Physics P1 Energy Transfer by Heating

• Infrared radiation is energy transfer by electromagnetic waves
• All objects emit infrared radiation
• The hotter an object is, the more infrared radiation it emits in a given time
• If the temperature difference between the hot object and its surroundings is reduced, the rate of energy transfer decreases
• The transfer of energy by infrared radiation does not involve particles
• Infrared radiation can travel through a vacuum i.e. space
• This is how heat reaches us from the Sun
• A vacuum is a sealed compartment with no particles

• Dark, matt surfaces emit and absorb more infrared radiation than light, shiny surfaces
• Light, shiny surfaces reflect more infrared radiation than dark, matt surfaces

• Flow, shape, volume and density are the properties used to describe each state of matter
• The particles in a solid are held next to each other in fixed positions
• A solid has a fixed shape and volume, and cannot flow
• The particles in a liquid move about at random but remain in contact with one another
• A liquid has a fixed volume but no fixed shape, and can flow easily
• The particles in a gas move about randomly and are much further apart than particles in a solid or liquid
• A gas can flow, and does not have a fixed volume or shape
• The density of a gas is much less than that of a solid or liquid
• In general, the particles of a gas have more energy than that of a solid or liquid
• The particles in a liquid have more energy than those in a solid

• Metals are the best conductors of energy 
• Materials such as wool and fibreglass are the best insulators
• This is because they contain air trapped between the fibres, and air is a poor conductor of energy
• Conduction of energy in a metal is due mainly to free electrons transferring energy inside the metal
• When a metal 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

• Convection is the circulation of a fluid caused by heating
• When a fluid is heated, it expands
• The hot fluid becomes less dense and rises
• The warm fluid is replaces by cooler, denser fluid
• The resulting convection current transfers energy throughout the fluid

• Cooling by evaporation of a liquid is due to the faster moving molecules escaping from the liquid
• The average kinetic energy of the remaining molecules is reduced because the most energetic molecules have left

Evaporation can be increased by:

• Increasing the liquid's temperature
• Increasing the surface area of the liquid
• Creating a draught of air across the liquid's surface

Condensation on a surface can be increased by:

• Increasing the area of the surface
• Reducing the temperature of the surface

The rate of energy transfer to or from an object depends on:

• the shape, size and type
• the materials the object is in contact with
• the temperature difference between the object and its surroundings

To maximise the rate of energy transfer to keep things cool, we use materials that:

• are good conductors 
• are painted dark and matt
• have the air flow around them maximised

To minimise the rate of energy transfer to keep things warm, we use materials that:

• are good insulators
• are light and shiny
• prevent conduction, convection and radiation

• The specific heat capacity of an object is the energy need to raise 1kg of a substance by 1C
• The greater the mass of an object, the more slowly its temperature increases when it is heated

The rate of temperature change of a substance when heated depends on:

• the energy supplied to it
• its mass
• its specific heat capacity

Energy transfer from our homes can be reduced by fitting:

• loft insulation
• cavity wall insulation
• double glazing
• draught proofing
• aluminium foil behind radiators

U-Values tell us how much energy per second passes through different materials

The lower the U-Value, the better an insulator it is