Metals and Heat
FoundationMetals conduct electricity and heat: copper for wires, iron for woks. Wood and plastic insulate, so wok handles are wooden or plastic — cooking fast without burning hands.
- Metals conduct electricity and heat and shine; non-metals usually do neither well.
- Copper's conduction makes wires; iron's hardness and heat make woks.
- Fast conductors carry heat; slow conductors insulate.
文體小知識：說明文把物料的特性與用途逐樣講解。這一篇教你分辨金屬與非金屬，看看導熱和隔熱怎樣影響生活設計。
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1The things around us are made of different materials: spoons and wires of metal, desks and toys of wood and plastic. Metals share common features: a shiny surface, the ability to be shaped, and the power to conduct both electricity and heat. Non-metal materials such as wood, plastic, glass and rubber usually do not conduct electricity and conduct heat much more slowly.
2A material's features decide its uses. Copper conducts electricity especially well and draws easily into thin wires, so it makes electric wires. Iron is hard and conducts heat, so it makes woks and building materials. Aluminium is light and does not rust, so it makes drink cans and window frames. People choose the best material for each product by its features.
3Heat travels at different speeds. Metals conduct heat fast and are called conductors of heat; wood, plastic and air conduct slowly and are called insulators. Try a small test: butter the ends of a metal spoon, a wooden chopstick and a plastic spoon, stand them in hot water together, and the butter falls off the metal spoon first — the fastest conductor of all.
4Conducting and insulating both serve us. We want food cooked quickly, so we use a metal wok that conducts heat; we do not want burnt hands, so the handle is made of insulating wood or heat-proof plastic. A vacuum flask slows heat loss with a vacuum layer and plastic cap; in winter a down jacket keeps us warm by locking insulating air among the feathers.
5Knowing how materials conduct heat also keeps us safe. Be careful near hot woks and kettles: metal surfaces far from the flame may still be burning hot. Use oven gloves to carry hot things, and you will not be scalded. Using conductors and insulators well makes life more convenient and safer.
1Summary 重點摘要
Metals conduct electricity and heat: copper for wires, iron for woks. Wood and plastic insulate, so wok handles are wooden or plastic — cooking fast without burning hands.
2Vocabulary cards Vocabulary
A shiny material that conducts electricity and heat, such as copper, iron and aluminium.
A material that usually does not conduct electricity and conducts heat slowly, such as wood, plastic and glass.
Heat travelling along an object from the hot end to the cool end.
How well a material lets current pass; copper conducts very well.
A metal that conducts both electricity and heat well, used for wires and cookware.
A hard, heat-conducting metal used for woks and construction.
A material that conducts heat very slowly, such as wood, plastic and air, used to block heat.
A light metal that hardly rusts, used for cans and window frames.
A light man-made material that conducts heat slowly, used for handles and casings.
The part of a utensil held by hand, usually made of insulating material.
3Step by step Steps
- Step 1: Telling metals from non-metalsLook carefully and the materials around us fall roughly into metals and non-metals. Metals include copper, iron, aluminium, gold and silver. They share features: a metallic shine, a clear ring when tapped, and the ability to be drawn into wire or hammered into sheets; they conduct both electricity and heat. Non-metal materials such as wood, bamboo, plastic, glass, rubber and ceramics usually lack metallic shine, barely conduct electricity, and conduct heat slowly. Judge by shine and feel, and you can sort the things around you roughly into groups.
- Step 2: The common features of metalsA few simple tests reveal what metals share. Stand a metal spoon and a wooden chopstick in hot water together, and the spoon's handle soon warms — that is conduction of heat. Build a circuit of battery, wire and bulb: connect the bulb with copper wire and it lights; replace the wire with a plastic ruler and it goes dark — metals conduct electricity, plastic does not. Metals also shine and can be spread into thin foil. In short: metals conduct electricity and heat, shine, and can be shaped — and these features explain their uses.
- Step 3: Features decide usesA material's features and its uses go hand in hand. Copper is among the best of common metals at conducting electricity and draws easily into thin wire, so nearly every wire core is copper. Iron is hard, lasting and conducts heat fast, so woks are made of iron or steel, and skyscrapers stand on steel bars. Aluminium is light and hardly rusts, perfect for drink cans, aircraft skins and window frames. The other way round, wire coverings must insulate, so plastic is chosen. Learn the features first, then decide the use — that is the basic way to pick materials.
- Step 4: Testing conductors and insulatorsTo find which materials carry heat fastest, run your own experiment. Method one: butter one end of a metal spoon, a ceramic spoon, a wooden chopstick and a plastic spoon, stick a bean onto each buttered end, and stand them in hot water together. The material whose butter melts first and drops its bean first is the fastest conductor — usually the metal spoon. Method two: touch a metal ruler and a plastic ruler that have sat on the desk for ages. The metal one feels cooler because it carries the heat of your hand away fast — not because it is truly colder.
- Step 5: Conductors and insulators at workConductors and insulators each take their own jobs. To heat quickly, use metal: woks, pots and the heating parts of kettles are all metal. To hold heat back, use insulators: wok and pot handles of wood or heat-proof plastic, cloth pads and oven gloves for carrying hot soup, and a vacuum flask whose vacuum layer, plastic cap and silvered inner wall slow heat both ways. A down jacket keeps warm by locking air among the feathers — air conducts heat slowly, so it insulates. One utensil often uses both kinds together, each doing what it does best.
- Step 6: Using hot metalware safelyMetal conducts heat fast, so carelessness brings scalds. The rim of a wok and the area beside a pot handle, though not touching the flame, may be searing hot from conducted heat; before lifting, test with the back of your hand or use oven gloves. A metal spoon left long in hot soup heats up along its handle — never put it straight into your mouth after stirring. If home wires or sockets feel hot, do not touch them; tell an adult at once. Knowing how materials conduct heat makes life both easier and safer.
5Worked example Example
Example: Mum bought a new wok. The body is iron but the handle is wood. Why is the body not wood, and the handle not iron? Explain using what you know about heat conduction.
- The body must pass heat quickly from the stove to the food, so it needs a fast conductor; iron conducts fast and is hard — just right.
- Wood conducts heat very slowly; a wooden body would cook poorly and waste the flame, so the body is not wood.
- The handle must let us hold the wok bare-handed, so it must insulate and stop heat travelling from body to hand.
- Iron conducts fast and would burn the hand as a handle; wood conducts slowly and stays only warm, so the handle is wood.
- Answer: The body is iron because iron conducts heat fast and is hard, passing heat quickly to the food. The handle is wood because wood conducts slowly — an insulator — so heat hardly reaches the hand and holding it is safe. Conductors and insulators each play their part, making the wok handy and safe.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which metal is mostly used inside electric wires?
Q2What metal are woks usually made of?
Q3Why are wok handles made of wood or plastic?
Q4Which kind of material conducts electricity?
Q5How does a vacuum flask slow heat loss?
8Recap Recap
- Metals conduct electricity and heat and shine; non-metals usually do neither well.
- Copper's conduction makes wires; iron's hardness and heat make woks.
- Fast conductors carry heat; slow conductors insulate.
- A wooden or plastic wok handle is insulation in action.
- Using conduction and insulation well makes cooking fast and safe.