Preventing Unwanted Energy Transfers

Reducing transfers that are not useful

Whether an energy transfer is useful depends on what a system is meant to do. Heating food is useful when cooking, but transferring energy from hot food to the surrounding air is unwanted if we want the food to stay hot.

Unwanted transfers do not destroy energy. Instead, energy often spreads into the thermal stores of objects and their surroundings, where it is less useful. This is called energy dissipation. We can reduce these transfers by changing how a machine operates or by choosing suitable insulating materials.

Lubrication reduces frictional heating

Imagine pushing a supermarket trolley with stiff wheels. Friction in the moving parts opposes their movement, so you must do work against the frictional forces. Energy is transferred to the thermal stores of the parts, raising their temperature, and then spreads into the surroundings. That energy is not useful for moving the trolley.

Lubrication means applying a substance such as oil between moving surfaces. The lubricant reduces friction, so less work is done against friction as the parts move. Less energy is therefore dissipated by heating.

The same principle is used in bicycle chains and moving machine parts. Lubrication does not remove every unwanted transfer, but it reduces the energy needed to keep the parts moving.

Insulation slows transfer from warmer to cooler places

Energy transfers by heating from a warmer object or region to a cooler one. Thermal insulation reduces the rate of this transfer; it does not supply energy or keep an object at the same temperature indefinitely.

One way energy is transferred is conduction: energy passes through a material from its hotter region to its colder region, without the material moving as a whole. A material’s thermal conductivity describes how readily this happens. Under the same conditions, a material with higher thermal conductivity transfers energy by conduction at a higher rate. Good insulating materials have low thermal conductivity.

Wool clothing provides an everyday example. Wool and the air trapped between its fibres are poor thermal conductors, so they slow energy transfer from your warm body to colder surroundings.

Insulation can also keep something cold. Wrapping slows energy transfer out of hot fish and chips, but it slows energy transfer into ice cream from warmer surroundings. The direction differs; the insulating material reduces the rate in both cases.

How walls affect a building’s cooling

When a building is warmer than the outside, energy transfers through its walls to the colder surroundings. Two important factors affect this transfer:

• Wall thickness: for the same material and other conditions, a thicker wall reduces the rate of energy transfer by conduction. The building cools more slowly.
• Thermal conductivity: for the same thickness and other conditions, a wall made from a material with lower thermal conductivity reduces the rate of transfer. Again, the building cools more slowly.

Thickness and material must be considered together. A thick wall and a thin wall cannot be compared using thickness alone if they are made from different materials.

Applying the science in building design

Cavity wall insulation adds insulating material within the gap between wall layers. Loft insulation reduces energy transfer through the roof. Both use low-thermal-conductivity materials to slow unwanted transfer from the warm interior to the colder outside.

A well-insulated building needs less energy supplied by heating to maintain its indoor temperature. Choosing suitable insulation and an appropriate thickness therefore helps engineers reduce energy consumption and the resources needed to heat buildings.

Lubrication

• Friction opposes movement.
• Work done against friction raises the temperature of parts and dissipates useful energy.
• Oil reduces friction, so less energy is transferred by heating.

Thermal insulation

• Reduces the rate of energy transfer from warmer to cooler regions.
• Higher thermal conductivity → faster transfer by conduction.
• Good insulation has low thermal conductivity.
• Can keep hot objects hot or cold objects cold for longer.