Energy
by Chris Woodford. Last updated: October 29, 2021.
Try to think of something that doesn't involve energy and you
won't get very far. Even thinking—even thinking about energy!—needs
some energy to make it happen. In fact, everything that happens in the
world 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, yet
it is everywhere around us. Revving car engines burn energy, hot cups
of coffee hold energy, street lights that shine at night are using
energy, sleeping dogs are using energy too—absolutely everything you
can think of is using energy in one way or another. Energy is a magical
thing that makes other things happen. Everything in the world is either
energy or matter ("stuff" around us) and even matter, when you really
get down to it, is a kind of energy!
Picture: A supernova 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.
Potential energy and kinetic energy
Although there are many kinds of energy in the world, they all fall
into two broad categories: potential energy
and kinetic
energy. When energy is stored up and waiting to do things, we
call it potential energy; "potential" simply means the energy has the
ability to do something useful later on. When stored energy is being used to do
something, 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 kinetic
energy in the world around us. If you push a boulder up a hill, you'll
find it's a real effort to get to the top. This is because
the force of gravity is constantly trying to pull you (and the boulder)
back down. In science, we say you have to do work
against the force of gravity to push the boulder up the hill. Doing work means
you have to use energy: the muscles in your body have to convert sugar
and fat to make the energy you need to push the boulder. Where does
this energy go? Although you use energy as you climb, your body and the
boulder also gain energy—potential energy. When the boulder is at the
top of the hill, you can let it go so it rolls back down again. It can
roll down because it has stored potential energy. In other words, it
has the potential to roll down the hill all by itself.
Artwork: You have to "do work" against the force of gravity
when 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 it
had at the top is gradually converted into kinetic energy. When we talk
about kinetic energy, we usually mean the
energy something has because it is moving. Anything that has mass
(contains some matter that takes up a volume) and moves along at a
certain velocity (or speed) has kinetic energy. The more mass something
has and the faster it goes (the higher the velocity), the more kinetic
energy it has. If a truck and a car are driving parallel to one another
down the freeway, at the same speed, the truck has more kinetic energy
than the car because it has much more mass. (Read more about the
science of motion.)
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 to heat (your climbing
equipment and the rope will get surprisingly hot) and sound (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.
Other kinds of potential and kinetic energy
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 an
electric toaster constantly for about 30 years!
Picture of Apollo 8 taken in 1968 by US Air Force courtesy of NASA on the Commons.
Things can have potential and kinetic energy for other reasons. Here
are some more examples. A thundercloud passing overhead has "the
potential" to release electrical energy as huge bolts of lightning. In
other words, we say it has electrical potential
energy. Suppose
you want to fire an arrow from a bow. When you pull back the elastic
bowstring, you have to stretch it well beyond its natural shape. As you
do this, you give it what's known as elastic
potential energy
(it is sometimes also called mechanical potential
energy). When
you release the bowstring, it uses the stored potential energy to fire
the arrow through the air.
Just as there are several kinds of potential energy, so there are
different kinds of kinetic energy too. When a thundercloud releases its
electrical potential energy as lightning, giant sparks fly from the sky
to the ground. A bolt of lightning is a huge electric
current (flow
of electricity) moving through the air—in other words, it is what we
might refer to as "electrical kinetic energy". We can also think of
sound, heat, and light as examples of kinetic energy because they
involve energy moving from one place to another.
Photo: Lightning is a huge release of electrical potential energy.
Heat energy
Heat is one of the most familiar kinds of energy in our world—but
is it potential energy or kinetic energy? Actually, it can be both.
Suppose you heat an iron bar in a fire so it glows red hot. If you
plunge it into a bucketful of cold water, you'll make a huge amount of
steam. The energy from the hot bar goes into the water and heats that
up too, losing some of its own energy in the process. This means that a
hot bar—a bar with heat energy—has potential energy: it has the
potential to heat something else up.
But a hot bar also has kinetic energy. Inside an iron bar, there are
billions of iron atoms held together in a rigid structure called
a crystal lattice. It's a bit like a climbing frame with atoms at the
joints. Although the atoms are pretty much fixed in the same place, they are
constantly jiggling about. Each atom has a little bit of kinetic
energy. The more you heat an iron bar and the hotter it becomes, the
more
the atoms jiggle about—and the more kinetic energy they have. In other
words, heat is held inside the bar by the jiggling atoms and their
kinetic energy. The idea that heat is caused by atoms and molecules
moving around is known as the kinetic
theory 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—and
you get burned.
This is called heat conduction. But you
don't have to touch
something to feel its heat. If you sit some distance from a roaring
fire, you'll be able to feel its heat energy on your cheeks even though
the
flames are not actually touching you. This happens because the fire
passes
its energy through empty space by a process called heat
radiation.
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 8
minutes.
Heat energy also moves in a third way, known as heat
convection.
If you put a pan of soup on top of the stove and heat it up, heat
travels from the stove to the pan by conduction. The soup at the bottom
of the pan quickly warms up. This makes it less dense ("thinner") than
the soup
above it, so it rises upward. As the warm soup rises, it pushes the
colder soup at the top out of the way, and the cold soup falls back
down
to take its place. Pretty soon, there's a kind of invisible loop
forming inside the soup, with heat energy constantly being carried up
from the stove and circulating through the liquid up above. This
process is also how heat travels through a hot air balloon, from the
burner at the bottom, so it systematically heats up all the gas inside.
You can read more about this topic in our main article on heat.
Making and using energy
Photo: The Sun is a spectacular collection of heat energy.
Most of our energy comes directly or indirectly from the Sun.
This image was taken by a telescope called the Extreme Ultraviolet Imaging Telescope,
part of the Solar and Heliospheric Observatory (SOHO), which is a joint project of the European Space Agency and NASA.
Picture courtesy of NASA Goddard Space Flight Center.
Where does energy come from? Well, if you have a hot cup of coffee
sitting on your desk, the heat energy it contains originally came from
the hot water you used to make it. The hot water got its energy from
the kettle you put on the stove or plugged into the electricity outlet.
And where did the electricity come from? Most likely, from a
power plant, which burned a fuel such as gas, coal, or oil to release the
energy it contained. But where did the energy in that fuel come from
originally?
You can play this energy game forever, tracing energy from one
thing to another—all the way back to its original source. Wherever you
start from and however you go, you pretty much always end up at the
same point: the Sun. This giant fireball in space provides over 99
percent of the energy we use on earth. You may think
solar power is
futuristic and impractical, but in fact the world has been solar
powered ever since it was created. Playing the energy game reveals
something else as well: we can never actually create energy or destroy
it. Instead, all we can do is convert it from one form to another. This
idea, which is one of the most basic laws of physics, is known as the
conservation of energy.
The energy we use in our daily lives falls into three broad
categories: the food we eat to keep our bodies going, the energy we use
in 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 a
sugary substance called glucose that blood transports around our bodies
to power our muscles. All animals ultimately get their energy from
plants, which are themselves powered by sunlight.
Plants are like
living solar panels that absorb the Sun's energy and convert it into
food. The energy we use in our homes tends to be provided by coal, gas,
and oil. These three "fossil fuels" are
underground supplies of
energy, created millions of years ago, that we drill, mine, or pipe to
the surface to satisfy our energy needs today. Most of the energy we
use in our vehicles also comes from oil. The trouble with fossil fuels
is that we are using them much more quickly than we are creating them.
Another problem is that burning fossil fuels creates a gas called
carbon dioxide that is building up in Earth's atmosphere and causing a
problem known as global warming (climate
change).
Photo: Plants are like living solar panels. It's amazing to think that nature produced
something that can automatically capture and store solar energy in a very efficient way—something that
the world's best scientists and engineers are still struggling to do!
Electricity—the best kind of energy?
Fossil fuels such as oil, gas, and coal have been enormously helpful
to humankind's economic development. Coal powered the industrial
revolution in the 18th and 19th centuries, while
oil made possible a huge growth in personal transportation following
the invention of the internal combustion engine.
Natural gas, a much cleaner and more efficient fuel, has become an increasingly important source of
power since the mid-20th-century. Yet all these fuels have
their drawbacks. Coal is dirty and inefficient. Oil exists in limited
supplies in places such as the Middle East and growing demand for it is
a major source of world tensions and wars. Gas, though easy to move from
place to place, can be dangerous when it leaks or escapes. Turning
coal, gas, oil and other fuels into electricity is a way to make them
much more versatile and useful.
Electricity is a kind of energy
usually made in power plants by
burning fuels. According to the US EIA, just over 60 percent of the electricity made in the United States comes from
burning gas (40 percent), coal (19.3 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 a
rotating propeller-like mechanism called a turbine.
The turbine is connected to an electricity maker or generator,
which produces electricity as the turbine spins it around.
The great thing about electricity is that it is so versatile. Almost
any kind of fuel can be turned into electricity. Once electricity has
been made in a power plant, it is easy to transmit from one place to
another either overground or underground along cables. Inside homes,
factories, and offices, electricity is turned back into other kinds of
energy by a wide range of appliances. If you have an electrical stove
or toaster, it takes electricity supplied by a power plant and converts
it back into heat energy for cooking food. The lights in your home
convert electrical energy into light energy (and, unless you are using
energy-efficient light bulbs,
quite a lot of heat). Your stereo or MP3 player turns electricity
back into light, while your cellphone
(mobile
phone) uses it to make radio waves.
Energy future
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 the US EIA, world energy use is forecast to grow by 50 percent
(half as much again) between 2020 and 2050.
Around 83 percent of the energy we use on Earth today comes from
fossil fuels,
[1]
but that cannot continue much longer. Fossil fuels will
run 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 from wind power, for example, or
solar panels.
We can incinerate trash to generate heat that will drive a power
station (though at the risk of producing air pollution). We can grow so-called "energy crops" (biomass) to burn in our power
stations instead of fossil fuels. And we can harness the huge reserves
of heat trapped inside Earth, known as geothermal energy. Together,
these energy sources are known as renewable 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 wave
has enough power per meter (3.3ft) of its width to power 1000 light bulbs.
[2]
If we could cover just one percent of the
Sahara Desert with solar panels (an area slightly smaller than the
United
States), we could make more than enough electricity for our entire
planet.
[3]
Photo: In the future, we'll need to get better at using renewable energy
sources, such as Earth's internal heat (geothermal energy).
Picture by Robert Blackett, Utah Geological Survey, courtesy of
National Renewable Energy Laboratory (NREL).
We'll also need to be smarter in the way we use energy. By designing
machines and appliances that do the same jobs but use less power, we
can make the energy we have go much further. This is called energy
efficiency (saving energy) and it's like a completely free way
of making
power. Energy companies often find it cheaper to give away thousands of
energy-efficient light bulbs than build new power plants.
What about cars? In the future, most of our vehicles will be powered
by electricity from onboard batteries or battery-like devices called
fuel cells, which use
hydrogen gas to generate electricity and power electric motors.
Electric vehicles first became popular in places like
California and are now finally taking off worldwide.
Hybrid vehicles are also helping to make
oil go further. Unlike a conventional car, a hybrid car has two "engines": one of them, a
standard petrol engine, is used for high-speed driving—down the
freeway, for example; the other, a compact electric motor, powers the
car cleanly, quietly, and efficiently in cities.
Today, most of our electricity comes from far-off power plants
transmitted down huge lengths of cable. It takes energy to move energy
from one place to another. Making electricity in remote power plants
and 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, you
waste two tons of it getting the energy out of the coal, making
electricity, and
transmitting the electric power to customers. This is why buildings of
the future are
likely to make more of their own local power,
for example, with solar panels, community wind turbines,
or heat pumps that "suck" stored energy from the ground beneath out feet.
Each second, the Sun sends out more power than all the energy people
on Earth would use in about three quarters of a million years.
[4]
Not all of this energy reaches
our planet and it's not all in a form we can capture. But if we think
about the energy we use, and use it more wisely, there's no reason why
we should ever run out—or why we should spoil our planet for tomorrow's
children when we make the energy we use today.
World of energy
Which world regions use most energy?
This chart shows that developed countries use much more energy than
developing 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 from
BP Statistical Review of World Energy 2021: Primary Energy (Consumption), p11, showing 2020 figures.
"Europe/Eurasia" includes BP's figures for Europe and CIS.
Where does the world's oil come from?
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 than
any other country—and almost 50 percent more than China. Although people assume most of the world's oil comes
from the Middle East, two thirds is supplied by other parts of the world.
Source: Drawn by Explainthatstuff.com using data from
BP Statistical Review of World Energy 2021: Oil Production, p18 (2020 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).
Which fuels supply the world's energy?
Despite all the talk of "green energy", fossil fuels still supply
about 83 percent of all world energy. Use of coal is now falling (down from 30 percent in 2015
to 27 percent in 2020), while renewables are increasing (up from 2 percent in 2015 and 3 percent in 2016
to 6 percent in 2020).
Source: Drawn by Explainthatstuff.com using data from BP Statistical Review of World Energy 2021: Consumption by Fuel, p11, showing 2020 figures.
How much energy will the world use in future?
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 the
OECD).
Source: Drawn by Explainthatstuff.com using data
for world energy consumption 1990–2040,
from US Energy Information Administration (EIA):
International Energy Outlook 2016. Figures are in quadrillion BTU (British thermal units).
Look here for more energy statistics...
