Energy Stores & Transfers

The three modes of heat transfer

• Three distinct processes carry thermal energy from a hot region to a cold one:
  • Conduction: the main heat transfer in solids; energy passes from particle to particle along the material without the particles moving overall
  • Convection: the main heat transfer in fluids (liquids and gases); whole regions of hot fluid actually move from one place to another, carrying their energy with them
  • Radiation: heat transfer through infrared electromagnetic waves; the only one of the three that can carry energy through a vacuum, because no medium is needed
• A real-world example often uses all three at once. A hot mug of coffee: conducts into the table beneath it, sets up a convection current in the steam rising above the surface, and radiates infrared from the outside of the mug

Conduction in detail

• In any solid, the particles vibrate about fixed positions; the hotter the material, the larger the vibrations
• When one end of a solid is heated, the particles at that end vibrate more strongly. They collide with neighbouring particles, passing the extra vibrational energy along the line
• Metals are especially good thermal conductors because they have delocalised electrons that drift freely through the lattice:
  • the free electrons collide far more often with the vibrating ions than the ions collide with each other
  • they carry energy across the metal much faster than vibration alone
  • this is why a stainless-steel teaspoon left in a hot cup of tea quickly becomes too warm to hold, while a wooden stirrer of the same shape stays cool

Why insulators insulate

• A thermal insulator is a material in which conduction happens only slowly
• Common insulators include wood, plastic, rubber, glass, dry paper, and any material with lots of trapped air
• Trapped air is one of the best insulators:
  • the air molecules are far apart, so very few collisions pass energy along
  • if the air is trapped in pockets (within wool, fibreglass, foam, feathers), it cannot move as a convection current either, so the convection pathway is also shut down

Convection in detail

• When a region of a fluid is heated:
  • the particles in that region gain kinetic energy and push each other further apart
  • the fluid in that region expands and becomes less dense than the cooler fluid around it
  • the less-dense hot fluid rises, while the cooler, denser fluid sinks to take its place
• The rising and sinking sets up a closed loop of moving fluid called a convection current
• Convection currents drive everyday heating: radiators warm rooms by setting up a current of air around the room; the sun warms the atmosphere by setting up large convection cells; ocean currents move warm and cool water around the globe

Radiation in detail

• Every object at any temperature above absolute zero emits infrared (thermal) radiation; the hotter the object, the more it emits per second
• Thermal radiation is a transverse electromagnetic wave, so it carries energy through a vacuum (the way sunlight reaches Earth across 150 million km of empty space) as well as through transparent media
• The colour and finish of a surface strongly affects how well it emits and absorbs thermal radiation:

• Practical consequences:
  • solar panels and solar-heating absorbers are painted matt black to absorb as much sunlight as possible
  • emergency space blankets are made of shiny silver foil to reflect a person's body-heat radiation back inwards
  • houses in hot climates are often painted white to reduce both absorption from the sun and emission of heat back outwards
  • vacuum flasks have silvered inner walls to cut radiation losses (alongside the vacuum that blocks conduction and convection)