Heat transfer

Infrared Radiation is the emission of electromagnetic waves 

• All objects are continually emitting and absorbing infrared radiation
• Infrared radiation is emitted from the surface of an object
• An object that is hotter than it's surroundings emits more radiation than it absorbs
• An object that is cooler than it's surroundings absorbs more radiation than it emits
• The hotter an object is the more radiation it radiates in a given time

Radiation depends on surface colour and texture

• Dark matt surfaces absorb infrared radiation much better than light, shiny surfaces
• Dark matt surfaces also emit much more radiation than light shiny surfaces
• Light shiny surfaces reflect a lot of the infrared falling on them

Kinetic theory explains the three states of matter

SOLIDS - strong forces of attraction hold the particles close together in a fixed regular arrangement. The particles don't have much energy so they can only vibrate about their fixed positions

LIQUIDS - There are weaker forces of attraction between the particles. The particles are close together, but can move past each other and form irregular arrangements. They have more energy than the particles in a solid

GASES - There are almost no forces of attraction between the particles. The particles have more energy than those of liquids and solids. They are free to move and travel in random directions and at high speeds

Conduction of heat in solids

• Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles
• Usually conduction is faster in denser solids, because the particles are closer together and so will collide more often and pass energy between them. Materials that have larger spaces between their particles conduct heat energy much more slowly - these materials are insulators

Why are metals the best conductors of heat energy?

• 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
• Because the electrons can move freely, this is a much faster way of transferring the energy through the metal than slowly passing it through neighbouring atoms
• This is why heat energy travels so fast through metals
• Conduction is more efficient through a short fat rod than through a long thin rod because the electrons have a shorter distance to transfer the energy

How does convection occur?

How does convection occur?

• Heat is tranfserred from the heat source to the water air by conduction
• The air/water particles near the heat sorurce get more energy so they start moving around faster
• There is more distance between the particles, causing the air/water to expand and become less dense
• The lower density means that the hotter air/water will rise above the cold, denser air/water (or the colder denser air/water falls and pushes the hotter, less dense air/water up)
• The cold air/water is then heated and rises - repeating the cycle
• You end up with convection currents going up round and down, moving the heat through the water

 CONVECTION CURRENTS ARE ALL ABOUT CHANGES IN DENSITY

Changes between states of matter

Changing between states of matter

Condensation is when Gas turns into Liquid

• When a gas cools, the particles in the gas slow down and lose energy
• The attractive forces between the particles pull them closer together
• If the gas particles get close enough together and the temperature is cold enough then condensation happens and the gas becomes a liquid
• Water vapour in the air condenses when it touches a cold surface eg bathroom mirrors

Evaporation is when Liquid turns into Gas

• Evaporation is when particles escape from a liquid
• Particles near the surface of a liquid can escape and become gas particles if they are travelling in the right direction and have enough energy to break free from the attractive forces of the other particles in the liquid
• The particles with the most energy are most likely to evaporate from the liquid
• When the particles with the most energy evaporate, the average energy of the particles left behind decreases
• This means the temperature of the liquid left behind falls - the liquid cools

The speed of evaporation and condensation can vary

The rate (speed) of evaporation will be faster if the....

• temperature of the liquid is higher
• density of the liquid is lower
• surface area of the liquid is larger - more of the particles will be closer to the surface of the liquid so more can escape
• airflow over the liquid is greater - the air around the liquid will change more often, so there will be less liquid in the air, making it easier for evaporation to happen

The rate (speed) of condensation will be faster if the....

• temperature of the surface the gas touches is lower
• surface area of the surface the gas touches is larger

The rate at which an object transfers heat energy depends on....

1. The surface area and volume

• Heat is radiated from the surface of an object
• The bigger the surface area, the more infrared waves can be emitted from the surface (or absorbed by the surface), speeding up heat transfer
• Radiators have large surface areas to increase the amount of heat they transfer
• Car and motorbike engines have fins to increase the surface area so heat is radiated away quicker, cooling the engine down quicker
• Heat sinks are devices designed to transfer heat away from objects they are in contact with eg computer parts. They also have fins so they can radiate heat as quickly as possible
• If two objects with the same temperature have the same surface area but different volumes, the object with the smaller volume will cool more quickly

The rate at which an object transfers heat energy depends on....

2. The colour

• Dark, matt surfaces emit and absorb radiation better than bright, shiny surfaces

3. The material of the object and what it's touching

• Objects made from conductors will transfer heat more quickly than objects made from insulators
• Heat will be conducted away much faster from an object in contact with a conductor than an insulator
• The bigger the temperature difference, the faster heat is transferred between the object and what it is touching

Vacuum flasks are designed to reduce heat transfer

Vacuum flasks are designed to reduce heat transfer

• The cap is made of plastic and filled with cork or foam to reduce any conduction through it
• The bottle is held in place using insulating foam. This reduces conduction to or from the outer glass bottle
• The case is make of plastic (a good insulator). This reduces heat transfer by conduction
• The glass bottle has two silver shiny walls to reduce heat loss by radiation
• The glass bottle has a vacuum between the two walls. This stops all conduction and convection from the liquid inside
• The air between the plastic casing and the glass bottle is a good insulator so it reduces heat transfer by conduction

Humans and animals have ways of controlling heat transfer

• In the cold, the haires on your skin stand up and trap a thicker layer of insulating air around your body. This reduces the amount of heat you lose by conduction and convection
• Coats and duvets also keep us warm by trapping an insulating laye
• Generally, animals in warm climates have larger ears than those in cold climates to help control heat transfer

Humans and animals have ways of controlling heat transfer

• Arctic foxes have small ears, with a small surface area to reduce heat loss byradiation
• Desert foxes on the other hand have huge ears with a large surface area so they can lose heat by radiation and keep cool

Insulation reduces heat loss from homes

Insulation reduces heat loss from homes

You need to know which types of heat transfer are reduced by different types of home insulation

• LOFT INSULATION - a thick layer of fibreglass wool laid out across the whole loft floor reduces conduction and radiation into the roof space from the ceiling

• CAVITY WALL INSULATION - foam squirted into the gap between the bricks reduces convection by trapping pockets of air, conduction because the trapped air and the foam are insulators and radiation across the gap

• HOT WATER TANK JACKET - insulating materials eg fibreglass and wool reduce conduction and radiation

Payback time calculations

Effectiveness and cost effectiveness are not the same thing

• The most effective methods of insulation are the ones that give you the biggest annual saving ie they save you the most money each year on your heating bills
• The money you have saved on heating bills will equal the initial cost of putting in the insulation - PAYBACK TIME
• The most cost effective methods tend to be the cheapest
• They are cost effective because they have a short payback time - the money you save covers the amount you paid really quick
• Payback time can be calculated using this equation:

                                                Payback time  =    initial cost                                                                                                                            annual saving

eg If it costs £200 to install some loft insulation, with an annual payback of £50, how long is the payback time?      Payback time  =  200  =  4 years

                                                      50

U - values

U - values show how fast heat can transfer through a material

• Heat transfers faster through materials with higher  U - values than through materials with low U - values

• So the better the insulator, the lower the U - value

• For example, the U - value of a duvet is about 0.75 W/m2K, whereas the U - value of loft insulation material is around 0.15 W/m2K

Specific Heat Capacity

Specific heat capacity tells you how much heat a solid, liquid or gas can store

• It takes more heat energy to increase the temperature of some materials than others
• Materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. They can store lots of heat
• The measure of how much energy a substance (material) can store is call its specific heat capacity
• Specific heat capacity is defined as the amount of energy needed to raise the temperature of 1kg of a substance by 1 degree C

• The formula for specific heat capacity is:

                                E   =   M  x  C  x  

where: E = Energy transferred (J), M = Mass (kg), C = specific heat capacity (J/kg C),      = temperature change   C

Specific heat capacity formula triangle

                    E

           M      C

Specific heat capacity

Heaters have high specific heat capacities to store lots of energy

• The materials used in heaters usually have high specific heat capacities

• Water has a really high specific heat capacity. It is also a liquid, so it can easily be pumped around in pipes - ideal for central heating systems in buildings

• Electric storage heaters store heat energy at night and then release it during the day. They store the heat using concrete or bricks, which have a high specific heat capacity

• Some heaters are filled with oil. Oil has a lower specific heat capacity than water - this means that they are often not as good as water heating systems