Why Does Heat Flow From Hot to Cold?


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Ice cream begins to melt as soon as it’s in the cone; chocolate chip cookies start to lose their gooey goodness as soon as they’re out of the oven. But why? Why does everything warm eventually become colder?

Heat flows from hot to cold because warmer molecules move faster. When warmer molecules bump into cooler ones, two things change: cooler molecules speed up (get warmer), and warmer molecules slow down (get cooler.) The flow of kinetic energy from warmer objects to cooler objects is “conduction.”

Keep reading to find out more about this process. Learn what atoms and molecules are, why they’re always moving around, and how heat affects motion. Finally, know why conduction only happens in one direction—why cold objects don’t just keep getting colder.

Why Does Heat  Flow From Hot  to Cold?

Everything Is Made of Particles

The kinetic molecular theory tells us that everything around us is created from tiny particles called atoms. Atoms can join together to make molecules. In fact, most of the substances around us are made of molecules. Our oceans and atmosphere are mostly made up of molecules, and so are we—our DNA, blood, bones, and tissues are all made of molecules. 

For more about atoms, see this Khan Academy video: 

Here’s a great visualization of how molecules form: 

Particles Are Always Moving

We also know from the kinetic molecular theory that atoms and molecules are always moving, unless they’re extremely cold. Any atom or molecule that’s warmer than “absolute zero” is in motion. “Absolute zero” is defined as “zero kelvins,” a temperature equal to −459.67° F (−273.15° C). You can see that it’s very unlikely that you will encounter any matter that is at absolute zero, so for all practical purposes, we can say that matter is always moving.

Particles of matter move in three basic ways: they rotate, vibrate, and translate. “Rotate” means spin, “vibrate” means shake, and “translate” means to move from one place to another. The atoms and molecules in the floor underneath you, in the air around you, and even in your own flesh and bones are vibrating, shaking, and translating right now. But when we talk about the movement of heat being due to the movement of particles, it’s really only their vibrating motion that we’re talking about.

Hear Neil deGrasse Tyson explains why absolute zero is more theoretical than practical: 

Watch kids perform a basic experiment that demonstrates the motion of matter: 

Read more about particle movement in Reactions: An Illustrated Exploration of Elements, Molecules, and Change in the Universe. The photographs in this beautiful book explore how energy, time, and motion create molecular changes in the world around us.

You could also conduct some delicious experiments with molecules by learning about molecular gastronomy, using the Molecule-R – Molecular Gastronomy Starter Kit.

Particles That Move Faster Have More Heat

Applying energy to a substance makes its particles move faster, increasing its heat and temperature. To understand how this works, it helps to know the scientific meanings of heat, energy, and temperature.

  • Energy is what needs to be transferred to a substance in order to increase its heat. 
  • Heat is the total energy contained in the movement of a substance’s particles.
  • Temperature is what you get when you divide heat by the number of molecules present in a substance. 

This may seem confusing but think of a bowl of soup that’s been heated to 200°F (93°C). 

If you scoop some onto a spoon, the soup in the spoon—at least at first—is the same temperature as the soup in the bowl. They’re both 200°F (93°C) because they contain the same average heat per molecule. 

But the bowl contains a much larger volume of soup than the spoon does—so the soup in the bowl has more heat than the soup in the spoon because heat is a measure of the total energy contained in the vibration of the soup’s molecules.

If you put the bowl of soup in the microwave, the microwave energy will make its particles move faster, adding more energy to the soup and increasing its heat. A full bowl will take longer to heat to the same temperature than a half-full bowl because the greater number of molecules in the full bowl means more energy is needed to increase the average heat per molecule.

If these terms are still confusing, check out this quick and easy-to-follow video: 

Collision Creates Heat Transfer

When two substances are in contact with one another–the air, a bowl, and some hot soup, for example—their atoms and molecules collide as they vibrate. In this collision, energy is transferred, flowing from the substance with the faster molecules and higher temperature to the substance with the slower molecules and lower temperature. That is, heat flows from the hot soup to the air and bowl and from the bowl to the air. 

This process of heat transfer between things that touch is called thermal conduction

To visualize why this happens, think about when another person bumps into you. If you’re both essentially the same size, the person moving more slowly will be knocked farther, faster, of course, than the speedier person.

Thermal conduction doesn’t always occur at the same rate. The thermal conductivity of the two substances can make a big difference—some substances are just better than others at transferring energy. Starting temperatures matter, too. A large temperature difference between the two substances will create faster heat transfer; in addition, some substances become better conductors as they get warmer.

For a visual explanation of the process of thermal conduction, see this video:

You can experiment with thermal conductivity yourself, with the Arbor Scientific Ice Melting Blocks, Thermal Conductivity Experiment Kit, or the Thermal Conductivity Experiment from Eisco Labs.

For some hands-on learning about heat transfer and other physical and chemical properties of matter, try Theodore Gray’s Completely Mad Science: Experiments You Can Do at Home but Probably Shouldn’t.

Heat Transfer Only Works in One Direction

It’s logical to wonder why, in the collision between substances of different temperatures, the colder particles can’t transfer some of their energy to the warmer particles. 

We know that this doesn’t happen because when we drop ice cubes into a glass of water, the water always gets cooler and the ice always melts. If conduction could work in the reverse direction, sometimes the water would get warmer, and the ice would become even colder. But knowing that it doesn’t happen isn’t the same as knowing why it doesn’t happen.

This principle—that when two substances are in contact, heat will always flow from the warmer substance to the cooler substance—is called the Second Law of Thermodynamics

The laws of thermodynamics tell us that something called entropy is at work in the universe. Entropy is a kind of measurement of randomness or disorder, and it can only stay the same or increase—it can never decrease. The Second Law of Thermodynamics tells us that heat will flow from warmer to cooler systems because of entropy—because warmer systems have more entropy. 

Conclusion

Whenever two substances touch, the Second Law of Thermodynamics says that heat will flow from the warmer substance to the cooler substance. This happens because the faster-moving particles in the warmer substance collide with the slower-moving particles in the cooler substance. 

The collision causes energy to be transferred from the particles of the warmer substance to the particles of the cooler substance. This collision resulting in heat transfer is called conduction.

Chris Hewitt

Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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