byChris Woodford.Last updated: April 11, 2024.
Try to think of something that doesn't involveenergy and youwon't get very far. Even thinking—even thinking about energy!—needssome energy to make it happen. In fact, everything that happens in theworld uses energy of one kind or another. But what exactly is energy?
Energy is a bit of a mystery. Most of the time we can't see it, yetit is everywhere around us. Revving car engines burn energy, hot cupsof coffee hold energy, street lights that shine at night are usingenergy, sleeping dogs are using energy too—absolutely everything youcan think of is using energy in one way or another. Energy is a magicalthing that makes other things happen. Everything in the world is eitherenergy or matter ("stuff" around us) and even matter, when you reallyget down to it, is a kind of energy!
Picture: Asupernova is the remains of an exploding star and it's just about the most spectacular release of energy you can get.This particular one is a gigantic explosion of dusty gas 14 light-years across(roughly 132 billion kilometers) and booming outward at 2,000 km per second (or 4 million mph).Composite photo of Kepler's Supernova courtesy of NASA/ESA/JHU/R.Sankrit & W.Blair,andNASA Chandra X-Ray Observatory.
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Although there are many kinds of energy in the world, they all fallinto two broad categories:potential energyandkineticenergy. When energy is stored up and waiting to do things, wecall it potential energy; "potential" simply means the energy has theability to do something useful later on. When stored energy is being used to dosomething, we call it kinetic energy; "kinetic" means movement and, generally,when stored energy is being used up, it is making things move or happen.
It's easy to find examples of both potential energy and kineticenergy in the world around us. If you push a boulder up a hill, you'llfind it's a real effort to get to the top. This is becausethe force ofgravity is constantly trying to pull you (and the boulder)back down. In science, we say you have to doworkagainst the force of gravity to push the boulder up the hill. Doing work meansyou have to use energy: the muscles in your body have to convert sugarand fat to make the energy you need to push the boulder. Where doesthis energy go? Although you use energy as you climb, your body and theboulder also gain energy—potential energy. When the boulder is at thetop of the hill, you can let it go so it rolls back down again. It canroll down because it has stored potential energy. In other words, ithas the potential to roll down the hill all by itself.
Artwork: You have to "do work" against the force of gravitywhen you push a boulder up a hill and lose energy as you do so; the boulder gains this "potential" energy as it climbs.
As the boulder starts to roll down the hill, the potential energy ithad at the top is gradually converted into kinetic energy. When we talkabout kinetic energy, we usually mean theenergy something has because it is moving. Anything that has mass(contains some matter that takes up a volume) and moves along at acertain velocity (or speed) has kinetic energy. The more mass somethinghas and the faster it goes (the higher the velocity), the more kineticenergy it has. If a truck and a car are driving parallel to one anotherdown the freeway, at the same speed, the truck has more kinetic energythan the car because it has much more mass. (Read more about thescience ofmotion.)
A lot of things we do each day involve converting energy between potential and kinetic.Pull yourself up a cliff on a rope and you have more potential energy the higher you go up.If you abseil down, your potential energy is converted into kinetic energy as you move. By the time you reach the bottom, the kinetic energy has turned toheat (your climbingequipment and the rope will get surprisingly hot) andsound (the rope will make a noise as you whiz down).
Artwork: You gain potential energy every time you walk up stairs. Your muscles pull your body against the force of gravity, doing work. In theory, the potential energy your body gains as you climb is exactly the same as the food energy it loses: one form of energy is simply converted into another. (In practice, you need to use more energy than you might think because your body wastes quite a lot of energy in the process.) At the top of a flight of stairs, you could turn your stored potential energy back into kinetic energy (movement) in various ways, such as sliding down the banisters or jumping down a fireman's pole! You can trace every bit of energy your body uses back to the food you eat, which comes from animals and plants and ultimately from the Sun.
Photo: Now that's what I call kinetic energy!A spacecraft travels at something like 40,000 km/h (25,000 mph or 11,000 m/s)as it re-enters Earth's orbit.Assuming it weighs about 30,000 kg, then, according to my calculations, it has enough energy to power anelectric toaster constantly for about 30 years!Picture of Apollo 8 taken in 1968 by US Air Force courtesy ofNASA on the Commons.
Things can have potential and kinetic energy for other reasons. Hereare some more examples. A thundercloud passing overhead has "thepotential" to release electrical energy as huge bolts of lightning. Inother words, we say it haselectrical potentialenergy. Supposeyou want to fire an arrow from a bow. When you pull back the elasticbowstring, you have to stretch it well beyond its natural shape. As youdo this, you give it what's known aselasticpotential energy(it is sometimes also calledmechanical potentialenergy). Whenyou release the bowstring, it uses the stored potential energy to firethe arrow through the air.
Photo: A bow and arrow converts elastic potential energy (stored in the bow) into kinetic energy (in the shooting arrow).Photo by Robert A. Whetstone courtesy of US Army andDVIDS.
Just as there are several kinds of potential energy, so there aredifferent kinds of kinetic energy too. When a thundercloud releases itselectrical potential energy as lightning, giant sparks fly from the skyto the ground. A bolt of lightning is a hugeelectriccurrent(flowof electricity) moving through the air—in other words, it is what wemight refer to as "electrical kinetic energy". We can also think ofsound, heat, and light as examples of kinetic energy because theyinvolve energy moving from one place to another.
Photo: Lightning is a huge release of electrical potential energy.
Heat is one of the most familiar kinds of energy in our world—butis it potential energy or kinetic energy? Actually, it can be both.Suppose you heat aniron bar in a fire so it glows red hot. If youplunge it into a bucketful of cold water, you'll make a huge amount ofsteam. The energy from the hot bar goes into the water and heats thatup too, losing some of its own energy in the process. This means that ahot bar—a bar with heat energy—has potential energy: it has thepotential to heat something else up.
But a hot bar also has kinetic energy. Inside an iron bar, there arebillions of ironatoms held together in a rigid structure calleda crystal lattice. It's a bit like a climbing frame with atoms at thejoints. Although the atoms are pretty much fixed in the same place, they areconstantly jiggling about. Each atom has a little bit of kineticenergy. The more you heat an iron bar and the hotter it becomes, themorethe atoms jiggle about—and the more kinetic energy they have. In otherwords, heat is held inside the bar by the jiggling atoms and theirkinetic energy. The idea that heat is caused by atoms and moleculesmoving around is known as thekinetictheory of matter.
Hot objects like to pass their heat energy to other things nearby.If you touch something hot, some of its heat energy flows into you—andyou get burned.This is calledheat conduction. But youdon't have to touchsomething to feel its heat. If you sit some distance from a roaringfire, you'll be able to feel its heat energy on your cheeks even thoughtheflames are not actually touching you. This happens because the firepassesits energy through empty space by a process calledheatradiation.Radiation is the way the Sun passes its energy through about 150 million km (93 million miles)of empty space to earth in a journey that takes a little over 8minutes.
Heat energy also moves in a third way, known asheatconvection.If you put a pan of soup on top of the stove and heat it up, heattravels from the stove to the pan by conduction. The soup at the bottomof the pan quickly warms up. This makes it less dense ("thinner") thanthe soupabove it, so it rises upward. As the warm soup rises, it pushes thecolder soup at the top out of the way, and the cold soup falls backdownto take its place. Pretty soon, there's a kind of invisible loopforming inside the soup, with heat energy constantly being carried upfrom the stove and circulating through the liquid up above. Thisprocess is also how heat travels through ahot air balloon, from theburner at the bottom, so it systematically heats up all the gas inside.
You can read more about this topic in our main article onheat.
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Photo: The Sun is a blazing red ball of heat energy.Most of our energy comes directly or indirectly from it.Picture courtesy of NASA/GSFC/SDO andNASA on the Commons.
Where does energy come from? Well, if you have a hot cup ofcoffeesitting on your desk, the heat energy it contains originally came fromthe hot water you used to make it. The hot water got its energy fromthekettle you put on the stove or plugged into the electricity outlet.And where did the electricity come from? Most likely, from apower plant, which burned a fuel such as gas, coal, or oil to release theenergy it contained. But where did the energy in that fuel come fromoriginally?
You can play this energy game forever, tracing energy from onething to another—all the way back to its original source. Wherever youstart from and however you go, you pretty much always end up at thesame point: the Sun. This giant fireball in space provides over 99percent of the energy we use on earth. You may thinksolar power isfuturistic and impractical, but in fact the world has been solarpowered ever since it was created. Playing the energy game revealssomething else as well: we can never actually create energy or destroyit. Instead, all we can do is convert it from one form to another. Thisidea, which is one of the most basic laws of physics, is known as theconservation of energy.
The energy we use in our daily lives falls into three broadcategories: the food we eat to keep our bodies going, the energy we usein our homes, and the fuel we put in our vehicles. The food we eat comes from plants and animals, which our stomachs digest to make asugary substance called glucose that blood transports around our bodiesto power our muscles. All animals ultimately get their energy fromplants, which are themselves powered by sunlight. Plants are likelivingsolar panels that absorb the Sun's energy and convert it intofood. The energy we use in our homes tends to be provided by coal, gas,and oil. These three "fossil fuels" areunderground supplies ofenergy, created millions of years ago, that wedrill, mine, or pipe tothe surface to satisfy our energy needs today. Most of the energy weuse in our vehicles also comes from oil. The trouble with fossil fuelsis that we are using them much more quickly than we are creating them.Another problem is that burning fossil fuels creates a gas calledcarbon dioxide that is building up in Earth's atmosphere and causing aproblem known asglobal warming (climatechange).
Photo: Plants are like living solar panels. It's amazing to think that nature producedsomething that can automatically capture and store solar energy in a very efficient way—something thatthe world's best scientists and engineers are still struggling to do!
Fossil fuels such as oil, gas, and coal have been enormously helpfulto humankind's economic development. Coal powered the industrialrevolution in the 18th and 19th centuries, whileoil made possible a huge growth in personal transportation followingthe invention of theinternal combustion engine. Natural gas, a much cleaner and more efficient fuel, has become an increasingly important source ofpower since the mid-20th-century. Yet all these fuels havetheir drawbacks. Coal is dirty and inefficient. Oil exists in limitedsupplies in places such as the Middle East and growing demand for it isa major source of world tensions and wars. Gas, though easy to move fromplace to place, can be dangerous when it leaks or escapes. Turningcoal, gas, oil and other fuels into electricity is a way to make themmuch more versatile and useful.
Electricity is a kind of energyusually made inpower plants byburning fuels. According to theUS EIA, about 60 percent of the electricity made in the United States comes from burning gas (43.1 percent), coal (16.2 percent), and oil (0.4 percent). Inside a power plant, fuel is burned in a huge furnace to release the energy it contains as heat.The heat is used to boil water and produce steam, which turns arotating propeller-like mechanism called aturbine.The turbine is connected to an electricity maker orgenerator,which produces electricity as the turbine spins it around.
The great thing about electricity is that it is so versatile. Almostany kind of fuel can be turned into electricity. Once electricity hasbeen made in a power plant, it is easy to transmit from one place toanother either overground or underground along cables. Inside homes,factories, and offices, electricity is turned back into other kinds ofenergy by a wide range of appliances. If you have an electrical stoveortoaster, it takes electricity supplied by a power plant and convertsit back into heat energy for cooking food. The lights in your homeconvert electrical energy into light energy (and, unless you are usingenergy-efficient light bulbs,quite a lot of heat). Your stereo orMP3 player turns electricityback into light, while yourcellphone(mobilephone) uses it to makeradio waves.
Photo: Petroleum refineries like this may shut down in future as oil supplies start to run dry.Picture by David Parsons courtesy of National Renewable Energy Laboratory (NREL).
According to theUS EIA, world energy use could grow by 50 percent (half as much again) between 2020 and 2050.Around 82 percent of the energy we use on Earth today comes fromfossil fuels,[1]but that cannot continue much longer. Fossil fuels willrun out sooner or later and, even if they last longer than expected,they could make global warming run out of control.
Fortunately, because much of the power we use comes from electricity, we have alternatives.We can make electricity fromwind power, for example, orsolar panels.We can incinerate trash to generate heat that will drive a powerstation (though at the risk of producingair pollution). We can grow so-called "energy crops" (biomass) to burn in our powerstations instead of fossil fuels. And we can harness the huge reservesof heat trapped inside Earth, known as geothermal energy. Together,these energy sources are known asrenewable energy,because they will last forever(or, at least as long as the Sun keeps shining) without running out.Earth's supplies of renewable energy are vast. A 3m (10ft) high ocean wavehas enough power per meter (3.3ft) of its width to power 1000 light bulbs.[2]If we could cover just one percent of theSahara Desert with solar panels (an area slightly smaller than theUnitedStates), we could make more than enough electricity for our entireplanet.[3]
Photo: In the future, we'll need to get better at using renewable energysources, such as Earth's internal heat (geothermal energy). Picture by Carol M. Highsmith, courtesy of Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division..
We'll also need to be smarter in the way we use energy. By designingmachines and appliances that do the same jobs but use less power, wecan make the energy we have go much further. This is calledenergyefficiency (saving energy) and it's like a completely free wayof makingpower. Energy companies often find it cheaper to give away thousands ofenergy-efficient light bulbs than build new power plants.
What about cars? In the future, most of our vehicles will be poweredby electricity from onboardbatteries or battery-like devices calledfuel cells, which usehydrogen gas to generate electricity and powerelectric motors.Electric vehicles first became popular in places likeCalifornia and are now finally taking off worldwide. Hybrid vehicles are also helping to makeoil go further. Unlike a conventional car, a hybrid car has two "engines": one of them, astandard petrol engine, is used for high-speed driving—down thefreeway, for example; the other, a compact electric motor, powers thecar cleanly, quietly, and efficiently in cities.
Today, most of our electricity comes from far-off power plantstransmitted down huge lengths of cable. It takes energy to move energyfrom one place to another. Making electricity in remote power plantsand transmitting it down wires wastes around two thirds of its energy.In other words, if you burn three tons of coal in a power plant, youwaste two tons of it getting the energy out of the coal, makingelectricity, andtransmitting the electric power to customers. This is why buildings ofthe future arelikely to make more of their ownlocal power,for example, with solar panels, communitywind turbines,orheat pumps that "suck" stored energy from the ground beneath out feet.
Each second, the Sun sends out more power than all the energy peopleon Earth would use in about three quarters of a million years.[4]Not all of this energy reachesour planet and it's not all in a form we can capture. But if we thinkabout the energy we use, and use it more wisely, there's no reason whywe should ever run out—or why we should spoil our planet for tomorrow'schildren when we make the energy we use today.
This chart shows that developed countries use much more energy thandeveloping countries. Since 2009, China has used more energy in total than any other country in the world(including the United States).
Source: Drawn by Explainthatstuff.com using data fromEnergy Institute Statistical Review of World Energy 2023: Primary Energy (Consumption), p8, showing 2022 figures. "Europe/Eurasia" includes figures for Europe and CIS.
Just eleven countries produce three quarters of the world's oil(in order of production, they are: United States, Saudi Arabia, Russian Federation, Iraq, Canada, United Arab Emirates, Kuwait, China, Iran, Brazil, and Nigeria). Although the United States is one of the world's biggest oil producers,it's also the world's biggest oil consumer by far. It imports more oil thanany other country—and almost 50 percent more than China. Although people assume most of the world's oil comesfrom the Middle East, two thirds is supplied by other parts of the world.
Source: Drawn by Explainthatstuff.com using data from Energy Institute Statistical Review of World Energy 2023: Oil Production, p15 (2022 figures)and BP Statistical Review of World Energy 2018: Oil Production, p14 (2017 figures).
Even so, the Middle East still has almost half of the world's total proved oil reserves:
Source: Drawn by Explainthatstuff.com using data from BP Statistical Review of World Energy 2021: Oil (Total proved reserves), p16 (2020 figures).
Despite all the talk of "green energy", fossil fuels still supplyabout 82 percent of all world energy. Use of coal is now falling (down from 30 percent in 2015to 27 percent in 2022), while renewables are increasing (up from 2 percent in 2015, 3 percent in 2016,and 6 percent in 2020 to 7 percent in 2022).
Source: Drawn by Explainthatstuff.com using data from Energy Institute Statistical Review of World Energy 2023: Consumption by Fuel, p9, showing 2022 figures.
According to the US government's Energy Information Administration,world energy consumption will increase by about three quarters between 2000 and 2030,and double between 2000 and 2040.The biggest growth will be in developing countries such as China and India (and other nations outside theOECD).
Source: Drawn by Explainthatstuff.com using datafor world energy consumption 1990–2040,fromUS Energy Information Administration (EIA):International Energy Outlook 2016. Figures are in quadrillion BTU (British thermal units).
Chris Woodford is the author and editor of dozens of science and technology books for adults and children, including DK's worldwide bestsellingCool Stuff series andAtoms Under the Floorboards, which won the American Institute of Physics Science Writing award in 2016. You can hire him to write books, articles, scripts, corporate copy, and more via his websitechriswoodford.com.
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