When a lamp turns on, water boils, a bicycle moves, or a phone charges, energy is being transferred or transformed. In physics, energy is the ability to do work or cause change.
That definition matters because energy is not always visible as an object. We read energy from its effects: an object moves, temperature changes, light appears, sound is produced, or electric charge flows.
energy→ability to do work or cause change
This explanation follows the definition used by the U.S. Energy Information Administration (EIA). You can open EIA's Energy Explained page about energy through this source link.
Energy is often calculated through work. In physics, work happens when a force causes displacement in the direction of that force.
W=FΔs
From here on, we use the International System of Units (SI) so force, displacement, and energy use one consistent unit system.
Symbol guide:
Symbol
Meaning
SI unit
W
work or transferred energy
J
F
force
N
Δs
displacement
m
Suppose you push a box with a force of 15 N until it moves 2 m. The energy transferred to the box through work is:
W=FΔs=15 N×2 m=30 J
So, the box receives 30 J of energy through that push.
If a force is applied but the object does not move, the mechanical work on the object is 0 J. The person pushing may still feel tired, but that energy does not become mechanical work on the object that stays still.
In SI, the unit for energy is the joule, with symbol J. Because energy can be calculated from force times displacement, a joule can be read as:
1 J=1 N⋅1 m=1 kg m2s−2
The National Institute of Standards and Technology (NIST) lists the joule as an SI derived unit for energy, work, and heat. NIST SP 330 section 2 can be opened through the SI reference, while the NIST joule glossary can be opened through the joule reference.
Energy moves from a source into a device, then changes into the form we need.
Follow Energy from Source to Use
The diagram shows a pattern, not every technical detail. In a solar panel, light energy changes into electrical energy. In hydropower, gravitational potential energy of water changes into turbine motion, then into electrical energy. In the body, chemical energy from food helps muscles do work.
Renewable energy is not just a list of energy sources. Its foundation is a more basic physics question: where does the energy come from, what does it become, and how much of it is actually useful?
Situation
Physics question
Solar panels are installed on a roof
How much light energy becomes electrical energy?
Flowing water turns a turbine
How much water motion energy can be transferred to a generator?
Home lamps turn on every night
How much electrical energy is used in kWh?
Fuel burns inside an engine
How much chemical energy becomes motion and heat?
An energy source is called renewable when it can be replenished naturally on a human timescale, such as sunlight, wind, flowing water, geothermal heat, and properly managed biomass. An energy source is called non-renewable when it forms much more slowly than it is used, such as coal, oil, and natural gas. The classification reference comes from the U.S. Energy Information Administration (EIA), on EIA's Renewable Energy Explained page through this source link.
The International Energy Agency (IEA) reported that global renewable capacity additions reached nearly 510 GW, or 510 gigawatts, in 2023. You do not need to memorize that number here, but it shows why energy, power, and energy transformation matter when discussing the future of electricity. The number comes from IEA's Renewables 2023 report through this source link.
These ideas give the basic language for forms of energy, conservation of energy, energy transformation, and why energy demand should be met with cleaner sources.
