4PH1

Energy Stores & Transfers

Energy Resources & Energy 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 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

Cavity wall insulation: a filled cavity traps the air and gives less heat loss, while an unfilled cavity lets warm air circulate and gives more heat loss for the same inside and outside temperatures
Source: Reducing Energy Loss by Save My Exams
- 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 exam question

Explaining how a design reduces energy transfer

Question: Explain how a design feature (air-filled layers, a lid, a shiny or white surface) reduces energy transfer by conduction, convection or radiation, or suggest changes to cut losses (2 marks per pathway).

Asked in 4 of the 24 papers. One physics reason per mark. Conduction: the material is a poor conductor; air is a gas with particles far apart that rarely collide; a thicker layer conducts more slowly. Convection: the air in the pockets or under the lid cannot move, so no convection current forms. Radiation: white or silver surfaces are poor emitters of infrared.

Explain the pathway named: in a conduction part "trapped air" alone was ignored by one scheme (it is the convection answer): say the air is a poor conductor. A lid on a cooling experiment cuts convection and evaporation, so less energy leaves: higher final temperature, longer cooling time. Improvements score as named changes: insulate the beaker, make the outside shiny.

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 ; 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