Answers · Conduction and convection

Conduction and convection — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. TK1 · Final check

Model answer. (a) The brass rod is thicker than the plastic rod, and a thicker rod carries more energy at a time: use rods of the same thickness. The pins are fixed with different things, and butter melts at a much lower temperature than candle wax: fix both pins with the same wax. (The pins are both 6 cm from the tank, so that part is fair.) (b) Supplement. Brass is a good thermal conductor, glass is in between and plastic is a thermal insulator. The top of the glass took about 18 times as long as the brass to reach 40 °C (70 s against 4 s), so glass conducts far less well than the metal. But the plastic took about 5 times as long as the glass (360 s against 70 s), so glass conducts clearly better than the insulator: it is one of the solids in between. (c) Supplement. The particles of glass vibrate and pass energy on too: lattice vibrations happen in every solid. Brass conducts better because it is a metal and also has free electrons, which gain kinetic energy at the hot end and carry it quickly through the brass. (d) Supplement. The particles of water are close together, but they are not held in a fixed lattice: they slide past each other, so vibrations are not passed on well. Water has no free electrons either, so energy is passed on only slowly, by collisions between neighbouring particles. Water is a bad thermal conductor.

A full answer includes

(a) I identified the different thicknesses of the rods (8 mm and 4 mm).
(a) I said both rods should have the same thickness.
(a) I identified the different fixings: butter on one pin, candle wax on the other.
(a) I said both pins should be fixed with the same wax.
(b) I identified brass as a good thermal conductor.
(b) I identified glass as in between.
(b) I identified plastic as a thermal insulator.
(b) I said glass took far longer than brass: 70 s against 4 s.
(b) I said plastic took about 5 times as long as glass: 360 s against 70 s.
(c) I said the particles of both brass and glass vibrate and pass energy on.
(c) I said brass, a metal, also has free electrons.
(c) I said the free electrons carry energy quickly through the brass.
(d) I said the particles of water are not held in a fixed lattice.
(d) I said they slide past each other, so vibrations are not passed on well.
(d) I said water has no free electrons.
I did not name the pin distance as a fault: both pins are 6 cm from the tank.
I did not put glass with the plastic as an insulator.

Also accept

(a) different thicknesses: same thickness; butter and wax: same wax (b) brass good conductor, glass in between, plastic insulator: glass far slower than brass but about 5 times faster than plastic (c) both vibrate; brass also has free electrons (d) no fixed lattice, no free electrons

Watch for

“The pins at the same distance are a fault too.” — Both pins are 6 cm from the tank, so that part of the plan is fair.
“Glass goes with plastic, because glass is an insulator.” — Next to brass, glass is a bad conductor. But the plastic took about 5 times as long as the glass, so with three groups glass goes in between.
“Brass conducts better because its particles vibrate faster.” — The particles of glass vibrate and pass energy on too. Brass is better because it is a metal and also has free electrons, which carry energy quickly.
“Water conducts well because its particles touch each other.” — Close particles are not enough: in a liquid they are not held in a lattice, and there are no free electrons.

2. TK2 · Final check

Model answer. (a) The far side of the room stays much colder than before, or warms far more slowly. Air warmed in the heater expands and becomes less dense, so it must rise from the grille and spread out under the ceiling, while cooler, denser air sinks on the far side and flows back along the floor to the heater. With the sofa over the grille, the warm air cannot rise freely into the room: much of it stays trapped behind the sofa, so the convection current that carried energy across the room is weakened. (b) Air is a very bad thermal conductor, so very little energy would reach the far side by conduction. Air is a fluid: energy crosses a room mainly by convection, when the warmed air itself moves and carries its energy with it. (c) The cold water from the ice is denser than the warmer water around it, so it sinks below the ice cube to the bottom, then spreads along the bottom of the tank, while warmer water moves up to take its place. (d) Thick porridge in a saucepan is too stiff to flow, so no convection current can carry energy away from the base, and porridge conducts badly. Energy builds up in the layer touching the base, which gets hot enough to burn. Thin soup flows, so warmed soup rises and cooler soup sinks to the base: a convection current spreads the energy. Stirring does the same job for the porridge.

A full answer includes

(a) I predicted that the far side stays colder than before (or warms much more slowly).
(a) I said the warmed air is less dense, so it has to rise from the grille into the room.
(a) I said the sofa stops the warm air rising freely.
(a) I said the convection current that carries energy across the room is weakened.
(b) I said air is a very bad thermal conductor.
(b) I said the energy crosses the room mainly when the warmed air itself moves: convection.
(c) I predicted the coloured water sinks below the ice cube.
(c) I predicted it then spreads along the bottom of the tank.
(c) I said the cold water is denser than the water around it.
(d) I said thick porridge cannot flow, so there is no convection current in it.
(d) I said the porridge conducts badly.
(d) I said the layer at the base gets hot enough to burn.
(d) I said the thin soup flows, so a convection current carries energy away from the base.
I did not write that heat rises or cold sinks by itself.

Also accept

(a) the far side stays colder: the warm, less dense air cannot rise freely, so the convection current is weakened (b) air conducts very badly; energy crosses the room when the warm air moves (c) sinks below the cube, spreads along the bottom: cold water denser (d) porridge cannot flow, no convection; the base layer overheats

Watch for

“The far side warms as before, because the heater still gives out the same energy.” — The energy still leaves the heater, but the warm air cannot rise freely into the room, so it stays near the heater and the sofa instead of being carried across the room.
“The heater warms the room mainly by conduction through the air.” — Air is a very bad conductor; the energy is carried round by the moving air: convection.
“The cold water from the ice floats, like the ice.” — Ice floats because ice is less dense than water. Once it has melted, the very cold water is denser than the warmer water around it, so it sinks.
“Porridge burns because it needs more heat than soup.” — The difference is that soup can flow and porridge cannot, so only the soup has a convection current.