Introduction to Heat Transfer

What is heat?

Think about all the ways that you can heat something up. You can boil water on the stove, rub your hands together quickly, or stand in front of a fire. But what is heat? 

Heat is related to thermal energy. Thermal energy comes from the movement of tiny particles inside all matter. All solids, liquids, and gases are made up of small particles such as atoms and molecules. These particles have kinetic energy and are constantly moving. When these particles move more quickly, the amount of thermal energy increases.

Heat is thermal energy that is moving from one place to another. Heat flows from warmer objects to cooler objects. Since heat is a form of energy it is measured in Joules or sometimes in calories. 

Misconception Alert

Objects don’t contain heat. They can contain thermal energy. 

So what’s the difference between heat and temperature? Temperature tells us how hot or cold something is. Temperature is a measurement of an object’s average kinetic energy. Basically, it is a measure of the average motion of an object’s particles. Temperature is measured in degrees Celsius, degrees Fahrenheit, or using the Kelvin scale. Temperature and heat are connected. Heat is the flow of thermal energy between objects with different temperatures. 

Did you know?

A calorie is the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius. The energy in the food you eat is measured in calories.

How is heat transferred?

Have you ever held a cup of hot chocolate after being outside in the cold? Holding a hot cup makes your hands feel warmer. What you are experiencing is the transfer of heat from one object to another. Heat energy from the hot chocolate is transferred to your hands. 

When two objects have different temperatures, heat is transferred. The cooler object gets warmer until the two objects have the same temperature. Heat energy always flows from the warmer object to the cooler object. 

Heat can be transferred in three ways:

1. Conduction
2. Convection
3. Radiation

Conduction

Conduction happens when materials or objects are in direct contact with each other. The molecules in the warmer object vibrate faster than the ones in the cooler object. The faster vibrating molecules collide with the slower molecules. This makes the cooler molecules vibrate more quickly, and the object gets warmer. For example, have you ever sat on a cool couch? Did you notice how the seat was much warmer when you stood up? Heat from your skin was transferred to the couch through the vibration of molecules.

Conduction can also happen within a single object. Think of a metal rod that has just been poking around in a fireplace. The end of the rod that’s been touching the hot embers becomes very hot. Energy from the hot end will move through the rod to the colder end. Eventually, the temperature of the entire rod will be the same. This is why it is important to wear a glove when handling a hot metal rod!

Some materials are better than others at conducting heat. You might have noticed this walking around your house in the winter. Have you ever noticed that your feet get much colder walking on bathroom tile than on carpet? This happens even though both the tile and the carpet are the same temperature as your house. However, tile is a much better conductor than carpet. More heat flows from your foot to the floor when walking on tile than carpet. 

Thermal conductivity is a measure of how well a material conducts heat. Materials that are good at conducting heat are known as conductors. Metals, such as silver, copper, and aluminum are conductors. Materials that are not good at conducting heat and are known as insulators. Styrofoam, snow and fiberglass are examples of insulators. Many homes have insulation. Insulation keeps homes from losing too much heat energy to the surrounding air. Many common objects also provide insulation from air such as coolers, insulated flasks and sleeping bags.

Did you know?

Chefs like to use wooden spoons because wood is not a good conductor of heat. This means the spoons won't heat up too quickly and burn their hands.

Conduction usually happens in solids. The particles in liquids or gases are farther apart than in solids. This makes it easier for gas and liquid molecules to move around. Thus, liquids and gases more often transfer heat through convection.

Convection

Convection is another way that heat can be transferred. Convection is motion in a gas or liquid that is caused by temperature differences. This motion transfers heat throughout the gas and liquid. The molecules in liquids and gases are farther apart and have more room to move around than in solids. Because of this, heated liquid or gas molecules can physically move. This is different from conduction, where the molecules just vibrate more quickly. 

Heating a pot of water on a burner is an example of convection. Heat transfers to water molecules at the bottom of the pot through conduction. These molecules start moving faster. The water at the bottom of the pot becomes less dense. It rises above the denser, cooler water. As the water rises, it carries heat energy upwards with it. Cooler water takes its place at the bottom of the pot where it is heated. This creates a circular cycle of heat transfer. This pattern is known as convection. 

Convection plays a very important role in wind and ocean currents. For example, air over land is generally warmer than air over the ocean. The warmer air heats up and rises. It is then replaced by cooler air from above the ocean. We experience this movement of air as wind. 

Radiation is the third type of heat transfer. Unlike convection and conduction, no matter is needed for radiation. Thermal radiation is the transfer of energy via electromagnetic waves. Electromagnetic waves carry energy across space. Thermal radiation is the way that the Sun heats the Earth. The Sun’s energy travels in waves through space, not through atoms or molecules. Other warm objects, such as a toaster or your body, also radiate heat energy. A microwave also uses radiation to heat your food.

Heat Transfer in a House

An example of all three heat transfer processes occurring at the same time is the heating or cooling of a house. 

1. Conduction can either heat or cool the house. In the summer, heat is transferred from the warm air outside into the house through the walls or roof. In the winter, heat is transferred from the warm air inside the house out through the wall or roof.
2. Convection occurs inside each room. Warmer air rises towards the ceiling and cooler air sinks towards the floor. Convection is also why the second floor of a house feels hotter than the basement.
3. Thermal radiation from the Sun heats the roof of the house. Radiation can also transfer heat energy through windows. 

We experience these different forms of heat transfer everyday. Understanding these concepts can lead to innovative uses of heat energy. For example, a Canadian teenager created a flashlight powered by the heat of your hand. Who knows what other ways we will use our knowledge of heat in the future.