P1.1 Heat Transfer

Solids

• Tightly packed in a regular pattern
• Particles have low energy
• Their only movement is vibrating about their fixed positions

Liquids

• Particles are less tightly packed and can move around eachother
• As the particles move more they have more energy than the particles in the solid

Gas

• Particles are spread out with large spaces between them
• Particles can move in all directions
• So they have lots of energy

Slow Method - All solids

• Particles which are heated gain energy
• This means they vibrate more
• They pass on increased vibrations to their neighbouring particles

Fast Method - Metals only

• Metals contain free electrons  which can move freely throughout the metal
• When the metal is heated the free electrons gain energy
• They pass the energy on by colliding with another particle

Convection is a heat transfer that occurs in fluids. A fluid can be a gas or a liquid.

Explanation

• The particles in the part of the fluid which is heated gain energy and they move around more/vibrate more
• This means they spread out and the warm fluid becomes less dense
• The warmer fluid rises and colder, more dense fluid takes its place
• This process repeats forming a convection current

Convection cannot occur in solids because the particles are fixed and unable to move past each other. 

• Between the Earth and the Sun there is a vacuum
• In that vacuum there are no particles
• So heat transfer by Conduction or Convection is not possible
• Heat is instead transferred by radiation
• Radiation as an electromagnetic wave
• All objects give off (emit) radiation
• The hotter an object is the radiation it emits
• All objects take in (absorb) radiation

A dull, black surface is the best at absorbing radiation

A shiny, silver surface is the best at reflecting radiation

Evaporation

Evaporation is a process of when a liquid turns into a gas/vapour

Factors affecting the rate of evaporation

• Increasing the surface area of the liquid
• Increasing the temperature of the liquid
• Creating a draft across the surface of the liquid

Condensation

Condensation is the process of when a vapour/gas turns back into a liquid

Factors affecting the rate of condensation

• Increasing the surface area
• Reducing the surface temperature

The Vacuum Flask

The vacuum flask allows no heat to escape from or enter its contents. It has double walls with a vacuum between to prevent heat loss by conduction and convection. Radiation is prevented by silvering the walls. A vacuum is therefore equally efficient at keeping cold liquids cold and hot liquids hot.

The factors that affect the rate of energy transfer

• Materials the object is in contact with
• The objects shape
• The objects surface area

When we heat an object its temperature rise depends on 3 things:

1. The mass of the object - the less mass there is, the bigger the temperature rise

2. How much energy is supplied to the object - the more energy supplied, the bigger the temperature rise

3. The material the object is made from - All materials heat up at different rates depending on their specific heat capacity (SHC)

What is Specific Heat Capacity?

The SHC of a substance is the energy needed to raise the temperature of 1kg of the substance by 1℃. The units for SHC is J/kg℃ 

The formula used to calculate energy, mass SHC or temperature

E=m*c* θ             E=Energy (J)  M=Mass (kg)  C= SHC(J/kg℃)  θ=Temperature Change (℃)  

Energy lost from...

Roof... Method of Insulation - Fibreglass in loft: Fibreglass is a good insulator. Air between the fibres also reduces energy transfer by conduction and convection

Walls... Method of Insulation - Cavity Wall Insulation: Insulation pumped into space between two layer of brick. It's a better insulator than the air it replaces reducing heat loss by conduction. It traps air in small pockets reducing convection.

Windows... Method of Insulation - Double Glazing: Two glass panes with a vacuum between prevent heat loss by conduction and convection 

Floor... Method of Insulation - Carpets: Carpets made of an insulating material, preventing heat loss by conduction

Doors... Method of Insulation - Draught proof strips: Insulators placed around doors to reduce heat loss by conduction.

This is the time taken to get back the money spent on Insulation through savings on heating bills.

Example

Double Glazing costs £1000 and the annual saving is £50. What is the payback time?

Double Glazing =1000/50

Double Glazing =20 years

All insulators are given a U-Value. The U-Value is the amount of heat that passes through 1m² of that material. The lower the U-Value, the better the material is at insulating.

• The metal plate is matte black as this is the best absorber of radiation. The pipes containing the liquid to be heated  are made from copper because it is a good conductor
• A transparent cover allows the radiation through. The back of the panel is insulated to stop the heat being transferred through the back of the panel.