Heat

Hot Stuff

You can’t see heat, but you sure can feel it when it’s there – and when it’s not. Heat is a type of energy. Sometimes it is called heat energy, or thermal energy.

Heat moves from place to place to warm us up or cool us down. It can move from a heating vent in the wall all around a room. It can move from the flames of a fire to the air near your hands or face. It can move from inside your body to outside it, making you feel colder. But how is heat made? There isn’t just one answer to that question.

When cold air is breathed in, it is warmed by an animal’s body. You can see exhaled breath when the air is much colder than the air being breathed out.

When cold air is breathed in, it is warmed by an animal’s body. You can see exhaled breath when the air is much colder than the air being breathed out.

©Douglas M. Paine

Giving Off and Taking in Heat

This ice cream is taking heat in from the surrounding air, which is warmer than the ice cream. Uh oh, better eat it fast before it melts away!

This ice cream is taking heat in from the surrounding air, which is warmer than the ice cream. Uh oh, better eat it fast before it melts away!

©Alissa Sanderson

Chemical reactions and processes can give off heat or take in heat from their surroundings. The type of chemical reaction that gives off heat is called an exothermic reaction. Burning a log in the fireplace causes an exothermic reaction that gives off heat. Leaving an ice cube to melt in the sun involves the taking in of heat, which makes it an endothermic process.

Nuclear Reactions Give Off a LOT of Heat

Nuclear reactions give off heat too, an amazing amount of it. One type of nuclear reaction, a fission reaction, splits apart the atoms in the element uranium. That process gives off a tremendous amount of energy. In fact, it’s the same process used to build a nuclear bomb! Power plants with nuclear reactors supply electricity to millions of people around the world.

Electricity: You’re Toast

Electricity is used to heat many appliances and tools in homes around the world. Once the electricity heats up the coils in the toaster, they give off enough extra heat to toast bread.

Electricity is used to heat many appliances and tools in homes around the world. Once the electricity heats up the coils in the toaster, they give off enough extra heat to toast bread.

©Gary Prichard

Electrical current traveling through the circuits in your home heats up appliances like clothes dryers and toasters. In many homes, the whole heating system is electric. How hot does electric heat get? Pop a piece of bread in your toaster. Wait a few seconds. Now peek inside. You’ll see that the metal side pieces are so hot they’re glowing red. This is the same thing that happens inside a light bulb. Electricity heats up a filament in the bulb, which makes it glow.

Friction Makes Heat, but Don’t Rub It in

There are different ways to make fire but all use energy. The Maasai men are using friction to start a small fire. In this picture, they are spinning a stick fast enough in one spot that enough heat will build up to make a small flame.

There are different ways to make fire but all use energy. The Maasai men are using friction to start a small fire. In this picture, they are spinning a stick fast enough in one spot that enough heat will build up to make a small flame.

©K.Campbell/GLOBIO.org

Rub your hands together fast. Feel the heat? That’s friction. It’s the resistance to movement of one object – your hand -- by the other object – your other hand. In the process, kinetic energy changes into heat energy. The more resistance there is, the more friction produced. More friction means more heat. A polished surface, like a gym floor or a lane in a bowling alley, reduces friction. That’s why a bowling ball rolls fast down the lane towards the pins. (As long as your aim is good!) If the alley was covered in carpet, there would be more friction. The ball would slow and wobble and could even stop partway to its destination.

How Does Heat Move?

Have you ever started to sip a cup of very hot chocolate, said “Ouch,” and quickly put it back down? The rim of the cup had become hot enough to burn your lip. The heat had moved from the hot chocolate inside the cup to the outside of the cup. Heat doesn’t just sit there. It moves. It always moves in the same direction: from hot to cold. But it has different ways of getting around.

Conduction: A Ticket to Travel

Cooking can be done over a fire or on an electric stove but when we cook, most of us use metal pans. Metal is a good conductor which helps make sure our food is hot, perfectly done, and delicious.

Cooking can be done over a fire or on an electric stove but when we cook, most of us use metal pans. Metal is a good conductor which helps make sure our food is hot, perfectly done, and delicious.

©Patrick Moore

Conduction of heat is what made that mug hot on the outside. If you put a pan of milk to warm on the stove, the heat moves from the burner up through the pan to the milk inside. The pan conducts heat. Your cup had conducted heat too.

Not all materials conduct heat well. Metals are good conductors. That’s why pots and pans are usually made of metals, such as iron or aluminum. Wood is not a good conductor. That’s why a wooden spoon is a safe stirrer for a hot pot of soup. It protects your hand by not conducting the heat from the pot.

Convection: Going With the Flow

Convection is another way that heat moves. Through convection, a small space heater in a corner can carry heat in the air to a whole room. Through convection all the water in a tea kettle gets heated, not just the water at the bottom closest to the burner.

This way heat has of moving is all about flow. Convection needs something flowing to carry heat. That means a liquid, such as water, or a gas, such as air. Solids don’t flow. They block convection. Think about baking a cake. When the oven door (a solid) is closed, the heat stays inside. When you open the door to see if your cake is done, a blast of delicious, cake-scented, hot air comes rushing out. That air has been heated by convection, in a convection oven. 

Radiation: Riding the Waves

This sunbathing lemur probably doesn’t know why it’s so nice and warm in the sun but we do. The sun sends out heat in invisible waves. These waves are what make standing in the sun pleasant on cooler days and downright hot other times.

This sunbathing lemur probably doesn’t know why it’s so nice and warm in the sun but we do. The sun sends out heat in invisible waves. These waves are what make standing in the sun pleasant on cooler days and downright hot other times.

©G.Ellis/GLOBIO.org

A final way heat travels is by radiation. That’s movement by invisible waves called rays. A fire in a fireplace radiates heat so you can feel it on your hands and face if you’re sitting close enough. Heat comes from the sun in rays of light.

How Do We Measure Heat?

We measure heat for many reasons. Most of us use thermometers to tell what temperature it is outside so we know what to wear. Chefs check the temperature of the foods they are cooking and scientists take the temperature of many different things, including super hot volcanoes and the icy water in Antarctica!

We measure heat for many reasons. Most of us use thermometers to tell what temperature it is outside so we know what to wear. Chefs check the temperature of the foods they are cooking and scientists take the temperature of many different things, including super hot volcanoes and the icy water in Antarctica!

©G.Ellis/GLOBIO.org

Heat can be measured using one of three systems: Fahrenheit, Celsius, and Kelvin. Fahrenheit and Celsius thermometers are used to measure many, many things: the temperature of food, the temperature of human bodies (to tell if we’re running a fever), the temperature of the air outside.

The United States is about the only place in the world that uses the Fahrenheit system. Celsius is preferred everywhere else, and even in the United States it is the system scientists use. Kelvin is also used by scientists everywhere, to measure an astronomical amount of heat, like the temperature of the sun.

Scientists can measure the amount of heat an object gives off as it changes states. For instance, as water changes from a liquid to a gas. This is called latent heat. They can also measure the heat capacity of an object. That means its ability to store heat energy.

Turn on the Steam

By using heat to turn water into steam, train engines around the world use heat energy to do work. Trains carry people, animals, and goods all over.

By using heat to turn water into steam, train engines around the world use heat energy to do work. Trains carry people, animals, and goods all over.

©Keith Davenport

Producing and using heat is a big part of everyday life. So is keeping too much heat away. We heat food on the stove, but we also keep heat away from food in the refrigerator. We heat our homes in the winter, and use air conditioners to keep heat out in the summer. The engines in our cars and trucks create heat too.

For wider scale transportation, steam engines power locomotives and steamships. Within a steam engine, water is heated by burning coal, wood, or oil. The heated water turns to steam. Steam moves a piston or blade, which turns a wheel. In this way, a steam engine uses heat and turns it into work.

The First Law . . .

The study of heat and its movement is called thermodynamics. The rules that describe how heat works are the Laws of Thermodynamics. The First Law of Thermodynamics tells us that energy, including heat energy, cannot be created or destroyed. But energy can shift from one form to another.
Think of jumping into a pool or lake on a hot day. Your body cools because some heat energy shifts from your body to the water. The temperature of the water rises a little bit. But the energy (heat) isn’t destroyed. It just shifts from your body to the water.

. . . And the Last Law

This special photo, called infrared, shows the temperature of these cups. Can you tell which one is hot and which is cold?

This special photo, called infrared, shows the temperature of these cups. Can you tell which one is hot and which is cold?

©NASA/IPAC

The last Law of Thermodynamics is called the zeroth, because it was thought of after the others. It says that two systems will exchange energy until they are in equilibrium. What does that mean? 

If you dropped ice cubes (one system) into that hot, hot chocolate (another system), the systems will exchange energy until they are both the same temperature. Eventually you will have a mug of lukewarm chocolate. This state, in which both systems have the same temperature, is called equilibrium. If you just leave the chocolate alone for awhile, though, it will reach equilibrium another way. The air in the room (one system) will exchange energy with the hot chocolate (another system) until the chocolate comes to room temperature. But drink it before it cools down too much!