You Can't Create It, You Can't Destroy It
The most important rule in the universe concerning energy is the Law of Conservation of Energy. It states that:
Energy cannot be created or destroyed, only transformed from one form to another.
This means the total amount of energy in the universe is constant. When we 'use' energy, we are not getting rid of it; we are just changing it into a different form. These changes are called energy transformations.
Tracing Energy Transformations
Let's trace the flow of energy in a few examples.
A Flashlight:
1.Starts as chemical potential energy stored in the batteries.
2.A chemical reaction in the battery converts this to electrical energy that flows through the wires.
3.The electrical energy flows into the bulb, which transforms it into radiant energy (the light you see) and thermal energy (the bulb gets hot).
Transformation Chain: Chemical → Electrical → Radiant + Thermal
A Gasoline-Powered Car:
1.Starts as chemical potential energy in the gasoline.
2.The engine burns the gasoline, releasing the energy as thermal energy (a hot explosion).
3.This thermal energy pushes pistons, creating mechanical energy that turns the wheels and moves the car. The moving car now has kinetic energy.
Transformation Chain: Chemical → Thermal → Mechanical (Kinetic)
A Person Eating an Apple and Running:
1.Starts as chemical potential energy stored in the sugars of the apple.
2.Your body digests the apple through cellular respiration, converting the food's chemical energy into a usable chemical energy molecule called ATP.
3.Your muscle cells use the energy from ATP to contract, creating mechanical energy to move your legs. As you run, you also generate a lot of thermal energy, which is why you get hot.
Transformation Chain: Chemical → Chemical (ATP) → Mechanical + Thermal
The Problem of Wasted Energy: Efficiency
Notice that in almost every transformation, thermal energy (heat) is one of the products. Usually, this heat is not useful for the intended purpose of the device. It is considered wasted energy.
Energy efficiency is a measure of how much of the input energy is converted into useful output energy.
Efficiency = (Useful Energy Out) / (Total Energy In) × 100%
No energy transformation is ever 100% efficient. Due to friction and other factors, some energy is always lost as waste heat.
An old incandescent light bulb is very inefficient. About 90% of the electrical energy it uses is wasted as heat, and only 10% becomes useful light.
An LED bulb is much more efficient. It converts most of the electrical energy into light and wastes very little as heat.