Energy

Energy comes from a very old word in Ancient Greek, meaning "activity." It is a special kind of property that can be moved to a body or a physical system. We can see energy when something does work, gives off heat, or makes light. One important rule about energy is that it cannot be created or destroyed—it can only change its shape. We measure energy using a unit called the joule.

There are many kinds of energy. For example, when something moves, it has kinetic energy. An object that is held high up has potential energy because of its position. Energy is also stored in the chemicals that make up food and fuel—this is called chemical energy. The warmth from the Sun and the light we see are forms of energy too, carried by electromagnetic radiation. Even the tiniest pieces of matter have a special kind of energy called rest energy.

All living things, like plants, animals, and humans, need energy to grow, move, and stay alive. They get this energy from food and the Sun. The whole planet, with its weather and living things, runs on the energy that comes from the Sun. Understanding energy helps us explain how everything in our world works.

Forms

The total energy of a system can be divided into different types, such as potential energy and kinetic energy. Kinetic energy comes from the movement of an object or its parts, while potential energy is the stored energy an object has because of its position or what is around it.

We often group these types together to talk about specific kinds of energy. For example, when we look at the movement and position of parts of a system, we call this mechanical energy. Another type is nuclear energy, which comes from the forces inside atoms.

History

Main articles: History of energy and timeline of thermodynamics, statistical mechanics, and random processes

The word energy comes from an ancient Greek word meaning "activity." Long ago, people thought about energy in simple ways, like feeling happy or pleased.

Later, scientists began to study energy more closely. They learned that energy can change forms but cannot be created or destroyed. They also discovered that heat, light, and movement are all related to energy. Today, we know that even the tiny particles that make up atoms have energy, and scientists continue to explore how energy works in the universe.

Units of measure

Main article: Units of energy

Energy is measured in a special unit called the joule. One joule is the amount of energy needed to push something with a force of one newton over a distance of one meter. Another way to think about it is like counting how much power we use each second. For example, a kilowatt-hour—which is the energy used by a machine that runs at one kilowatt for one hour—equals 3.6 million joules.

Different fields sometimes use other units to measure energy. Scientists might use electronvolts when talking about the energy of tiny particles. We use food calories to talk about the energy we get from eating, and BTUs are used to measure energy used in heating and cooling. Each of these units just helps us understand and compare energy in different ways.

Scientific use

Classical mechanics

Main articles: Mechanics, Mechanical work, and Thermodynamics

In classical mechanics, energy is a useful idea because it stays the same no matter how things change. It helps us understand how things move and work.

Work is a way to move energy around. It happens when a force pushes something over a distance.

The total energy of a group of things is sometimes called the Hamiltonian. This helps us describe how things move, even very complicated ones.

Another idea is the Lagrangian. It looks at the difference between two kinds of energy to figure out how things move.

Chemistry

Main article: Chemical energy

In chemistry, energy is tied to the tiny parts that make up substances. When chemicals change, their energy changes too. Some reactions give off energy, like when wood burns. Others need energy to happen, like when plants grow.

Chemical reactions need a little energy to get started. This is called activation energy. The speed of a reaction depends on how much energy the chemicals have.

Biology

Main articles: Bioenergetics and Food energy

In biology, energy is important for all living things. It helps plants grow and animals move. Living things get energy from food or sunlight.

Plants use sunlight to make food in a process called photosynthesis. Animals get energy by eating plants or other animals. Our bodies use this energy to stay alive and do things.

Earth sciences

In Earth sciences, energy explains things like mountains, volcanoes, and weather. The Sun's energy drives weather patterns like wind and rain. Inside the Earth, heat from radioactive elements moves pieces of the Earth's crust.

Cosmology

In space, huge amounts of energy are released when stars are born or explode. The Sun's energy comes from nuclear fusion, where tiny parts called atoms join together.

Quantum mechanics

In quantum mechanics, energy is linked to how tiny particles behave. It helps us understand how light and matter work together.

Relativity

When things move very fast, their energy changes in surprising ways. Even things that are not moving have energy because of their mass. This is shown in the famous equation E=mc².

Transformation

Energy can change from one type to another. For example, a battery changes chemical energy into electric energy. A dam uses the gravitational energy of water to spin a turbine, which then creates electric energy. The Sun changes nuclear energy into light and heat.

When an object falls, its stored energy changes into moving energy. In a pendulum, energy swings between moving energy and stored energy. These changes show that energy cannot be created or destroyed, only changed from one form to another.

Conservation of energy

Main article: Conservation of energy

Energy cannot be created or destroyed — this idea is called the law of conservation of energy. It means that the total amount of energy in a closed space stays the same, unless energy moves in or out as work or heat.

When we look at the total energy of a group of particles, we always find that the energy stays constant. This law is a key idea in physics, showing that energy can change forms but the total amount never changes.

Energy transfer

Energy can move from one place to another. When we talk about systems that don't let anything in or out except energy, we call them closed systems. In these systems, energy can be transferred as work — like pushing a toy car — or as heat — like feeling warmth from a heater. Energy can travel in many ways, such as through light, moving objects bumping into each other, or even through water moving because of tides.

In open systems, where matter can come in or out, energy can also change along with the matter. For example, when fuel is added to a car engine, it brings extra energy into the system. This helps us understand how cars and many other machines work.

Thermodynamics

Energy is like a hidden power inside everything. It can change from one type to another, but it never disappears or appears out of nowhere. This idea is called the law of conservation of energy.

One important type of energy is called internal energy. It includes all the tiny ways energy can hide inside a group of particles, like how they move or how they are arranged. When we heat something up or squeeze it, we change its internal energy. Scientists have special rules to describe how this energy changes, which help us understand how things work in the world around us.