Lesson overview
Conduction and convection: Why does a pan handle stay cool while the water boils?
Fit a fair copper rod beside rods of aluminium, steel and glass, predict which drawing pin drops first, then pour hot water into the tank and time the pins as the wax melts; look inside the copper and the glass with a magnifier. Then drop a dye crystal into a beaker of cold water, heat one corner and follow the dye round its loop, and decide where to put the flame to turn the loop the other way.
Suggested sequence
- Lesson 1 · Conduction. Good and bad thermal conductors, and a fair test to compare them; Supplement: conduction by vibrating particles and free electrons, why liquids and gases conduct badly, and the solids in between. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Convection. Convection in liquids and gases, explained by density changes, with an experiment to show it; heating a room, a sea breeze and heating a pan. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Conduction
- Describe experiments to demonstrate the properties of good thermal conductors and bad thermal conductors (thermal insulators) (2.3.1.1)
- Supplement: Describe thermal conduction in all solids in terms of lattice vibrations, and in metals also in terms of the movement of free (delocalised) electrons (2.3.1.2)
- Supplement: Describe, in terms of particles, why thermal conduction is bad in gases and most liquids; know that many solids conduct thermal energy better than thermal insulators but less well than good thermal conductors (2.3.1.3, 2.3.1.4)
Part 2: Convection
- Know that convection is an important method of thermal energy transfer in liquids and gases; explain convection in terms of density changes and describe experiments to illustrate it (2.3.2.1, 2.3.2.2)
- Explain everyday applications and consequences of conduction, convection and radiation, including heating kitchen pans and heating a room by convection (2.3.4.1)
Misconceptions to listen for
- “The radiator panel should be an insulator, so that it keeps its heat in.” — A radiator is there to pass energy from the hot water inside it to the room, so it must let energy through quickly: a good conductor.
- “The walls of the cool box should be a good conductor, to let the cold reach the food.” — Nothing called “cold” flows. The walls must slow down the energy that flows in from the warm air outside, so they need an insulator.
- “The oven glove should be a good conductor, so that it takes the heat away from the tray.” — If the glove conducted well, energy from the hot tray would reach your hand quickly. An insulator lets it through only slowly.
- “Plastic reflects more sunlight, so less energy reaches your hand.” — The sunlight has already warmed both slides to the same 45 °C. Your hand gets its energy from the slide you touch, and steel passes it on much faster.
- “Steel stores more heat than plastic, so it feels hotter.” — What you feel is the rate at which energy flows into your skin. Steel conducts well, so energy reaches your skin quickly.
- “Energy flows from your hand into the plastic, so it feels cool.” — The plastic is at 45 °C and your skin at about 33 °C, so energy flows into your hand from both slides: only slowly from the plastic.
- “Steel is the better conductor: it kept its energy in for longer.” — A good conductor lets energy through quickly. The steel let less energy reach its far end, so it is the worse conductor.
- “They conduct equally well: both rods started at 21 °C.” — The same start is what makes the test fair. What you compare is how fast each far end warms after that.
- “Neither result counts: the far ends never reached the water temperature.” — You do not need to wait that long. In the same time, the rod whose far end warms more conducts better.
- “The rod whose pin takes longest conducts best, because it holds on to the energy.” — A good conductor lets energy through quickly, so its pin drops first.
Scope
Experiments that show good and bad thermal conductors (2.3.1.1); Supplement: conduction in solids by lattice vibrations and, in metals, by free electrons (2.3.1.2), why most liquids and gases conduct badly (2.3.1.3), and solids that conduct less well than good conductors but better than insulators (2.3.1.4); convection in liquids and gases, explained by density changes, with an experiment to show it (2.3.2.1, 2.3.2.2); everyday applications and consequences of conduction, convection and radiation, including heating a pan and heating a room (2.3.4.1). Thermal radiation and the vacuum flask are in Thermal radiation; conductivity numbers and rates are not covered.