- kinetic theory
- heat energy v. temperature
- thermometers
- temperature scales
- equilibrium
- specific heat
- Heat flows in three ways:
- Heat flow within changes of state
- Video Archive
Kinetic Theory 
Heat energy occurs when moving particles generate friction through collisions. Everything is made up particles and they are constantly in motion. This idea is called KINETIC THEORY.
Pick a particle in the simulation and follow it for a cycle of random motion (before it resets). “Heat” occurs whenever two particles collide with each other or the wall of the container. Is your particle fast? Is it slow? When does it change speeds?
The average speed of the particles is called the object’s temperature. Faster particles overall will make the temperature of the object increase while slower particles make it cooler to the touch. Particles have more energy when they move faster and spend that energy as heat when they collide.
Imagine if the particles in the box were perfectly still. None are touching each other and none are touching the walls. They would have no heat energy and as a result have no temperature at all. This is the coldest possible temperature in the universe and we have given it a name…ABSOLUTE ZERO. It’s nearly impossible to reach absolute zero because even the coldest of cold things still have some heat energy. (As of 2025, Scientists have reached to 38 trillionths of a degree Celsius above absolute zero)
Kinetic theory can be summed up pretty easily with the following statement: Adding heat to particles will cause TWO things to happen…
- The particles will speed up
- The particles will spread out
This helps us understand why MOST materials will EXPAND when heated. The particles may be the same size but there is more space between these faster moving particles. The wax near the heat lamp is expanding which means it is lowering in density. This is why heated materials tend to rise.
This classic demonstration shows the expansion and contraction of matter. The brass sphere and hoop were machined so that they perfectly fit together. When the particles are given heat they speed up and spread out…that means they take up more space. With enough heat added the sphere will not fit in the hoop. This also shows how MOST materials will CONTRACT when heat is removed. The space between the particles gets smaller and smaller as the substance is cooled off.
Watch the size of the balloon change as the liquid Nitrogen removes heat from the gas inside. Blowing warm air over the balloon gives the inside gases more heat energy. The particles speed up and spread out. As a result, the balloon returns to its normal shape and size.
Most materials expand when heated and concrete is no exception to the rule. Our roads and sidewalks stretch out in the summer and then contract in the winter. Gaps are intentionally placed between sections of concrete and then filled with a treated corkboard to allow for this seasonal motion. Without room to expand and contract, roads and sidewalks would crack (more than they already do).
“Street Creep” is what occurs when the road by your house expands and pushes on your driveway concrete which in turn can push the foundation of your home causing significant damage. Cutting a gap in the concrete and filling with treated expandable material can prevent this kind of damage.
A bridge needs finger joints that allow for the length of concrete slabs to change during the year. Without these gaps the bridge would eventually crumble to the water below.
Heat is different from temperature 
“Heat” and “temperature” are not the same thing. The heat of a substance is the total kinetic energy (or motion) of particles and is measured in Joules. The temperature of the object is a measurement of the average speed of those particles.
A flaming match has very fast moving particles and is a high temperature. It would be easy to think the match has more heat energy than an iceberg. That isn’t the case however. There are so many more particles in an iceberg that even though they are moving slowly, the TOTAL kinetic energy far surpasses that of the match.
This comparison is like asking the following question: Would you rather have 10 one hundred dollar bills or a collection of over a QUINTILLION pennies? The penny might seem small but in large numbers they greatly outnumber $1000!
Let’s continue to describe heat like it is money. If I gave you a few million dollars we might call you “rich”. If we took away those millions and then some, we could use words like “poor”. It doesn’t make sense in our language to give people RICH or even take away POOR. These words aren’t the thing being measured. Instead they describe the amount of money that is being transferred.
Similarly, words like “hot” and “cold” are just descriptors. They help us compare the temperatures between objects or let someone communicate how comfortable they are but they are not precise measurements nor to they even exist. HEAT is the thing being measured. A place with more heat is described as hot, while an area with less heat can be described as cool, or even cold.
Thermometers
We can measure the temperature of a object or area with a thermometer. A thermometer often has a reservoir of a liquid (Mercury or an alcohol colored red) that expands when heated. The liquid moves up a small capillary tube where the level can be measured.
Some thermometers are measuring a wave of light called infrared. These IR thermometers are placed near the blood vessels in our face and the heat from our bodies radiates to the device to be recorded.
Temperature Scales
We measure the speed of particles on one of several temperature scales. America uses one called Fahrenheit while the metric scales for temperature are Celsius (aka Centigrade) and Kelvin. Let’s start with a bit of history to see what Fahrenheit is all about.
Equilibrium
Heat flows from hot to cold until temperatures are the same.
Place your hand on a table that is “room temperature”. The heat will flow from your hand and into the table. This loss of heat will make your hand feel cooler. If you keep your hand in this spot it will continue to warm up until the temperatures are the same. HEAT Energy is moving…not the cold.
When you open a window in the wintertime it would not be unusual for someone to say “please close that window. You are letting in the cold”. While this may feel right, there is no such thing as cold. It is correct physics to say “please close that window, you are letting out the heat from the house.”
specific heat
Substances don’t all heat up and cool off at the same speed however. This is rate for each substance is called its SPECIFIC HEAT. We typically call substances with a low specific heat to be good conductors of heat. Substances with higher a specific heat are classified as insulators.
Using money as our metaphor again, specific heat is a measure of how expensive it is to purchase a degree of temperature change for each substance. Many of you have played Monopoly before and bought properties. The same piece of plastic from the game box will cost you different amounts of money depending on the property it is for.
One gram of Iron needs 0.449 Joules to heat it up 1 OC. One gram of Copper only needs 0.385 Joules to do the same thing. This means that a Copper skillet can heat up and transfer energy to the food faster than an Iron skillet. This also means it cools off quicker than Iron.
Water has an extremely large specific heat when you compare it to metals. One gram of water needs 4.18 Joules to heat up a 1OC. A degree of Celsius for water is almost 10 times more expensive than a degree Celsius for Iron! This means water has a great ability to cool things off. It can absorb heat without a huge temperature change.
Remember that time you were at the beach in the afternoon? The sand crazy hot on your bare feet but as you ran into the water you cooled off quick. Even though the sun is beating down equally on the ocean and the shore, SAND has a specific heat of only 0.8 Joules per gram for each degree. A bit of division tells us that the beach heats up about 5 TIMES FASTER than the water!!
Conduction
Conduction is the flow of heat through direct contact. When two things that have different temperatures collide, heat is transferred until the temperatures are the same.
The paper cup that has water in CANNOT burn away. As long as the water is there to pull heat from the paper cup it will never have enough to catch fire. What would happen if the water boils away?
Convection
Convection is the flow of heat through a fluid. Liquids and gases both qualify as being fluid because they “flow” around objects they come into contact with.
Fluids heat up and become less dense causing them to rise. As the particles get away from the heat source they slow down and get closer together.
Radiation
Radiation is the flow of heat through electromagnetic waves that travel at the speed of light. This form of heat is really just a wavelength of LIGHT that we can’t see. “Infra” means lesser than or below in Latin so the name tells us that this wavelength is just a bit lower in energy than the color red. I go into more detail over on the Radiant Energy page
Moving heat by changing state
Review the states of matter with the section on the physical change of matter.
When is heat added? When is heat removed? Where does that heat go when it is removed?
You buy a window air conditioning unit and take it out of the box. If you plug it in and let it run in the room you’ll find that it doesn’t work at all. In fact, the temperature in the room would go up! Why would this be.
Imagine getting COLD water on a hot day from your water bottle…without using any ice! Get a canteen with fabric on the outside and keep the fabric wet. As the water on the material evaporates, it pulls heat from the water bottle. As the water bottle gets colder, the water that is touching the bottle gets colder too. Heat moves from the inside and all the way out as long as evaporation is taking place.
Consider for a moment that you are an Orange farmer in Florida. Why would spraying your crops BEFORE a frost be a way to temporarily protecting your oranges from the cold? Water has a lot of energy. As the temperature outside drops, the water freezes. The unused heat is released into the air around the crops and directly into the orange before the protective ice coats the fruit.
Video Archive
The following PBS video is EXCELLENT but a bit long. It is NOT part of our quiz.