Reducing unwanted transfers keeps energy useful
Definition
Lubrication
Lubrication is the use of a substance, such as oil or grease, between moving surfaces to reduce friction and the unwanted transfer of energy by heating.
Definition
Thermal insulation
Thermal insulation is the use of materials or structures that reduce the rate of energy transfer by heating.
- Reducing unwanted transfers increases the fraction of the input transferred usefully without creating energy.
- The correct method depends on whether the unwanted pathway is friction, conduction, convection or infrared radiation.
Lubrication reduces heating by friction
- Oil or grease separates moving surfaces so their microscopic irregularities make less direct contact.
- The frictional force falls, so less work is done against friction and less energy enters thermal stores.
- Bearings, gears and engine parts therefore run cooler and more input energy remains available for useful motion.
Key Idea
Lubrication reduces dissipation by reducing friction between moving surfaces.
Building insulation slows cooling
- Loft insulation traps air and reduces conduction through the roof.
- Cavity-wall insulation traps air in small pockets, reducing conduction and preventing large convection currents.
- Double glazing uses a trapped gas layer to reduce conduction and convection through windows.
- Draught proofing reduces convection by stopping warm air escaping through gaps.
- Reflective foil reduces net infrared transfer into an outside wall because its shiny surface has low emissivity.
Thickness and conductivity control the rate
Definition
Thermal conductivity
Thermal conductivity is a measure of how readily a material transfers energy by conduction.
- Lower thermal conductivity means less energy is transferred each second for the same area, thickness and temperature difference.
- Greater wall thickness increases the conduction distance, so the rate of energy transfer and rate of cooling decrease.
- A thick layer of low-conductivity material therefore slows cooling more than a thin layer of a good conductor.
Example
- Two identical houses have the same wall area and temperature difference.
- The house with a thicker layer of low-conductivity foam cools more slowly because the conduction path is longer and the foam transfers energy less readily.
Insulation choices have costs and limits
- Payback time compares installation cost with the annual saving:
- payback time=installation costsaving per year\text{payback time}=\frac{\text{installation cost}}{\text{saving per year}}payback time=saving per yearinstallation cost
- Thicker insulation usually gives progressively smaller extra savings, so cost and available space affect the choice.
Exam technique
- For a wall question, link greater thickness to a longer conduction path and lower thermal conductivity to less energy transferred each second.
- Use rate of energy transfer or rate of cooling, not only that insulation keeps heat in.
Keep the mechanisms distinct
Common Mistake
- Insulation reduces energy transfer; it does not stop it completely.
- Trapped air insulates only when it cannot circulate freely and form convection currents.
- Thermal conductivity is a material property, not a temperature.
- Lubrication applies to moving surfaces, while insulation applies where there is a temperature difference.
- A valid improvement must act on the named unwanted transfer pathway.
Self review
- How does lubrication reduce unwanted transfers?
- What does thermal conductivity measure?
- How does increasing wall thickness affect cooling rate?
- Give two insulation methods and the transfer each reduces.
