--- title: "How does heat get through?" subtitle: "Year 9 Physics · Energy transfer · Lesson 1" --- # How does heat get through? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Leave a metal spoon in hot soup and the handle soon burns your fingers. Heat a pot of water from the bottom and the top gets hot too. Two different journeys for the energy. :::teacher-note **In one sentence:** Students explain conduction (energy passed by particle collisions, particles staying in place) and convection (the heated fluid itself moving, driven by density differences), and apply them to conductors and insulators (AC9S9U04, node 1). **Students should be able to say:** I can explain conduction as energy passed between particles by collisions. I can explain convection as the movement of heated fluid, driven by density differences. I can explain why a named material is a good conductor or a good insulator. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A metal spoon stands in hot soup. Soon the handle is hot. What do you predict is happening in the spoon?** ☐ Heat flows up like a liquid ☐ Hot particles travel up the handle ☐ Particles vibrate harder and pass the energy to their neighbours, staying in place ☐ Cold leaves the spoon My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** The particles stay in place. Those at the hot end vibrate harder and bump their neighbours, passing the energy along the handle. That is conduction. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: two ways to travel :::keep-together ### In the bar **In the bar, how did the energy reach the far end?** ☐ Hot particles moved to the far end ☐ Particles vibrated harder and bumped their neighbours, passing the energy along ☐ The heater's flame travelled along the bar ::: :::teacher-note **Answer:** Particles vibrated harder and bumped their neighbours, passing the energy along Conduction: energy passes from particle to particle through collisions. The particles stay where they are. ::: :::keep-together ### In the water **In the water, how did the energy reach the top?** ☐ The cold water went down and pushed heat up ☐ The heated water itself moved up, carrying the energy ☐ Particles passed energy up without moving ::: :::teacher-note **Answer:** The heated water itself moved up, carrying the energy Convection: the heated fluid rises, carrying its energy, and cooler fluid sinks to replace it. ::: :::keep-together ### Why it rises **Why does the heated water rise?** ☐ The heater pushes it up ☐ Hot particles are lighter than cold ones ☐ Its particles spread out, so it is less dense than the cooler water around it ☐ Heat always rises ::: :::teacher-note **Answer:** Its particles spread out, so it is less dense than the cooler water around it Heated water expands: the same particles take up more space, so it is less dense (density, from Year 7) and floats up through the cooler, denser water. The board pushes hot particles up directly; that part is a simplification. ::: :::keep-together ### Which is which **Conduction or convection?** Groups: Conduction, Convection - A saucepan handle getting hot: {{field-15}} - Warm air from a heater spreading round a room: {{field-15}} - Your hand warming up on a hot mug: {{field-15}} - Water in a kettle heating all the way through: {{field-15}} - A sea breeze on a hot afternoon: {{field-15}} - Ice cream melting on a metal spoon faster than on a plastic one: {{field-15}} ::: :::teacher-note **Answer:** A saucepan handle getting hot → Conduction; Warm air from a heater spreading round a room → Convection; Your hand warming up on a hot mug → Conduction; Water in a kettle heating all the way through → Convection; A sea breeze on a hot afternoon → Convection; Ice cream melting on a metal spoon faster than on a plastic one → Conduction ::: :::keep-together ### Solids **Why can't convection happen in a solid?** ☐ The particles are held in place, so the material can't flow ☐ Solids can't be heated ☐ Particles in a solid don't move at all, so they can't carry energy ::: :::teacher-note **Answer:** The particles are held in place, so the material can't flow Convection needs the material to flow. Only liquids and gases can. ::: :::keep-together ### A foam cup **Why does a foam (polystyrene) cup keep a drink hot so well?** ☐ Its particles are packed very tightly, so energy can't squeeze through ☐ The foam reflects the heat back into the drink ☐ It is mostly trapped air: gas particles are far apart, so they rarely collide to pass energy on, and air in the tiny bubbles can't flow to convect ::: :::teacher-note **Answer:** It is mostly trapped air: gas particles are far apart, so they rarely collide to pass energy on, and air in the tiny bubbles can't flow to convect Insulators pass energy on poorly. In a gas the particles are far apart, so collisions that pass energy along are rare; trapping the gas in small bubbles stops convection too. ::: :::typical ## Elaborate: turn the heat off :::keep-together **Turn the heat off once the current is going and watch for a while. What happens to the current?** ☐ It reverses direction ☐ It keeps going forever ☐ It slows down and stops as the water ends up the same temperature all through ::: :::teacher-note **Answer:** It slows down and stops as the water ends up the same temperature all through A convection current needs one part of the fluid to be hotter (less dense) than another. With no heater, the difference fades and the current stops. ::: ::: :::wider ## Elaborate: conductors and insulators :::keep-together ### Saucepans **Saucepans are metal with plastic or wooden handles. Why?** ☐ Metal conducts heat well to cook the food; plastic and wood conduct poorly, so the handle stays cool ☐ Metal handles would melt ☐ Plastic handles are lighter ::: :::teacher-note **Answer:** Metal conducts heat well to cook the food; plastic and wood conduct poorly, so the handle stays cool Metals are good conductors; plastic and wood are insulators. ::: :::keep-together ### Trapped air **Wool, fur and feathers keep animals warm mostly because they trap air. Why does trapped air insulate so well?** ☐ Air is warm ☐ Air reflects heat back ☐ Air conducts poorly, and trapped in small pockets it can't form convection currents ::: :::teacher-note **Answer:** Air conducts poorly, and trapped in small pockets it can't form convection currents Gas particles are far apart, so conduction is poor; in tiny pockets the air can't circulate, so convection is blocked too. Possum-skin cloaks, made and worn by many First Nations peoples of south-eastern Australia, use the same idea: dense fur trapping air. ::: ::: :::typical ## Elaborate: a question to test :::keep-together ### The question **You want to find out how wrapping a cup changes how fast hot water in it cools. Which is an investigable question?** ☐ Does a thicker wool wrap make hot water in a cup cool more slowly? ☐ What is heat? ☐ Is a wool wrap nicer to hold than a foam cup? ☐ Why is wool the best insulator? ::: :::teacher-note **Answer:** Does a thicker wool wrap make hot water in a cup cool more slowly? An investigable question names one thing you change (the thickness of the wrap) and one thing you measure (how fast the temperature falls). ::: :::keep-together ### The hypothesis **Which is the best hypothesis for that question?** ☐ The wrap will make a difference ☐ Thicker wraps are better ☐ If the wrap is thicker, the water will cool more slowly, because more trapped air slows conduction and blocks convection ☐ The water will cool down ::: :::teacher-note **Answer:** If the wrap is thicker, the water will cool more slowly, because more trapped air slows conduction and blocks convection A good hypothesis states a relationship (thicker wrap, slower cooling), could be shown wrong by your data, and gives a reason from the model. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Conduction:** particles vibrate harder and pass energy to their neighbours by collisions. **The particles stay in place.** Mostly in solids; metals conduct well. - **Convection:** heated fluid becomes less dense and rises, cooler fluid sinks: a **convection current**. **The particles move**, carrying the energy. Only in liquids and gases. - **Insulators** (wood, plastic, trapped air) pass energy on poorly. Trapped air blocks convection too. - It is the hot fluid that rises, not "heat". ## Exit ticket :::keep-together **Why can convection happen in water but not in a metal spoon?** {{lines-3}} ::: :::teacher-note **Answer:** Convection needs the material to flow so heated parts can rise. Water's particles can move; a spoon's are held in place, so it can only conduct. :::