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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

Why is there no 2nd harmonic in the classical electron orbit?
Shree Reply
how to reform magnet after been demagneted
Inuwa Reply
A petrol engine has a output of 20 kilowatts and uses 4.5 kg of fuel for each hour of running. The energy given out when 1 kg of petrol is burnt is 4.8 × 10 to the power of 7 Joules. a) What is the energy output of the engine every hour? b) What is the energy input of the engine every hour?
Morris Reply
what is the error during taking work done of a body..
Aliyu Reply
what kind of error do you think? and work is held by which force?
Daniela
I am now in this group
smart
theory,laws,principles and what-a-view are not defined. why? you
Douglas Reply
A simple pendulum is used in a physics laboratory experiment to obtain an experimental value for the gravitational acceleration, g . A student measures the length of the pendulum to be 0.510 meters, displaces it 10 o from the equilibrium position, and releases it. Using a s
Emmanuel Reply
so what question are you passing across... sir?
Olalekan
Two masses of 2 kg and 4 kg are held with a compressed spring between them. If the masses are released, the spring will push them away from each other. If the smaller mass moves off with a velocity of 6m/s, what is the stored energy in the spring when it is compressed?
Emmanuel Reply
54 joule
babar
how?
rakesh
Reduce that two body problem into one body problem. Apply potential and k. E formula to get total energy of the system
rakesh
i dont think dere is any potential energy... by d virtue of no height present
Olalekan
there is compressed energy,dats only potential energy na?
rakesh
yes.. but... how will u approach that question without The Height in the question?
Olalekan
Can you explain how you get 54J?
Emmanuel
Because mine is 36J
Emmanuel
got 36J too
Douglas
OK the answer is 54J Babar is correct
Emmanuel
Conservation of Momentum
Emmanuel
woow i see.. can you give the formula for this
joshua
Two masses of 2 kg and 4 kg are held with a compressed spring between them. If the masses are released, the spring will push them away from each other. If the smaller mass moves off with a velocity of 6m/s, what is the stored energy in the spring when it is compressed? Asume there is no external force.
Emmanuel Reply
Please help!
Emmanuel
please help find dy/dx 2x-y/x+y
Inuwa
By using the Quotient Rule dy/dx = 3y/(x +y)²
Emmanuel
3y/(x+y)²
Emmanuel
may be by using MC^2=MC^2 and Total energy=kinetic energy +potential energy so 1st find kinetic energy and den find potential energy which is stored energy
rakesh
i think i m correct
rakesh
But how?
Emmanuel
3y/(x+y)²
Douglas
what's the big bang?
kwame Reply
yes what is it?
LamaBbake
it is the explanation of how the universe began
Zainab
yes
Ana
explain
Chinagorom
in
Chinagorom
it is a theory on how the universe began. to understand more I would suggest researching the topic online.
david
thanks guys
kwame
if a force of 12N is applied to load of 200g what us the work done
Joshua Reply
We can seek accelation first
Nancy
we are given f=12 m=200g which is 0.2kg now from 2nd law of newton a= f/m=60m/s*2 work done=force applied x displacement cos (theta) w= 12x60 =720nm/s*2
Mudang
this very interesting question very complicated for me, í need urgent help. 1,two buses A and B travel along the same road in the same direction from Harper city (asume They both started from the same point) to Monrovia. if bus A maintains a Speedy of 60km/h and bus B a Speedy of 75km/h, how many
mohammed
hours Will it take bus B to overtake bus A assuming bus B starts One hour after bus A started. what is the distance travelled by the buses when They meet?.
mohammed
pls í need help
mohammed
4000 work is done
Ana
speed=distance /time distance=speed/time
Ana
now use this formula
Ana
what's the answer then
Julius
great Mudang
Kossi
please Ana explain 4000 ?
babar
hey mudang there is a product of force and acceleration not force and displacement
babar
@Mohammed answer is 0.8hours or 48mins
Douglas
nice
A.d
its not possible
Olalekan
í want the working procedure
mohammed
the answer is given but how Will One arrive at it. the answers are 4hours and 300m.
mohammed
physics is the science that studies the non living nature
isidor Reply
ancient greek language physis = nature
isidor
what is phyacs
technical Reply
if i am going to start studying physics where should i start?
BRIAN Reply
I think from kinematics
Nancy
You can find physics books at the library or online. That's how I started.
Chelsea
And yes, kinematics is usually where you can begin.
Chelsea
study basic algebra and calculus and can start from classical mechanics
Mudang
yes think so but dimension is the best starting point
Obed
3 formula's of equations of motion
benjamin Reply
vf=vi+at........1 s=vit+1/2(at)2 vf2=vi2+2as
Ana
solve the formula's please
benjamin
those are the three .. what you wanna solve ?
Nihrantz
For first equation simply integrate formula of acceleration in the limit v and u
Tripti
For second itegrate velocity formula by ising first equation
Tripti
similarly for 3 one integrate acceleration again by multiplying and dividing term ds
Tripti
any methods can take to solve this eqtions
a=vf-vi/t vf-vi=at vf=vi+at......1
Ana
suppose a body starts with an initial velocity vi and travels with uniform acceleration a for a period of time t.the distance covered by a body in this time is "s" and its final velocity becomes vf
Ana
what is the question dear
Zeeshan
average velocity=(vi+vf)/2 distance travelled=average velocity ×time therefore s=vi+vf/2×t from the first equation of motion ,we have vf =vi+at s=[vi+(vi+at)]/2×t s=(2vi+at)/2×t s=bit+1/2at2
Ana
find the distance
Ana
how
Zeeshan
Two speakers are arranged so that sound waves with the same frequency are produced and radiated through a room. An interference pattern is created. Calculate the distance between the two speakers?
Hayne Reply
How can we calculate without any information?
Amir
I think the formulae used for this question is lambda=(ax)/D
Amir
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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