Energy & Heat
Energy can't be created or destroyed — only transformed. From the warmth of the Sun to the food you eat, energy is the currency of the universe.
"Energy is this abstract thing that nature keeps track of. It's like money — it can change form (from dollars to euros to yen), but the total amount never changes. You can't create it or destroy it, only convert it from one form to another."
Energy is the ability to do work — to make things move, change temperature, or transform. It comes in many forms:
Kinetic Energy — the energy of motion. A rolling ball, a flying bird, and a speeding car all have kinetic energy.
Potential Energy — stored energy waiting to be used. A ball held high has gravitational potential energy. A stretched rubber band has elastic potential energy.
Thermal Energy — the energy of heat, which is really just the kinetic energy of jiggling atoms and molecules.
Chemical Energy — energy stored in chemical bonds, like in food or gasoline.
The most important rule in all of physics is the Law of Conservation of Energy: energy cannot be created or destroyed, only transformed from one form to another. When you eat food, chemical energy becomes kinetic energy (movement) and thermal energy (body heat).
The energy in a single candy bar (about 1 million joules) could theoretically power a 60-watt light bulb for about 4.5 hours!
"Heat is not a thing — it's energy moving from something hot to something cold. When you touch a cold window in winter, heat flows from your warm hand to the cold glass. Your hand feels cold not because 'cold' comes in, but because heat goes out!"
Heat is thermal energy being transferred from a hotter object to a cooler one. It always flows from hot to cold — never the other way around (this is related to the Second Law of Thermodynamics).
Heat transfers in three ways:
Conduction — direct contact. Touch a hot pan and heat flows into your hand through the metal. Metals are good conductors; wood and plastic are poor conductors (insulators).
Convection — through moving fluids. Hot air rises because it's less dense, creating currents. This is how a room heater warms an entire room.
Radiation — through electromagnetic waves. The Sun warms the Earth through radiation across 93 million miles of empty space. No contact or fluid needed!
Temperature measures the average kinetic energy of molecules. Hot things have fast-moving molecules; cold things have slow-moving molecules. At absolute zero (-273.15°C), molecules would theoretically stop moving entirely.
The hottest temperature ever recorded on Earth was 56.7°C (134°F) in Death Valley, California. But the surface of the Sun is about 5,500°C — and its core is 15 million°C!
"The laws of thermodynamics are nature's accounting rules. The first law says you can't get something for nothing — energy is always conserved. The second law says things naturally go from order to disorder. Your room gets messy on its own, but it never cleans itself!"
The Laws of Thermodynamics are among the most fundamental rules in all of physics:
First Law: Energy cannot be created or destroyed, only converted. This is the conservation of energy applied to heat and work. A car engine converts chemical energy (gasoline) into kinetic energy (motion) and thermal energy (heat).
Second Law: In any energy transfer, some energy becomes unusable (usually as waste heat). This is why no engine can be 100% efficient. It also means that disorder (entropy) in the universe always increases over time.
Third Law: You can never reach absolute zero temperature. You can get incredibly close, but never quite there.
These laws explain why perpetual motion machines are impossible, why ice melts in warm water (never the reverse), and ultimately why the universe is slowly winding down.
Scientists have cooled atoms to within a billionth of a degree above absolute zero — but they can never quite reach zero itself!
Experiment & Discover
Play with the simulation below to see physics in action!
Pendulum — Energy Conservation
Watch potential energy (PE) convert to kinetic energy (KE) and back. The total stays constant!
Test Your Understanding
As Feynman said: "If you can't explain it simply, you don't understand it well enough."
A ball is sitting on top of a hill. What type of energy does it have?
Where You'll See This
Physics isn't just in textbooks — it's everywhere around you!
Solar panels convert light energy from the Sun into electrical energy
Your body converts chemical energy from food into kinetic energy (movement) and thermal energy (body heat at 37°C)
A roller coaster converts potential energy at the top into kinetic energy at the bottom, and back again
Insulation in your house walls slows heat transfer, keeping your home warm in winter and cool in summer