Uses of Energy

Energy, Work, Power, Climate

• We like to convert E to do work.

• Conservation of E says we cannot create E, but we can transfer E between objects.

• E can be transferred by inputting thermal E to object.

Fuels release Thermal Energy

We use released heat from fuels to transform to other types of E we use to do work.

Turn a piston or a turbine.

A Cyclic Process needed to do continuous work.

• Engines convert fuel to KE.

• In order to have continuous work, system must be returned to original state.

• E is “lost” when system returns to original state.

Hamper ex: hot air balloon rises to force rope to rotate a spool thus doing work.

Once rope is used up, balloon must cool to return to ground & be re-heated a cycle. Some heat is “lost” in the process when balloon cools to return to ground.

Steam turns a turbine. Convert Heat > KE > Electric + Heat

• Most processes involve significant E lost to (unwanted) heat.

• Degraded E define

• Flashlight battery puts out 100 units, only 10 units transferred to light the rest is degraded “lost” to heat.

Sankey Diagrams show E transfer.Straight arrow shows % useful work.Curved shows % degraded work.

Straight branch useful.

Branch width = proportion.

Coal Fired Plant

input

Sankey Diagrams – some more specific about heat losses.

Ex 1: In a car engine chemical E is converted to KE of the wheels as shown.

• a. Calculate the overall efficiency of the engine.• b. Calculate the engine efficiency ignoring

frictional losses.

Eff = useful work x 100 %tot work

• a. 300 J/ 1000 J x 100% = 30%

• ~ 550 J “lost” to friction.

• b. (300 + 550) J /1000 J x 100% = 85 %

We like to convert E to electricity.

• Need some fuel to get a turbine spinning.

Power Plant

Fuel source:Coal, Gas, OilNukes.

Heat/Steam

Coils MagnetsElectric

Recycle water

List some energy that depend on the sun.

• Coal

• Oil

• Wood

• Solar

• Wind

List some energy not from the sun.

• Tidal

• Nuclear

• geothermal

Energy Density

• Fuels rated by density

• The amount of energy per kg fuel.

• Oil much denser than wood.

Ex 2. Use the Energy density table Hamper to calculate the following:

A steam engine has 4500 kW of power. If we wish to run the engine for 5 hours, how many kg of each type of fuel will be needed?

Wood = 4765 kg

Coal = 2492 kg

Oil= 1727 kg

Energy Density 235U = 9 x 107 MJ/kg

Power Plants 5:30 min

http://www.youtube.com/watch?v=20Vb6hlLQSg&feature=endscreen&NR=1

Nuclear problem Hamper 187.