The Unstoppable Law: Conservation of Energy
What is Energy and Its Many Forms
Before we can understand how energy is conserved, we need to know what it is. In simple terms, energy is the ability to do work or cause change. It exists in many different forms all around us. Think about a battery powering a toy car, the wind blowing leaves, or the warmth from a campfire. All of these involve energy.
Here are some of the most common forms of energy:
| Form of Energy | Description | Example |
|---|---|---|
| Kinetic Energy | The energy of motion. | A rolling skateboard, a flying baseball. |
| Potential Energy | Stored energy due to position or state. | A book on a high shelf, a stretched rubber band. |
| Thermal Energy | The internal energy of an object due to the motion of its atoms. | A hot cup of cocoa, a warm sidewalk. |
| Chemical Energy | Energy stored in the bonds of atoms and molecules. | Food, batteries, burning wood. |
| Radiant (Light) Energy | Energy that travels in waves, including visible light. | Sunlight, light from a lamp. |
| Electrical Energy | The movement of electrons. | Power from a wall outlet, lightning. |
The Core Principle: Energy Transformation
The Law of Conservation of Energy tells us that while energy can change from one form to another, the total amount of energy always stays the same. Imagine energy as water. You can freeze it into ice, melt it back into water, or boil it into steam, but the amount of H$_2$O molecules remains constant. Energy works the same way.
The total energy in an isolated system is constant. This is often written as:
$E_{total} = KE + PE + TE + ... = constant$
Where $KE$ is kinetic energy, $PE$ is potential energy, and $TE$ is thermal energy. The sum of all these forms of energy at the start of a process equals the sum at the end.
For a falling object, we can use more specific formulas. The kinetic energy (KE) of a moving object is given by $KE = \frac{1}{2}mv^2$, where $m$ is mass and $v$ is velocity. The gravitational potential energy (PE) of an object at a height is $PE = mgh$, where $m$ is mass, $g$ is the acceleration due to gravity, and $h$ is the height.
Energy in Action: A Roller Coaster Ride
A roller coaster is a perfect example of the conservation of energy in action. The ride begins with the coaster being pulled up to the highest point of the first hill. This requires work, which is stored as a large amount of gravitational potential energy.
As the coaster plunges down the hill, this stored potential energy is transformed into kinetic energy, and the car speeds up. At the very bottom of the hill, the potential energy is at its minimum, and the kinetic energy (and speed) is at its maximum.
As the coaster climbs the next hill, it slows down because kinetic energy is being converted back into potential energy. It will never reach a height greater than the first hill because some energy is always transformed into other forms, like thermal energy and sound energy, due to friction with the track and air resistance. The total energy (potential + kinetic + thermal + sound) throughout the entire ride remains constant, perfectly illustrating the law.
Everyday Examples of Energy Conservation
This law isn't just for dramatic physics demonstrations; it's at work in your everyday life.
- A Pendulum: A swinging pendulum continuously converts potential energy at the highest points of its swing into kinetic energy at the lowest point. Over time, it stops because air resistance and friction at the pivot point transform its mechanical energy into thermal energy.
- Using a Battery: When you put batteries in a flashlight, the chemical energy stored inside them is transformed into electrical energy, which then becomes light energy (and a significant amount of thermal energy, which is why the flashlight gets warm).
- Eating Food: Your body is a master of energy transformation. The chemical energy stored in food is converted into kinetic energy when you run, thermal energy to keep you warm, and other forms needed to power your body's functions.
- Photosynthesis: Plants take radiant energy from the sun and convert it into chemical energy, which is stored in the plant's cells (as sugar). This energy later becomes available to animals that eat the plants, and to the animals that eat those animals.
Common Mistakes and Important Questions
If energy is conserved, why do we need to worry about saving energy?
This is a very common point of confusion. When people talk about "saving energy," they are really talking about conserving useful energy resources, like oil and gas. The law of conservation of energy still holds true. When we burn gasoline in a car, the total energy is conserved, but the concentrated chemical energy is transformed into a dispersed form, primarily thermal energy (heat) that dissipates into the environment and is difficult to use for practical work. So, we "save energy" by using efficient processes that convert more of the stored energy into the forms we want (like motion) and less into wasted heat.
Can energy be destroyed? For example, where does the energy go when a ball stops rolling?
No, energy cannot be destroyed. When a ball rolls on a flat surface and comes to a stop, its kinetic energy is not lost. It is transformed into other types of energy. Friction between the ball and the ground acts as a force that does work on the ball, converting its kinetic energy into thermal energy. The ball and the ground get very slightly warmer. Some energy is also converted into sound energy (the rolling noise). The total energy before and after is identical.
Does the Law of Conservation of Energy apply to living things?
Absolutely. Living organisms are excellent examples of open systems[1] that constantly exchange energy with their environment. The law still holds if you consider a larger system. For instance, a tree gets energy from the sun (radiant energy), converts it to chemical energy via photosynthesis, and uses it to grow. An animal eats the tree (or another animal that did), converting that chemical energy into movement and body heat. At every step, energy is being transformed, not created or destroyed.
Conclusion: A Universal and Unbreakable Rule
Footnote
[1] Open System: A system that can exchange both energy and matter with its surroundings. For example, a pot of boiling water with the lid off is an open system because water vapor (matter) and heat (energy) escape into the air.