冬天，气温下降，你是如何应对的呢？你也许会打开取暖器，多穿衣服或者披上一张毯子。不过你有没有想过为什么一件夹克能让你感到温暖呢？为什么我们用布料而不用金属箔来制作服装呢？在本活动中找到答案；<Find out the answers in this activity>；你的所获还能帮助你在天寒地冻中找到最有效的保温秘诀！
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.
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?
- Five glass jars with lids
- Scissors (and adult to help with cutting)
- Aluminum foil
- Bubble wrap
- Wool scarf or other wool clothes
- Hot tap water
- Paper for writing
- Pen or pencil
- Cut a piece of the aluminum foil, the bubble wrap and the paper (have an adult help if necessary). Each piece should be large enough to fit three times around the sides of the glass jar.
- Take the piece of aluminum foil and wrap it around the sides of one of the jars. You should have three layers of foil around the glass jar. Use the tape to attach the foil to the jar.
- Next, wrap another jar with the bubble wrap so that the glass is also covered in three layers. Make sure to tape the bubble wrap onto the jar.
- Use the cut paper to wrap a third jar in three layers of paper. Once again, attach the paper to the glass jar.
- Take another glass jar and wrap the scarf or other wool fabric around the jar. Only make three layers of wrapping and make sure that the scarf stays attached to the jar.
- Leave the last jar without any wrapping. This will be your control.
- Fill each jar with the same amount of hot water from your faucet.
- Use the thermometer to measure the temperature in each jar. Put your finger inside the water of each jar (use caution if your tap water is very hot)—how does the temperature of the water feel?
- Write down the temperature for each jar and close the lids. Are all the temperatures the same or are there differences? How big are the differences?
- Open your fridge and put all the five jars inside. Make sure they are still securely wrapped. Feel the temperature of the fridge—what does its temperature feel like?
- Put the thermometer in the fridge. What temperature does the thermometer read when you put it into the fridge?
- Once all the jars are in the fridge, close the fridge door and set your timer to 10 minutes. What do you think will happen with the jars and the hot water during that time?
- After 10 minutes open the fridge and take all the jars outside. Do the jars feel different?
- Open each jar, one at a time, and measure the water temperature with your thermometer. Also, feel the temperature with your finger. Did the temperature change? How did it change according to the thermometer?
- Repeat measuring the temperature for each jar and write down the temperature for each wrapping material. Did the temperature in each jar change the same way? Which wrapping material resulted in the lowest temperature change, and which resulted in the biggest?
- For a better comparison, calculate the temperature difference from the beginning and end of the test for each jar (temperature beginning versus temperature after 10 minutes in fridge). From your results, can you tell which material is the best or weakest thermal insulator?
- Extra: Will temperatures continue to change in a similar way for each material? You can close each jar again and put them back into the fridge for another 10 minutes. Are the results different this time or the same?
- Extra: Does the water temperature change the same in the fridge as in the freezer or at room temperature? Repeat the test, but this time instead of putting the glass jars into the fridge, put them into the freezer or keep them at room temperature. How much does the temperature of the water change within 10 minutes? Do the different wrapping materials behave differently?
- Extra: Try to find other materials that you think are good or bad thermal insulators and test them. Which material works the best? Can you think of a reason why?
- Extra: If you take the jars out of the fridge after 10 minutes, you probably still measure a temperature difference between the water inside the jar and the temperature inside the fridge. You can keep the glass jars longer in the fridge and measure their temperature every 15 to 30 minutes. How long does it take until the temperature of the water doesn't change anymore? What is the end temperature of the water inside the glass?
- Extra: Besides choosing the right insulator material, what are other ways to improve thermal insulation? Repeat this test with only one wrapping material. This time change the thickness of your insulating layer. Do you find a correlation between thickness of insulation layer and temperature change in the fridge?
Observations and results
Did your hot water cool down significantly during the 10 minutes inside the fridge? Although the fridge temperature is very low, your hot water has a high temperature. As heat energy 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 why you stay warmer when you put on an extra jacket. Paper and foil make it easier for the heat to escape because they don't have many air pockets.