P1.1 - The transfer of energy by heating processes and the factors that affect the rate at which that energy is transferred

• All objects emit and absorb infrared radiation
• The hotter an object is the more infrared radiation it radiates in a given time
• Dark, matt surfaces are good absorbers and good emitters of infrared radiation
• Light, shiny surfaces are poor absorbers and poor emitters but good reflectors of infrared radiation

• Everything around us is made of matter in one of three states - solid, liquid or gas
• The kinetic theory of matter - solids liquids and gases consist of particles; when the temperature of the substance is increased, the particles move faster
• The particles in a solid are held next to each other in fixed positions and vibrate abouth those positions so the solid keeps its own shape
• The particles in a liquid are in contact with each other but can move around at random so a liquid doesn't have its own shape and can flow
• The particles in a gas move about at random and are on average much farther apart from each other than in a liquid so the density of a gass is much less than that of a solid or liquid
• The particles in solids, liquids and gases have different amounts of energy
• In general, gas particles have more energy than liquid particles, which have more energy than solid particles

• Metals are the best conductors of energy as they contain lots of free electrons which move about at random inside the metal and hold the positive ions together. They collide with each other and with the positive ones.
• When a metal rod is heated at one end, the free electrons at the hot end gain kinetic energy and move faster. These electrons diffuse and collide with other free electrons and ions in the cooler parts of the metal and as a result, they transfer kinetic energy to these electrons and ions.
• In a non-metallic solid, all the electrons are held in the atoms so energy transfer only takes place because the atoms vibrate and shake each other. This is much less effective than energy transfer by free electrons therefore metals are better conductors than non-metals.

• Convection takes place only in fluids (liquids or gases) and is due to circulation (convection) currents within the fluid
• Convection currents transfer energy from the hotter to the cooler parts

The convection currents are caused because...

• Fluids rise when they are heated (hotter = less dense)
• Then they fall where they cool down (cooler = more dense)

• Evaporation is when a liquid turns into a gas
• Cooling by evaporation of a liquid is due to the faster-moving molecules escaping from the liquid
• Evaporation can be increased by increasing the surface area of the liquid; increasing the liquid's temperature; or creating a draught of air across the liquid's surface
• Condensation is when a gas turns into a liquid
• Condensation on a surface can be increased by increasing the surface area or reducing the temnperature of the surface

The rate at which an object transfers energy by heating depends on:

• Surface area and volume
• The material from which the object is made
• The nature of the surface with which the object is in contact

• The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating

• U-values measure how effective a mateial is as an insulator - they tell us how much energy per second passes through different materials
• The lower the U-value, the better the material as an insulator
• Energy transfer from our homes may be reduced by: loft insulation; double glazing; cavity wall insulation; draught proofing; aluminium foil behing radiators
• Solar panels may contain water that is heated by radiation from the Sun and this water may then be used to heat buildings or provide domestic hot water
• The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius: E = m x c x θ