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7.6 Conservation of energy  (Page 4/12)

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Another example of energy conversion occurs in a solar cell. Sunlight impinging on a solar cell (see [link] ) produces electricity, which in turn can be used to run an electric motor. Energy is converted from the primary source of solar energy into electrical energy and then into mechanical energy.

A solar-powered aircraft flying over the sea. Solar cells are on the upper surface of the wings, where they are exposed to sunlight.
Solar energy is converted into electrical energy by solar cells, which is used to run a motor in this solar-power aircraft. (credit: NASA)
Energy of various objects and phenomena
Object/phenomenon Energy in joules
Big Bang 10 68 size 12{"10" rSup { size 8{"68"} } } {}
Energy released in a supernova 10 44 size 12{"10" rSup { size 8{"44"} } } {}
Fusion of all the hydrogen in Earth’s oceans 10 34 size 12{"10" rSup { size 8{"34"} } } {}
Annual world energy use 4 × 10 20 size 12{4 times "10" rSup { size 8{"20"} } } {}
Large fusion bomb (9 megaton) 3 . 8 × 10 16 size 12{3 "." 8 times "10" rSup { size 8{"16"} } } {}
1 kg hydrogen (fusion to helium) 6 . 4 × 10 14 size 12{6 "." 4 times "10" rSup { size 8{"14"} } } {}
1 kg uranium (nuclear fission) 8 . 0 × 10 13 size 12{8 "." 0 times "10" rSup { size 8{"13"} } } {}
Hiroshima-size fission bomb (10 kiloton) 4 . 2 × 10 13 size 12{4 "." 2 times "10" rSup { size 8{"13"} } } {}
90,000-ton aircraft carrier at 30 knots 1 . 1 × 10 10 size 12{1 "." 1 times "10" rSup { size 8{"10"} } } {}
1 barrel crude oil 5 . 9 × 10 9 size 12{5 "." 9 times "10" rSup { size 8{9} } } {}
1 ton TNT 4 . 2 × 10 9 size 12{4 "." 2 times "10" rSup { size 8{9} } } {}
1 gallon of gasoline 1 . 2 × 10 8 size 12{1 "." 2 times "10" rSup { size 8{8} } } {}
Daily home electricity use (developed countries) 7 × 10 7 size 12{7 times "10" rSup { size 8{7} } } {}
Daily adult food intake (recommended) 1 . 2 × 10 7 size 12{1 "." 2 times "10" rSup { size 8{7} } } {}
1000-kg car at 90 km/h 3 . 1 × 10 5 size 12{3 "." 1 times "10" rSup { size 8{5} } } {}
1 g fat (9.3 kcal) 3 . 9 × 10 4 size 12{3 "." 9 times "10" rSup { size 8{4} } } {}
ATP hydrolysis reaction 3 . 2 × 10 4 size 12{3 "." 2 times "10" rSup { size 8{4} } } {}
1 g carbohydrate (4.1 kcal) 1 . 7 × 10 4 size 12{1 "." 7 times "10" rSup { size 8{4} } } {}
1 g protein (4.1 kcal) 1 . 7 × 10 4 size 12{1 "." 7 times "10" rSup { size 8{4} } } {}
Tennis ball at 100 km/h 22
Mosquito ( 10 –2 g at 0.5 m/s ) 1 . 3 × 10 − 6 size 12{1 "." 3 times "10" rSup { size 8{-6} } } {}
Single electron in a TV tube beam 4 . 0 × 10 − 15 size 12{4 "." 0 times "10" rSup { size 8{-"15"} } } {}
Energy to break one DNA strand 10 − 19 size 12{"10" rSup { size 8{-"19"} } } {}

Efficiency

Even though energy is conserved in an energy conversion process, the output of useful energy or work will be less than the energy input. The efficiency     Eff size 12{ ital "Eff"} {} of an energy conversion process is defined as

Efficiency ( Eff ) = useful energy or work output total energy input = W out E in . size 12{"Efficiency " \( ital "Eff" \) = { {"useful energy or work output"} over {"total energy input"} } = { {W rSub { size 8{"out"} } } over {E rSub { size 8{"in"} } } } "." } {}

[link] lists some efficiencies of mechanical devices and human activities. In a coal-fired power plant, for example, about 40% of the chemical energy in the coal becomes useful electrical energy. The other 60% transforms into other (perhaps less useful) energy forms, such as thermal energy, which is then released to the environment through combustion gases and cooling towers.

Efficiency of the human body and mechanical devices
Activity/device Efficiency (%) Representative values
Cycling and climbing 20
Swimming, surface 2
Swimming, submerged 4
Shoveling 3
Weightlifting 9
Steam engine 17
Gasoline engine 30
Diesel engine 35
Nuclear power plant 35
Coal power plant 42
Electric motor 98
Compact fluorescent light 20
Gas heater (residential) 90
Solar cell 10

Phet explorations: masses and springs

A realistic mass and spring laboratory. Hang masses from springs and adjust the spring stiffness and damping. You can even slow time. Transport the lab to different planets. A chart shows the kinetic, potential, and thermal energies for each spring.

Masses and Springs

Test prep for ap courses

You do 30 J of work to load a toy dart gun. However, the dart is 10 cm long and feels a frictional force of 10 N while going through the dart gun’s barrel. What is the kinetic energy of the fired dart?

  1. 30 J
  2. 29 J
  3. 28 J
  4. 27 J

(c)

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When an object is lifted by a crane, it begins and ends its motion at rest. The same is true of an object pushed across a rough surface. Explain why this happens. What are the differences between these systems?

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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