Stay warm with thermal insulation

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? What is the purpose of our clothes, and why are they made from fabrics and not foils? Find out the answers in this activity; your results may even tell you 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, a microwave or a fire. 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 – you will notice that it becomes cooler the longer you wait. This is due to a phenomenon called heat transfer, which is the flow of energy in 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 always 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 or gas is called convection.

Heat can also be transferred through radiation. You might have experienced that from sitting around a bonfire. Although you are not touching the fire, you can feel it radiating heat in your face even if it is cold outside. If you like to drink your tea hot, you might ask how heat transfer can be reduced, and how the tea keeps from cooling down? The answer is thermal insulation. Insulation means creating a barrier between the hot and the cold object that reduces heat transfer by either reflecting thermal radiation or decreasing thermal conduction and convection from one object to the other. Depending on the material of the barrier, the insulation will be more or less effective. Barriers that conduct heat very poorly are good thermal insulators, whereas materials that conduct heat very well have a low insulating capability. In this activity, you will test which materials make good or bad thermal insulators with the help of a glass of hot water. Which material do you think will be most effective?

Materials

Preparation

Procedure

Observations and Results

Did your hot water cool down significantly during the 10 minutes inside the fridge? While the fridge temperature is very low, your hot water has a high temperature. As heat energy always flows from the hot object to the cold object, the heat energy from your hot water will be transferred to the surrounding cold air inside the fridge once you put the glass jars inside. The most significant mechanism of heat transfer in this case is convection, which means that the air just next to the hot jar is warmed up by the hot water. Then, the warm air is replaced with cold air, which is also warmed up. At the same time, the cold air cools down the water inside the jar. The heat of the hot water is transported away by the flow of cold air around the cup. If you left the jars in the fridge long enough, you might have observed that the temperature changes until the hot water reaches the temperature inside the fridge. Without a temperature difference between the water and the fridge, the heat transfer will stop.

Heat from the water is also lost through conduction: the transfer of heat through the material, which is dependent on the thermal conductivity of the material itself. The glass jar can conduct heat relatively well. You notice that when you touch the glass jar with the hot water, the glass feels hot as well. What effect did the different wrapping materials have? You should have noticed that with wrapping materials, the temperature of the water after 10 minutes inside the fridge was higher compared to the unwrapped control. Why? Wrapping the glass jar reduces the heat transfer from the hot water to the cold air inside the fridge. Using wrapping materials that have a very low thermal conductivity reduces the heat loss through conduction. At the same time the insulator can also disrupt or reduce the flow of cold air around the glass jar, which results in less heat loss via convection.

One way of reducing convection is creating air pockets around the jar - for example, by using insulators such as bubble wrap, fabric, or wool, which have a lot of air pockets. Air in general is a good thermal insulator, but it can transmit heat through convection. However, if the air pockets inside the insulating material are separated from each other, heat flow from one air pocket to another cannot happen easily. This is the reason why you should have measured the highest temperature in the bubble wrapped jar and fabric wrapped jar. This also explains why most of our clothes are made of fabrics and you stay warmer when you put on an extra jacket. Paper and foil make it easier for the heat to escape as they don’t have that many air pockets.

Cleanup

More to Explore

• Heat Transfer - For Kids, from Real World Physics Problems
• Curious facts about insulation, from FunKids
• How Animals Stay Warm with Blubber, from Scientific American
• Science Activity for All Ages!, from Science Buddies

Credits