Heat Flow in Solids & Fluids

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn about heat flow in solids and fluids: discover heat flow by conduction, convection, and radiation. See if you can tell the difference in a quiz.

What is Heat Flow?

We all know what heat feels like. It's the feeling you get when the sun is shining on you, or when you put on warm clothes from the dryer, or when steam escapes after opening the dishwasher too early. But are all these kinds of heat flow the same?

Heat is a type of energy, measured in joules. It's energy in the form of random molecular motions. Like all energy, heat energy can flow from one place to another and cannot be created or destroyed. So heat flow is the movement of heat. Pretty simple, right?

Well, it does get a little more complicated when you start talking about the ways that heat can flow. Heat always flows from a hot place to a cold place when flowing spontaneously (you can force heat to flow the opposite way by using energy - like in a refrigerator).

Heat will continue to flow until the two objects reach the same temperature, which is called thermal equilibrium. For example, if you put your hand on a cold table, it won't feel cold forever. Eventually the table will have absorbed enough heat from your hand that the table and your hand will be the same temperature have reached thermal equilibrium.

Heat also flows in different ways depending on whether solids, liquids, or gases are involved. Heat can even flow in a vacuum. Heat flowing in a vacuum is called radiation, and is where heat transfers through electromagnetic waves. This is how heat gets to us from the Sun, and why you feel hot on a sunny day.

The heat from the sun transfers by radiation
The heat from the sun transfers by radiation

But today we're going to focus on the other two main types: heat flow in solids, and heat flow in fluids.

Heat Flow in Solids

Heat flow in solids can only happen when those solids are in contact with one another. The process of heat flow in solids is called conduction. Conduction is where two objects in contact with one another transfer heat between them due to a temperature difference, like the clothes from the dryer. This happens literally because the moving molecules hit each other. The molecules in the hot clothes hit the molecules on your cold skin, and this causes energy to transfer between them. The molecules themselves do not go anywhere, it's just energy going from one place to another.

Heat Flow in Fluids

Heat flow in fluids is a little more complicated. A fluid is any material in a state that flows, which includes both liquids and gases. Fluids transfer heat energy by a process called convection. Convection is when the hot part of a fluid expands and becomes less dense, causing it to rise above the cooler part, and this continues in a cycle that transfers energy. Let's break this down a little more.

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