Energy Transfers » Reducing Unwanted Energy Transfers

Why reduce unwanted energy transfers?

A warm house in winter is hotter than the air outside, so energy is always being transferred out of it by heating. The house then needs more fuel or electricity to stay warm.

We cannot stop this transfer, but we can slow it down. To explain how any method works, an examiner wants two things: which transfer it reduces (conduction, convection or radiation) and why.

Key terms:

• Thermal insulation: a layer of material that is a poor conductor, put in place to slow down the transfer of energy by heating.
• Cavity wall: an outside wall built as two layers of brick with a gap (the cavity) between them.
• Double glazing: a window made of two panes of glass with a narrow sealed gap between them.
• Draught: a flow of cold air into a room through a gap, such as under a door.
• Vacuum flask: a container with a double wall and a vacuum between the walls, used to keep drinks hot or cold.
• Lubrication: putting a substance such as oil or grease (a lubricant) between moving parts so that they slide over each other more easily.

Insulating a home

Remember, air is a very poor conductor. But it only insulates well when it is trapped, because air that is free to move carries energy away by convection. Most home insulation works by holding air still in small pockets.

Method	Mainly reduces	Why it works
Loft insulation	Conduction and convection	A thick layer of fibres on the loft floor traps air, so less energy is conducted through the ceiling. The air cannot move, so no convection currents form in the layer.
Cavity wall insulation	Conduction and convection	Foam or fibres in the cavity trap air in tiny pockets, so convection currents cannot form. The foam and trapped air are poor conductors.
Double glazing	Conduction	The gas trapped between the two panes is a much poorer conductor than glass. The gap is narrow, so the gas cannot circulate easily.
Draught excluders	Convection	Strips around doors and windows close the gaps. Warm air cannot flow out and cold air cannot flow in.
Carpets	Conduction	Carpet and underlay contain trapped air, so less energy is conducted from the room into the floor.
Curtains	Conduction and convection	Closed curtains trap a layer of air against the window. They also stop air in the room flowing over the cold glass.
Hot-water tank jacket	Conduction (and so convection)	A thick jacket of foam or fibres traps air around the tank, so less energy is conducted from the hot metal. The outside of the jacket is cooler than bare metal, so it warms the air around it less.
Reflective foil behind radiators	Radiation	A shiny surface is a good reflector. It reflects infrared back into the room, so less is absorbed by the wall.

The vacuum flask

A vacuum flask reduces all three transfers at once. To draw one:

1. Draw a tall bottle shape with a narrow neck. This is the inner glass wall, which holds the drink.
2. Draw a second bottle shape closely around the first, joined to it only at the top of the neck. This is the outer glass wall.
3. Label the thin space between the two walls vacuum.
4. Label the surfaces of the glass walls that face the vacuum silvered surfaces.
5. Draw a plug in the neck and label it stopper. A plastic case goes around the outside to protect the glass.

A flask slows the transfer of energy in either direction. So it keeps soup hot on a cold day and iced water cold on a hot day.

Layers of clothing

Your body is usually warmer than the air around you. Clothes keep you warm by trapping air, both in the fibres of the cloth and between the layers. This trapped air reduces conduction from your skin, and because it cannot move away it reduces convection too.

This is why several thin layers are often warmer than one thick layer: each extra layer traps another layer of air.

Lubrication

In a machine, moving parts rub against each other. Friction transfers energy mechanically to the thermal stores of the parts, so they heat up. This energy is wasted, and the machine is less efficient.

A lubricant, such as oil or grease, forms a thin layer between the surfaces so they slide more easily. There is less friction, so less energy is wasted heating the parts. Examples are oil on a bicycle chain, oil in a car engine and a drop of oil on a squeaky door hinge.

Testing insulation

You can compare how fast insulated and uninsulated beakers of hot water cool:

1. Take two identical beakers. Wrap one in a layer of insulation, such as bubble wrap, and leave the other bare.
2. Pour the same volume of hot water into each beaker.
3. Cover each beaker with a cardboard lid that has a thermometer through it.
4. Start timing each beaker when its thermometer reads 80 °C. Use a separate stopwatch for each beaker.
5. Record the temperature of each beaker every 2 minutes for 10 minutes.

Time (minutes)	0	2	4	6	8	10
Bare beaker (°C)	80	73	67	62	58	55
Insulated beaker (°C)	80	76	73	70	68	66

The insulation (present or not) is the independent variable and the temperature of the water is the dependent variable. To make the test fair, keep the same: the size and material of the beakers, the volume of water, the starting temperature, the lids and the time. To compare different insulating materials instead, also keep the thickness of each layer the same.

Exam-style question

A family wants to reduce unwanted energy transfers from their home.

(a) They put a thick layer of mineral wool on the floor of the loft. Explain how this reduces energy transfer through the ceiling. [3 marks]

(b) They take hot soup on a winter walk in a vacuum flask.

(i) Explain how the vacuum between the walls of the flask helps to keep the soup hot. [2 marks]

(ii) Explain why the walls of the flask are silvered. [2 marks]

(c) A student tests two insulating materials. She wraps one beaker in felt and an identical beaker in foam, then pours hot water into each. State two variables she should control. [2 marks]

(d) Explain why oiling the chain of a bicycle reduces unwanted energy transfer. [2 marks]