Pick one before you read on. It is fine to be wrong.
By the end you can
Test which material keeps hot water warm longest
Explain how a wet clay pot keeps water cool
Mei pours hot tea for her grandmother. It must still be warm when she wakes up. Mei wraps the cup in a cloth. Does that help?
Wool, cloth, feathers and foam are insulation. They hold many tiny pockets of still air. Still air is a poor conductor, and it cannot carry heat by convection. So heat leaves slowly.
Draw it first
Draw a cup wrapped in cloth
1 Draw a cup of hot water. Draw long arrows for the heat escaping from it.
2 Wrap it in a cloth. Dots show the tiny pockets of trapped air.
3 Now draw short arrows. The cloth slows the heat down, so the tea stays warm longer.
How can we find the best wrap? We do a fair test. We change only one thing, which is the wrap. We keep everything else the same.
A fair test. Use the same cup, the same amount of water and the same time for every wrap.Example results. Every cup started at 80 °C in a room at 20 °C. Cloth keeps the water warmest.
Worked example
Use the bars. How many degrees did the bare cup lose in 30 minutes? How many did the cloth cup lose? How much warmer is the cloth cup?
Bare cup: 80 - 42 = 38 °C lost.
Cloth cup: 80 - 58 = 22 °C lost.
The cloth cup lost 38 - 22 = 16 °C less.
Answer: bare 38 °C, cloth 22 °C, 16 °C warmer
The cloth cup cools slowly. Compare this with the bare cup in chapter 4, which had cooled to 42 °C by 30 minutes.
A vacuum flask uses every trick at once. Its double wall has a gap with no air, so heat cannot conduct or convect across it. Shiny walls send radiation back. The stopper stops warm air escaping from the top.
No air in the gap means no conduction or convection. Shiny walls cut radiation.
Now keeping things cool. In many hot places people store drinking water in a clay pot. The clay has tiny holes. A little water seeps through to the outside.
Water seeps through the clay and evaporates from the outside.
To evaporate, water needs energy. It takes that energy from the pot and the water inside. So the water in the pot gets cooler.
See it three ways
As a picture
In words
Energy leaves the pot with the water that evaporates. The pot loses energy, so it cools. It cools the water inside.
It works best when the air is dry and moving, because water evaporates fast. In humid air the water evaporates slowly, so the pot cools less.
Example readings. The pot water is cooler than the air, and cooler still on a dry day.
Worked example
The air is 34 °C. How many degrees cooler is the pot water on a dry day? On a humid day?
Dry day: 34 - 27 = 7 °C cooler.
Humid day: 34 - 32 = 2 °C cooler.
The pot does its job better on the dry day, by 5 °C.
Answer: dry day 7 °C, humid day 2 °C
Try it
Which wrap keeps water warm longest?
You need: three identical cups, a piece of cloth and a piece of paper, hot water, poured by an adult, a clock, a thermometer, if you have one.
Be safe: An adult pours the hot water and stays close. Never put a finger in the water. Carry nothing hot.
Wrap one cup in cloth and one in paper. Leave the third bare.
Ask an adult to pour the same amount of hot water into each cup. Mark the level with a pencil first.
Leave them for 20 minutes. Do not touch them.
Ask the adult to take off the wraps and touch the side of each cup for a moment. Or use a thermometer if you have one. Rank the cups from warmest to coolest.
What you will see: The cloth cup stays warmest and the bare cup is coolest. The paper cup is in between. Your cups may differ a little, but the order should be the same.
If you cannot try it, this is what happens.
A wrong idea many people have
A woollen jumper makes heat.
Why it feels right: You feel warm when you put one on, so the jumper seems to give out warmth.
The test that breaks it: Wrap one cup of cold water in a woollen sock and put a bare cup beside it. After an hour the wrapped water is no warmer than the bare one. The wool made no heat.
What is true: Wool is an insulator. It traps the heat your own body gives out, and it slows that heat from escaping.
Key words
insulation
a material that holds still air and slows heat down.
fair test
a test where only one thing is changed.
vacuum flask
a bottle with a double wall and no air in the gap.
Back to your guess
The answer is C: Wrap it in cloth or wool, which traps still air.
Wrap it in cloth or wool. These trap still air, and still air passes heat on very slowly. A metal tin would let the heat out fast.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why does a woollen cloth keep a cup of tea warm?
See the answer
It traps still air, and still air passes heat on very slowly.
Wool holds many pockets of still air. Air is a poor conductor and cannot circulate, so heat leaves slowly.
If you chose B: Wool makes no heat. It only slows down the heat that is already in the tea. Go to S3.8.6.
If you chose C: A good conductor would carry heat away fast. Wool is an insulator. Go to S3.8.5.
Amina wraps a cloth round one cup, and paper round a bigger cup with more water. She says cloth is best. Why is her test not fair?
See the answer
She changed two things: the wrap and the amount of water.
A fair test changes only one thing. Here both the wrap and the amount of water changed, so you cannot tell which made the difference.
If you chose A: That is not the problem. The problem is that she changed more than the wrap. Go to S3.8.6.
If you chose C: Paper is a fine thing to test. The test just needs the same cup and the same water. Go to S3.8.6.
Two cups start at 90 °C. After 30 minutes the cloth cup is 62 °C and the foil cup is 53 °C. How many degrees warmer is the cloth cup?
See the answer
9 °C
62 - 53 = 9 °C.
If you chose A: You added the two readings. To find how much warmer, subtract. Go to S3.8.6.
If you chose B: That is how much the cloth cup lost. The question asks how much warmer it is than the foil cup. Go to S3.8.6.
If you chose C: That is how much the foil cup lost. Compare the two readings instead. Go to S3.8.6.
How does a wet clay pot keep its water cool?
See the answer
Water seeps out and evaporates, and takes energy from the pot and the water.
Evaporating water needs energy. It takes it from the pot and the water inside, which cool as a result.
If you chose A: There is no such thing as made cold. Cooling is energy leaving, not cold arriving. Go to S3.8.6.
If you chose B: An insulator would only slow the warming. It could not make the water cooler than the air. Go to S3.8.6.
When does a wet clay pot cool its water best?
See the answer
In dry air that is moving.
Dry, moving air takes the water vapour away. The water evaporates fast, so more energy leaves the pot.
If you chose B: In humid air the water evaporates slowly. Slow evaporation takes away little energy. Go to S3.8.6.
If you chose C: A closed, damp room is humid. The water cannot evaporate quickly, so the pot cools little. Go to S3.8.6.
From an earlier chapter
Review: warm air rises above a heater and cool air sinks to take its place. What is this way of moving heat called?
See the answer
Convection
Convection is heat moving in a liquid or a gas as warm fluid rises and cool fluid sinks.
If you chose A: Conduction is heat moving through a solid. Air can flow, so this is something else. Go to S3.8.5.
If you chose C: Radiation is infrared that needs no material. Here the moving air carries the heat. Go to S3.8.5.
Keep it
Copy this into your exercise book.
Insulators trap still air and slow heat down. A fair test changes one thing. A wet clay pot cools water as water evaporates and takes energy away, best in dry air.
Draw: A cup in a cloth with short arrows. Next to it a wet pot with arrows going out.
