a physical quantity that follows precise natural laws.

Our universe has a finite amount of it;

it's neither created nor destroyed

but can take different forms,

such as kinetic or potential energy,

with different properties and formulas to remember.

For instance,

an LED desk lamp's 6 Watt bulb

transfers 6 Joules of light energy per second.

But let's jump back up into space

to look at our planet, its systems, and their energy flow.

Earth's physical systems include

the atmosphere,

hydrosphere,

lithosphere,

and biosphere.

Energy moves in and out of these systems,

and during any energy transfer between them,

some is lost to the surroundings,

as heat, light, sound, vibration, or movement.

Our planet's energy comes from

internal and external sources;

geothermal energy from radioactive isotopes

and rotational energy from the spinning of the Earth

are internal sources of energy,

while the Sun is the major external source,

driving certain systems,

like our weather and climate.

Sunlight warms the surface and atmosphere

in varying amounts,

and this causes convection,

producing winds and influencing ocean currents.

Infrared radiation, radiating out

from the warmed surface of the Earth,

gets trapped by greenhouse gases

and further affects the energy flow.

The Sun is also the major source of energy

for organisms.

Plants, algae, and cyanobacteria

use sunlight to produce organic matter

from carbon dioxide and water,

powering the biosphere's food chains.

We release this food energy

using chemical reactions,

like combustion and respiration.

At each level in a food chain,

some energy is stored

in newly made chemical structures,

but most is lost to the surroundings,

as heat, like your body heat,

released by your digestion of food.

Now, as plants are eaten by primary consumers,

only about 10% of their total energy

is passed on to the next level.

Since energy can only flow in one direction

in a food chain,

from producers onto consumers and decomposers,

an organism that eats lower on the food chain,

is more efficient than one higher up.

So eating producers is the most efficient level

at which an animal can get its energy,

but without continual input of energy

to those producers,

mostly from sunlight,

life on Earth as we know it

would cease to exist.

We humans, of course, spend our energy

doing a lot of things besides eating.

We travel, we build, we power all sorts of technology.

To do all this,

we use sources like fossil fuels:

coal, oil, and natural gas,

which contain energy

that plants captured from sunlight

long ago

and stored in the form of carbon.

When we burn fossil fuels in power plants,

we release this stored energy

to generate electricity.

To generate electricity,

heat from burning fossil fuels

is used to power turbines

that rotate magnets,

which, in turn, create magnetic field changes

relative to a coil of wire,

causing electrons to be induced to flow in the wire.

Modern civilization depends on our ability

to keep powering that flow of electrons.

Fortunately, we aren't limited

to burning non-renewable fossil fuels

to generate electricity.

Electrons can also be induced to flow

by direct interaction with light particles,

which is how a solar cell operates.

Other renewable energy sources,

such as wind, water, geothermal, and biofuels

can also be used to generate electricity.

Global demand for energy is increasing,

but the planet has limited energy resources

to access through a complex energy infrastructure.

As populations rise,

alongside rates of industrialization and development,

our energy decisions grow more and more important.

Access to energy

impacts health, education, political power, and socioeconomic status.

If we improve our energy efficiency,

we can use our natural resources more responsibly

and improve quality of life for everyone.