Why does the metal spoon in hot tea get hot, while the wooden spoon stays cool?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain conduction, convection and radiation with examples from home
Compare how fast metal and wooden spoons warm in hot water
Ravi stirs his tea and leaves a metal spoon in the cup. When he lifts it out, the handle is hot. His wooden spoon stood in the same cup, and its handle is cool.
Heat moves from hot things to cool things in three ways. They are called conduction, convection and radiation. We will take them one at a time.
Conduction is heat moving through a solid, from one particle to the next. In the hot end of a spoon the particles shake hard. They bump the next ones and pass the energy along.
Draw it first
Draw heat moving along a spoon
1 Draw a cup of hot water. Stand a metal spoon and a wooden spoon in it.
2 Draw an arrow up the metal handle. Heat moves along the metal from the water.
3 Write: heat travels up the metal. The wooden handle stays cool.
A conductor lets heat pass through it easily. Metals are good conductors. An insulator lets heat pass through slowly. Wood, plastic, cloth and air are insulators.
How fast heat moves through each material.One set of readings of the top of each handle, taken with a thermometer, once a minute.
Worked example
Use the table. By how many degrees did the metal handle warm in 4 minutes? How about the wooden handle?
After 4 minutes the metal handle is 38 °C. It started at 20 °C.
Metal: 38 - 20 = 18 °C.
Wood: 21 - 20 = 1 °C.
The metal handle warmed 18 times as much.
Answer: metal 18 °C, wood 1 °C
Convection is heat moving in a liquid or a gas. Water at the bottom of a pot is heated. It spreads out a little. A cupful of it then weighs less than a cupful of the cool water above, so it rises.
Warm water rises and cool water sinks to take its place. The loop spreads the heat through the pot.Air does the same. This is why the air near the ceiling is warmer than the air near the floor.
Radiation is heat that travels as invisible waves called infrared. It needs no material at all. It crosses empty space. The Sun warms the Earth with light and with infrared.
The space between the Sun and Earth is almost empty. Radiation crosses it, so no conduction or convection is needed.
A dark, dull surface takes in radiation best. It also gives out radiation best. A shiny, light surface sends most of it back. This is why a black can warms faster in the sun than a shiny one.
Example readings. Both cans started at 25 °C. The black can ends hotter.
See it three ways
As a picture
In words
Conduction moves heat through solids, such as a hot spoon. Convection moves heat in liquids and gases, such as warm air over a stove. Radiation moves heat as infrared, even through empty space, such as the Sun on your skin.
Try it
Race a metal and a wooden spoon
You need: a metal spoon and a wooden spoon of the same length, a cup, hot water, poured by an adult, a clock, or a friend who counts seconds.
Be safe: An adult pours the hot water and stays close. Never grab a spoon that has stood in hot water. Test it gently with one finger first, and take your finger away at once if it feels hot.
Ask an adult to pour hot water into the cup. Let it stand for five minutes first if it came from a kettle.
Stand both spoons in the cup with their handles leaning on the rim. Start counting.
After one minute, touch the top of each handle very briefly with one finger. Do not hold on.
Test again after two and three minutes. Write down which handle feels warmer each time.
What you will see: The metal handle warms up within a minute or two. The wooden handle stays cool for much longer. Heat travels fast along the metal and slowly along the wood.
If you cannot try it, this is what happens.
A wrong idea many people have
A metal chair is colder than a wooden chair in the same room.
Why it feels right: Metal feels much colder when you touch it, so it seems colder.
The test that breaks it: Both chairs have stood in the room all day. A thermometer reads the same on both. Metal feels colder because it carries heat away from your hand fast.
What is true: Both are at the same temperature. Metal is a good conductor, so it takes heat from your hand quickly. Wood is an insulator and takes it slowly.
Key words
conduction
heat moving through a solid.
convection
heat moving in a liquid or gas, as warm fluid rises.
radiation
heat moving as infrared, with no material needed.
conductor
a material that lets heat pass through it easily.
insulator
a material that lets heat pass through slowly.
infrared
invisible waves that carry heat.
Back to your guess
The answer is C: Heat travels along metal much better than along wood.
Heat moves along metal quickly. It moves along wood very slowly. Wood does warm up, but only after a long time.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
The handle of a metal spoon in hot tea gets hot. Which way is the heat travelling?
See the answer
By conduction
Heat moves through a solid by conduction, from particle to particle. Metal conducts heat well.
If you chose B: Convection happens in liquids and gases that can flow. A spoon is a solid, so its heat conducts through it. Go to S3.8.5.
If you chose C: Radiation travels as infrared through space. Here the heat passes through the metal itself. Go to S3.8.5.
The bottom of a pot of water is heated. What does the warm water do?
See the answer
It rises, and cooler water sinks to take its place.
Warm water has spread out, so a cupful of it weighs less, and it rises. Cooler water sinks in to replace it. This loop is convection.
If you chose A: Warm water has spread out, so a cupful of it weighs less than a cupful of cool water. So it rises. Go to S3.8.5.
If you chose C: The warm water is free to move. It has spread out, and it rises. Go to S3.8.5.
How does the Sun's warmth reach the Earth across empty space?
See the answer
By radiation
Radiation travels as infrared and needs no material. It crosses empty space.
If you chose A: Conduction needs a material to pass through. Space between the Sun and Earth is almost empty. Go to S3.8.5.
If you chose B: Convection needs a liquid or gas that can flow. Space has almost none. Go to S3.8.5.
Which is the best handle for a cooking pot?
See the answer
Wood, because it is an insulator.
Wood lets heat pass slowly, so the handle stays cool enough to hold.
If you chose B: Metal is strong, but it conducts heat to your hand fast. It could burn you. Go to S3.8.5.
If you chose C: They do not. Metal warms much faster than wood, as the spoons showed. Go to S3.8.5.
A learner tests three spoons in hot water. After 2 minutes the handles read as shown. How many degrees warmer is the copper handle than the steel handle?
See the answer
8 °C
Copper 41 °C and steel 33 °C. 41 - 33 = 8 °C.
If you chose A: You added the two readings. To find how much warmer, subtract. Go to S3.8.5.
If you chose B: You took the wooden spoon instead of the steel one. Read the steel row. Go to S3.8.5.
If you chose C: That is how much the copper handle warmed from 20 °C. The question compares it with steel. Go to S3.8.5.
From an earlier chapter
Review: a cup of tea is 88 °C when it is poured. Half an hour later it is 46 °C. How many degrees did it cool?
See the answer
42 °C
Cooling = 88 - 46 = 42 °C.
If you chose A: You added the two temperatures. The cooling is the difference, so subtract. Go to S3.8.4.
If you chose C: That is the temperature at the end. The question asks how much it fell. Go to S3.8.4.
Keep it
Copy this into your exercise book.
Conduction: heat through solids, fast in metals. Convection: warm liquid or gas rises, cool sinks. Radiation: infrared crosses empty space. Insulators slow heat down.
Draw: A metal spoon and a wooden spoon in a cup, with an arrow up the metal one.
