How does a paper bag wrapped around a hot water bottle create convection?

How does a paper bag wrapped around a hot water bottle create convection?

Heat is a form of energy. You need energy to heat something up: for example, a cup of tea. To make your tea you probably use energy from electricity or gas. However, once your tea is hot, it won't stay hot forever. Just leave the cup of tea out on the table for a while, and you already know that it will become cooler the longer you wait. This is due to a phenomenon called heat transfer, which is the flow of energy in the form of heat. If two objects have different temperatures, heat automatically flows from one object to the other once they are in contact. The heat energy is transferred from the hotter to the colder object. In the case of the tea, the heat of the liquid is transferred to its surrounding air, which is usually colder than the tea. Once both objects reach the same temperature, the heat transfer will stop. Heat transfer via movement of fluids (liquids or gases) is called convection.

Want to keep warm this winter? Try this "cool" activity and find out what types of insulation work best--and why. Credit: George Retseck

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Key concepts
Physics
Heat transfer
Insulation
Material science
Introduction
What do you do when it gets very cold in winter? You probably turn your heater on, put on an extra layer of clothes or cuddle under a warm blanket. But have you ever thought about why a jacket helps you stay warm? Why are our clothes made from fabrics and not foils? Find out the answers in this activity; your results might even help you find the best way to stay warm in the cold!
Background
Heat is a form of energy. You need energy to heat something up: for example, a cup of tea. To make your tea you probably use energy from electricity or gas. However, once your tea is hot, it won't stay hot forever. Just leave the cup of tea out on the table for a while, and you already know that it will become cooler the longer you wait. This is due to a phenomenon called heat transfer, which is the flow of energy in the form of heat. If two objects have different temperatures, heat automatically flows from one object to the other once they are in contact. The heat energy is transferred from the hotter to the colder object. In the case of the tea, the heat of the liquid is transferred to its surrounding air, which is usually colder than the tea. Once both objects reach the same temperature, the heat transfer will stop. Heat transfer via movement of fluids (liquids or gases) is called convection.

Another type of heat transfer is conduction, in which energy moves through a substance (usually a solid) from one particle to another (unlike in convection where it’s the heated matter itself that moves). A pot handle getting hot would be an example of conduction.