Physics P1

HEAT is TRANSFERRED in THREE DIFFERENT WAYS

Heat energy can be transferred by radiation, coduction or convection

• Heat radiation is the transfer of heat energy by infrared radiation (IR)
• Conduction & convection involve the transfer of energy by particles
• Conduction is the main form of heat transfer in solids
• Convection is the main form of heat transfer in liquids & gases

• Infrared radiation can be emitted by solids, liquids & gases
• All objects are always absorbing and emitting infrared radiation
• The bigger the temp. difference between a body and its surroundings, the faster energy is transferred by heating

Infrared Radiation -

• An object that's hotter than its surroundings emits more radiation than it absorbs (as it cools)
• An object that's cooler than its surroundings absorbs more radiation than it emits (as it warms)

The hotter an obect is, the more radiation it radiates in a given time

• Dark, Matt Surfaces are good at absorbing and emitting infrared radiation
• Light, Shiny Surfaces are good at reflecting infrared radiation (eg. gloss white or silver)
  

Vacuum flasks have silver inner surfaces to reflect the heat back in or out (depending on whether it is for storing hot or cold liquid)

Solar hot water panels contain water pipes under a black surface > radiation from the sun is absorbed by the black surface to heat the water in the pipes > this water can be used for washing or pumped to radiators

Three States of Matter - when heating a substance > the particles gain kinetic energy > vibrate & move faster

Solids - strong forces of attraction > to hold particles close together in a fixed, regular arrangement. The particles don't have much energy > they can only vibrate around their fixed positions

Liquids - weaker forces of attraction > particles are close together, but can move past each other, and form irregular arrangements. The particles have more energy than in solids > they move in random directions at low speeds

Gases - virtually no forces of attraction > particles have more energy than in solids and liquids > they are free to move, and travel in random directions, at high speeds

Conduction of Heat - occurs Mainly in Solids 

CONDUCTION of HEAT ENERGY is the process where VIBRATING PARTICLES pass on their EXTRA KINETIC ENERGY to NEIGHBOURING PARTICLES

This process continues throughout the solid > gradually the extra kinetic energy is passed throught the solid > causing a rise in temp. at the other side > hence an increase in the heat radiating from its surface

Conduction - usually faster in denser solids > particles closer together, so collide more often and pass energy between them.

Insulators - materials with larger spaces between the particles > conduct heat much slower

Metals are Good Conductors because of their Free Electrons

(conduction is more efficient in a short fat rod, than a long thin rod > less distance to travel)

Metals conduct so well because the electrons are free to move inside the metal

At the hot end, the electrons move faster and collide with other free electrons > transferring energy > these other electrons pass their extra energy to others, and so on

Electrons can move freely > faster way of transferring energy > faster than passing kinetic energy between jostling neighbouring atoms

This is why HEAT energy travels so FAST through METALS

Convection occurs in Liquids & Gases only (particles cant move in solids)

CONVECTION occurs when the more energetic particles MOVE from the HOTTER REGION to the COOLER REGION > AND TAKE THEIR HEAT ENERGY WITH THEM

eg. Immersion Heater (circulating convection currents)

• Heat energy's transferred from the heater coils to the water by conduction (particle collisions)
• The particles near the coils get more energy > so start moving faster
• Therefore there's more distance between them > becomes less dense
  
• This reduction in density means that the hotter water rises above the denser, cooler water
• Fast-moving particles collide with slow-moving particles and transfer heat
• As heat is transferred, the water cools and becomes more dense
• Denser water sinks again

CONVECTION CURRENTS are all about CHANGES IN DENSITY

Radiator example: Air is heated, less dense, rises > warm air displaces cooler air > cool denser air falls > cool air flows to fill the gap left by the rising heated air