energy
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Calories (food we eat)
calories (physics unit – heat. 1 Cal = 1000 cal)
Joules (J) – most often used by physicists
Kilowatt-hour (kW-hr) = 3600000 Joules
B.t.u. – British thermal unit
Thermals – 100,000 BTUs
Can kinetic energy be negative?
1. Yes
2. No
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Can spring potential energy be
negative?
1. Yes
2. No
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Fundamental relationships &
definitions
• Energy is the ability to cause change (damage)
• We’ll focus on Mechanical Energy, which
includes: KE + PEg + PEs.
• We’ll ignore non-mechanical energies, such as
nuclear, chemical, and “internal” (atomic kinetic).
• Fundamental relationship:
the energy of a system only changes if
outside influences act on the system.
• DME = S Wext + Qext + lightext. We’ll ignore last
two terms for now.
Is D KE = ½ m (vf – vi)2
1. False
2. True
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