Reducing Unwanted Energy Transfers

Why unwanted transfers matter

Energy cannot be destroyed, but it can be transferred away from where it is useful. For example, a machine is intended to transfer energy to moving parts. Friction may instead increase the thermal energy stores of those parts and their surroundings. The energy has not disappeared: it has been dissipated and is now less useful.

Reducing an unwanted transfer does not remove energy or stop transfer completely. It reduces the rate at which energy is transferred along an unwanted pathway. This means more of the supplied energy remains available for the intended purpose, or less energy must be supplied to maintain the desired result.

Reducing friction with lubrication

When two surfaces slide past one another, friction acts against their motion. A force must move the surfaces through a distance against this friction, so work is done. Energy is transferred mechanically from the moving system and ultimately increases the thermal energy stores of the surfaces and surroundings.

A lubricant such as oil forms a layer between the surfaces. This reduces direct contact between their irregular surfaces and therefore reduces friction. Less work is then done against friction, so less energy is dissipated by heating. Oil on bicycle gears and lubricating oil in an engine are practical examples.

The complete explanation is therefore:

lubrication → less friction → less work done against friction → a smaller unwanted transfer to thermal energy stores.

Reducing transfer by heating

A warm building has a greater temperature than its cooler surroundings. Energy is transferred from the warmer interior towards the cooler exterior, causing the building's thermal energy store and temperature to decrease unless heating replaces the energy being transferred away.

Within a solid wall, energy is transferred by conduction. Particles in a warmer region have more energy in their vibrations. Interactions with neighbouring particles pass energy through the material towards the cooler region.

Thermal insulation reduces this unwanted transfer. For walls, the two assessed properties are their thickness and the thermal conductivity of the material.

Wall thickness

For walls made from the same material under the same conditions, a thicker wall produces a lower rate of energy transfer by conduction. Energy must be transferred across a greater depth of material, so the building cools more slowly.

This comparison must be made fairly: the wall material, wall area and temperature difference between inside and outside should be the same. Increasing thickness reduces the rate of cooling; it does not mean the building can never cool.

Thermal conductivity

Different materials conduct energy at different rates. The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it. A wall material with lower thermal conductivity therefore reduces the rate at which energy leaves the building, so the building cools more slowly.

AQA does not require you to recall a formal definition of thermal conductivity. What matters here is being able to use the relationship correctly:

• higher thermal conductivity → faster energy transfer by conduction → faster cooling;
• lower thermal conductivity → slower energy transfer by conduction → slower cooling.

Combining the two factors

A thick wall made from a material with low thermal conductivity gives greater resistance to the unwanted transfer than a thin wall made from a highly conducting material. This reduces the energy transferred through the walls each second. Consequently, less energy from a heating system is needed to maintain the building at a chosen temperature.

Thickness and thermal conductivity are separate factors. A thick wall is not necessarily made from a low-conductivity material, and a low-conductivity wall is not necessarily thick. In a scientific comparison, change one factor while keeping the other constant.

Core idea

Unwanted transfers dissipate energy into less useful stores, often the thermal energy stores of objects and their surroundings.

Lubrication

Lubricant added → friction reduced → less work done against friction → less energy dissipated by heating.

Building walls

For the same wall area and inside–outside temperature difference:

• greater wall thickness → lower transfer rate by conduction → slower cooling;
• higher thermal conductivity → higher transfer rate by conduction → faster cooling;
• lower thermal conductivity → lower transfer rate → slower cooling.

Thermal insulation reduces the rate of transfer; it does not stop transfer completely. AQA does not require a formal definition of thermal conductivity, but you must use these relationships correctly.