Energy Stores & Transfers

Why bother?

• Real appliances waste energy in three common ways: conduction through casings, convection currents leaving hot regions, and radiation from hot surfaces. The energy is conserved, but the wasted share costs money and adds to greenhouse-gas emissions whenever the source is fossil-fuelled
• Reducing the wasted output raises the efficiency of the system: more of the same input ends up in the useful store

Reducing conduction

• Use materials with low thermal conductivity (good thermal insulators) wherever heat needs to stay in (or out)
• Make the insulating layer thicker, because heat takes longer to conduct through a thicker barrier
• Use materials that trap pockets of air, since air is itself a very poor conductor and pockets stop convection currents from forming inside the insulator
• Everyday examples:
  • double glazing sandwiches a thin layer of air or argon between two glass panes
  • loft insulation made of fibreglass mats placed between the joists
  • woolly jumpers and quilted jackets trap air between the fibres

Reducing convection

• Stop the fluid (usually air) from moving and you stop convection currents from forming
• Practical methods:
  • Cavity wall insulation: foam blown into the gap between the inner and outer walls of a house traps the air that would otherwise circulate as a current
  • Curtains and draught excluders cut down warm-air loss from windows and gaps around doors
  • A lid on a hot drink prevents warm air rising from the surface and being replaced by cool air

Reducing radiation

• Use surfaces that emit infrared poorly: polished, shiny silver
• Practical methods:
  • The inner walls of a vacuum flask are silvered on both sides; the inner silver reflects infrared back inside, the outer silver reflects infrared from outside back out
  • Foil-backed loft insulation reflects infrared from a warm room back downwards
  • Emergency space blankets wrap a person in shiny silver to retain body heat in a cold environment

A vacuum flask in one sentence per pathway

• A vacuum flask combines all three tricks:
  • the double-walled vacuum between the inner and outer flask stops both conduction and convection (no medium for either)
  • the silvered surfaces of the inner walls stop radiation
  • the insulating stopper at the top stops conduction and convection through what would otherwise be an open neck

Common-language traps to avoid

• "Heat rises": what actually rises is hot fluid that has become less dense. Heat itself is not a substance with a direction
• "Shiny things reflect heat": shiny surfaces reflect infrared radiation; they cannot reflect conduction or convection because neither bounces
• "Black things absorb heat": black surfaces absorb infrared radiation; they do not absorb conduction or convection separately
• A good exam answer almost always mentions more than one of conduction, convection and radiation