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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

Section summary

  • The law of conservation of energy states that the total energy is constant in any process. Energy may change in form or be transferred from one system to another, but the total remains the same.
  • When all forms of energy are considered, conservation of energy is written in equation form as KE i + PE i + W nc + OE i = KE f + PE f + OE f size 12{"KE" rSub { size 8{i} } +"PE" rSub { size 8{i} } +W rSub { size 8{"nc"} } +"OE" rSub { size 8{i} } ="KE" rSub { size 8{f} } +"PE" rSub { size 8{f} } +"OE" rSub { size 8{f} } } {} , where OE size 12{"OE"} {} is all other forms of energy besides mechanical energy.
  • Commonly encountered forms of energy include electric energy, chemical energy, radiant energy, nuclear energy, and thermal energy.
  • Energy is often utilized to do work, but it is not possible to convert all the energy of a system to work.
  • The efficiency Eff size 12{ ital "Eff"} {} of a machine or human is defined to be Eff = W out E in size 12{ ital "Eff"= { {W rSub { size 8{"out"} } } over {E rSub { size 8{"in"} } } } } {} , where W out size 12{W rSub { size 8{"out"} } } {} is useful work output and E in size 12{E rSub { size 8{"in"} } } {} is the energy consumed.

Conceptual questions

Consider the following scenario. A car for which friction is not negligible accelerates from rest down a hill, running out of gasoline after a short distance. The driver lets the car coast farther down the hill, then up and over a small crest. He then coasts down that hill into a gas station, where he brakes to a stop and fills the tank with gasoline. Identify the forms of energy the car has, and how they are changed and transferred in this series of events. (See [link] .)

A car coasting downhill, moving over a crest then again moving downhill and finally stopping at a gas station. Each of these positions is labeled with an arrow pointing downward.
A car experiencing non-negligible friction coasts down a hill, over a small crest, then downhill again, and comes to a stop at a gas station.
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Describe the energy transfers and transformations for a javelin, starting from the point at which an athlete picks up the javelin and ending when the javelin is stuck into the ground after being thrown.

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Do devices with efficiencies of less than one violate the law of conservation of energy? Explain.

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List four different forms or types of energy. Give one example of a conversion from each of these forms to another form.

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List the energy conversions that occur when riding a bicycle.

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Problems&Exercises

Using values from [link] , how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)

4 × 10 4  molecules size 12{4 times "10" rSup { size 8{4} } " molecules"} {}

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Using energy considerations and assuming negligible air resistance, show that a rock thrown from a bridge 20.0 m above water with an initial speed of 15.0 m/s strikes the water with a speed of 24.8 m/s independent of the direction thrown.

Equating ΔPE g size 12{Δ"PE" rSub { size 8{g} } } {} and ΔKE size 12{Δ"KE"} {} , we obtain v = 2 gh + v 0 2 = 2 ( 9.80 m /s 2 ) ( 20.0 m ) + ( 15.0 m/s ) 2 = 24.8 m/s size 12{v= sqrt {2 ital "gh"+v rSub { size 8{0} rSup { size 8{2} } } } = sqrt {2 \( 9 "." "80"" m/s" rSup { size 8{2} } \) \( "20" "." 0" m" \) + \( "15" "." "0 m/s" \) rSup { size 8{2} } } ="24" "." 8" m/s"} {}

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If the energy in fusion bombs were used to supply the energy needs of the world, how many of the 9-megaton variety would be needed for a year’s supply of energy (using data from [link] )? This is not as far-fetched as it may sound—there are thousands of nuclear bombs, and their energy can be trapped in underground explosions and converted to electricity, as natural geothermal energy is.

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(a) Use of hydrogen fusion to supply energy is a dream that may be realized in the next century. Fusion would be a relatively clean and almost limitless supply of energy, as can be seen from [link] . To illustrate this, calculate how many years the present energy needs of the world could be supplied by one millionth of the oceans’ hydrogen fusion energy. (b) How does this time compare with historically significant events, such as the duration of stable economic systems?

(a) 25 × 10 6 years size 12{"25" times "10" rSup { size 8{6} } `"years"} {}

(b) This is much, much longer than human time scales.

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Questions & Answers

the diagram of the digestive system
Assiatu Reply
allimentary cannel
Ogenrwot
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed
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Practice Key Terms 7

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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