Answers · Conduction and convection

Convection — answer key

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

1. TQ13 · Practice

Model answer. Convection is the transfer of thermal energy by the movement of the fluid itself, so it happens only in fluids: liquids and gases. In a solid the particles stay in their places, so a solid cannot flow and there is no convection. Liquids and gases are bad conductors, so convection is an important way for energy to travel through them. Row C.

A full answer includes

I chose row C.
I did not choose a row with convection in a solid.

Also accept

Row C

Watch for

“Row A.” — A solid cannot flow: its particles stay in their places, so there is no convection in a solid.
“Row B.” — A gas can flow too: warm air rising above a heater is convection.
“Row D.” — A liquid can flow too: warm water rising from the bottom of a hot-water tank is convection.

2. TQ14 · Practice

Options as numbered on the pupil sheet: 1 — The less dense water rises, pushed up by the denser water around it; 2 — Cooler, denser water sinks to take its place and is heated in turn; 3 — The water at the bottom is heated; 4 — Its density falls; 5 — Its particles move faster and spread out, so the water expands.

Model answer. The water at the bottom is heated; its particles move faster and spread out, so it expands; with the same mass in a bigger volume, its density falls; the denser water around it pushes it upwards, so it rises; cooler, denser water sinks to take its place, is heated in turn, and the current keeps going.

A full answer includes

I put “the water at the bottom is heated” first.
I put “the water expands” before “its density falls”.
I put “the less dense water rises” after “its density falls”.
I put “cooler, denser water sinks to take its place” last.
I did not put “rises” before “expands”.

Also accept

heated; expands; density falls; rises; cooler water sinks in

Watch for

“The water rises first and then expands.” — It rises because it has expanded and become less dense.

3. TQ15 · Practice

Model answer. When air is warmed, its particles move faster and spread out, so the air expands. Its mass stays the same but its volume grows, so its density falls. The balloon full of warm air is less dense than the cooler, denser air around it, which pushes it upwards. Heat itself does not rise, and each particle keeps the same mass.

A full answer includes

I chose: the warmed air expands, so it is less dense than the cooler air outside.
I did not choose that the heat rises and carries the balloon.

Also accept

expands, less dense than the air outside, pushed up

Watch for

“The heat rises, and it carries the balloon and the air inside with it.” — Heat is energy. What rises is warm air, because it is less dense than the air around it.
“Each air particle gets lighter when it is heated, so the air floats.” — Each particle keeps its mass; the same particles take up more space, so each cubic metre holds less mass.
“The flame from the burner pushes upwards on the top of the balloon.” — The flame heats the air near the burner; it does not reach the top. The cooler air outside pushes the balloon up.

4. TQ16 · Practice

Model answer. (a) Mass = density × volume = 1.23 kg/m³ × 2.40 m³ = 2.952 kg = 2.95 kg (3 s.f.). (b) Warming does not change the mass: it is still 2.95 kg. Density = mass ÷ volume = 2.95 kg ÷ 2.59 m³ = 1.14 kg/m³. That is less than the 1.23 kg/m³ of the cool air around it, so the warm air is pushed upwards and rises. (c) Volume = mass ÷ density = 2.95 kg ÷ 1.08 kg/m³ = 2.73 m³.

Checked automatically

(a) Mass of the air: 2.95 kg (3 significant figures)
- Common wrong answer 1.95 : You divided the volume by the density. Mass = density × volume.
(b) Density of the warm air: 1.14 kg/m³ (3 significant figures)
- Common wrong answer 0.475 : You divided 1.23 by the new volume, but 1.23 kg/m³ is a density, not the mass. Use the mass from (a).
- Common wrong answer 7.65 : You multiplied. Density = mass ÷ volume.
- Common wrong answer 0.877 : You divided the volume by the mass. Density = mass ÷ volume.
(c) Volume of the air: 2.73 m³ (3 significant figures)
- Common wrong answer 3.19 : You multiplied. Volume = mass ÷ density.
- Common wrong answer 0.366 : You divided the density by the mass. Volume = mass ÷ density.

A full answer includes

(a) I used mass = density × volume = 1.23 kg/m³ × 2.40 m³.
(a) My answer is 2.95 kg.
(b) I used density = mass ÷ volume = 2.95 kg ÷ 2.59 m³.
(b) My answer is 1.14 kg/m³.
(c) I used volume = mass ÷ density = 2.95 kg ÷ 1.08 kg/m³.
(c) My answer is 2.73 m³.
I did not use 1.23 as the mass of the air.

Also accept

(a) 2.95 kg (b) 1.14 kg/m³ (c) 2.73 m³

Watch for

“The mass of the air is 1.23 kg.” — 1.23 kg/m³ is the density: the mass of each cubic metre. 2.40 m³ of the air has a mass of 1.23 kg/m³ × 2.40 m³ = 2.95 kg.
“The density goes up, because the air has more energy.” — The mass stays the same and the volume grows, so the density falls.

5. TQ17 · Practice

Model answer. The water by the heater at D is warmed, expands, becomes less dense and rises from D to B. Cooler, denser water flows in along the bottom to take its place, and it carries the dye from C to D. The dye then rises to B, spreads across under the surface to A, cools, becomes denser and sinks down to C again. The dye traces this convection current.

A full answer includes

I chose: along the bottom from C to D, up to B, across to A, down to C.
I said the dye is carried along the bottom towards the heater first.
I said the warmed water rises above the heater, from D to B.
I did not choose a path where the dye rises from C or stays at C.

Also accept

C to D along the bottom, up to B, across to A, down to C

Watch for

“Up from C to A, across to B, down to D” — The water rises where it is heated, above the heater at D. The water at C is cool, so it does not rise there.
“It stays at C, because water conducts badly” — Water is a poor conductor, but here the water itself moves: cooler water flows along the bottom to the heater and carries the dye with it.
“Along the bottom from C to D, and it stays there by the heater” — At D the water is warmed and rises, so the dye rises with it and keeps moving round a loop.

6. TQ18 · Practice

Model answer. (a) Stand a beaker of cold water on a tripod and gauze. Drop a crystal of potassium manganate(VII), a purple dye, to the bottom next to one side, and heat gently with a small flame under that side only. (b) A purple stream rises above the flame, spreads across under the surface, sinks down the far side and flows back along the bottom: a loop. (c) The water over the flame is heated, expands and becomes less dense, so the cooler, denser water around it pushes it up. Cooler, denser water sinks on the far side and flows in to take its place, so a convection current forms, and the dye shows it.

A full answer includes

(a) I used a beaker of cold water.
(a) I used a crystal of a coloured dye (potassium manganate(VII)).
(a) I put the crystal at the bottom, by one side.
(a) I heated gently under that side only.
(b) I said the dye rises above the flame.
(b) I said it spreads across under the surface.
(b) I said it sinks down the far side.
(c) I said the heated water expands.
(c) I said its density falls, so it rises.
(c) I said cooler, denser water sinks to take its place.
I did not write that heat rises.

Also accept

(a) beaker of cold water, dye crystal by one side, small flame under that side (b) dye rises, crosses, sinks, returns (c) heated water less dense, rises; cooler, denser water sinks

Watch for

“Heat the whole bottom of the beaker evenly.” — With the flame under one side, the warm water rises in one place and the loop is clear to see.
“Heat rises, so the dye goes up.” — Heat is energy. The dye rises because the warmed water around it is less dense than the cooler water.

7. TQ19 · Practice

Model answer. The candle heats the air above it, which expands, becomes less dense and rises out through chimney Q. The paper is held over Q, so the smoke is carried up and away by this rising warm air. Cooler, denser air is drawn in through chimney P, but it has no smoke in it, so no smoke goes into the box.

A full answer includes

I chose: up and away above chimney Q.
I said the candle warms the air, which rises out of Q.
I said this rising air carries the smoke up, so no smoke goes into the box.

Also accept

up and away above Q

Watch for

“Down chimney Q, across the box and up out of chimney P” — That happens when the paper is over the chimney without the candle. Here air rises out of Q, so the smoke cannot go down it.
“Down chimney Q into the box, where it stays in a cloud” — Warm air is rising out of Q all the time, so the smoke is pushed up, not drawn down.
“Straight down chimney Q to the candle” — The candle draws cooler air in through P, not through Q. The air in Q moves up, away from the candle.

8. TQ20 · Practice

Model answer. Air that touches the cold glass is cooled, contracts, becomes denser and sinks. Without a radiator, this cold air would flow across the floor. The radiator under the window warms the sinking air, so it expands, becomes less dense and rises again, and a convection current carries the energy round the whole room.

A full answer includes

I chose: air cooled by the glass sinks, and the radiator warms it so it rises again.
I did not choose that the radiator stops cold coming in.

Also accept

air cooled by the glass sinks; the radiator warms it and it rises: a current round the room

Watch for

“The radiator stops the cold from coming into the room through the glass.” — Nothing called “cold” flows. Energy flows out through the glass; the radiator warms the air that the glass has cooled.
“Heat sinks from the radiator and warms the cold air near the floor.” — Warm air rises, because it is less dense. Heat does not sink or rise by itself.
“The cold glass draws warm air towards it, so the room heats evenly.” — Cold glass does not pull air. Air cooled by the glass becomes denser and sinks; the radiator then warms it.

9. TQ21 · Practice

Options as numbered on the pupil sheet: 1 — Convection; 2 — Thermal radiation; 3 — Conduction.

Model answer. The metal bottom and the metal handle stay in place and pass energy along: conduction. The water and the hot air above the fire carry energy by moving, and the rising hot air carries the sparks: convection. The warmth on her face reaches her sideways, without needing any material: thermal radiation, the subject of the next module.

A full answer includes

I matched the bottom of the pot to conduction.
I matched the handle to conduction.
I matched the water rising and sinking to convection.
I matched the sparks carried up to convection.
I matched the warmth on her face to thermal radiation.
I did not match the warmth on her face to convection.

Also accept

pot bottom, handle – conduction; water, sparks – convection; face – thermal radiation

Watch for

“The warmth on her face comes from convection.” — Hot air rises above the fire. The warmth reaching her face sideways is thermal radiation.
“The handle gets warm by convection.” — The metal handle cannot flow: energy is passed along it by conduction.
“The sparks are carried up by thermal radiation.” — Something is moving and carrying them: the rising hot air. That is convection.

10. TQ22 · Practice

Model answer. (a) The air next to the freezer walls is cooled, so it contracts and becomes denser. (b) The cold, dense air sinks and stays in the bottom of the chest. The warmer room air is less dense, so it stays on top, and no convection current carries the cold air up and out. (c) The cold, dense air pours out at the bottom of the open door and sinks to the floor, and warmer room air flows in at the top to take its place, so the freezer warms up much faster.

A full answer includes

(a) I said the air becomes denser.
(b) I said the cold, dense air sinks and stays in the bottom of the chest.
(b) I said the warmer room air above is less dense and stays on top.
(c) I predicted that the cold air pours out at the bottom of the open door.
(c) I said warmer room air flows in at the top to take its place.
I did not write that cold rises out of the freezer.

Also accept

(a) denser (b) cold dense air sinks and stays in the chest (c) cold air flows out at the bottom; warm air comes in at the top

Watch for

“Cold rises out of an open freezer.” — Cold air is denser, so it sinks. Nothing called “cold” flows.
“The cold air stays in the upright freezer as well.” — With the door open, nothing holds the cold, dense air in at the side: it flows out at the bottom.

11. TQ23 · Practice

Model answer. (a) Near the ground, the air moves from the park towards the car park. (b) The air above the hot tarmac is warmed, expands, becomes less dense and rises. Cooler, denser air from over the grass flows in along the ground to take its place. High up, the air flows back and sinks over the park: a convection current.

A full answer includes

(a) I said the air near the ground moves from the park to the car park.
(b) I said the air over the hot tarmac is warmed and becomes less dense.
(b) I said this air rises.
(b) I said cooler, denser air from the park flows in to take its place.
I did not say the warm air spreads out from the car park along the ground.

Also accept

(a) from the park to the car park (b) warm air over the tarmac rises; cooler air from the park flows in

Watch for

“The air moves from the car park to the park, because warm air spreads out.” — The warm air over the tarmac rises; the cooler air from over the park flows in along the ground to replace it.

12. TT2 · Apply

Model answer. (a) The water next to the heater is warmed, expands, becomes less dense and rises. Cooler, denser water sinks to the bottom to be heated in turn, so a convection current carries energy through all the water. (b) The hottest water is the least dense, so it collects at the top of the tank, where the pipe takes it off. (c) With the heater near the top, the warm water would stay at the top. No convection current would reach the water below the heater, and water is a bad conductor, so the bottom of the tank would stay cold.

A full answer includes

(a) I said the water by the heater expands.
(a) I said its density falls.
(a) I said the warmed water rises.
(a) I said cooler, denser water sinks to the heater.
(a) I named this a convection current.
(b) I said the hottest water is the least dense.
(b) I said it collects at the top of the tank.
(c) I said only the water above the heater would get hot.
(c) I said the warm water would stay at the top, so no current reaches the water below.
(c) I said water is a bad conductor, so little energy is conducted down.
I did not write that the heat would sink through the water.

Also accept

(a) a convection current from the bottom (b) the hottest water collects at the top (c) only the top would heat

Watch for

“A heater at the top would work just as well, because the heat would sink through the water.” — Warm water floats up, and water conducts badly, so the water below a high heater stays cold.