# P1.1 Heat Transfer

• Created by: Fiona S
• Created on: 16-02-15 09:31

## Particle Structure

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
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## Conduction

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
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## Convection

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.

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• 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

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## Evaporation and Condensation

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
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## Heat Transfer by Design

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.

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## Energy Transfer

The factors that affect the rate of energy transfer

• Materials the object is in contact with
• The objects shape
• The objects surface area
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## Specific Heat Capacity

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 (℃)

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## House Insulation

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 - CarpetsCarpets made of an insulating material, preventing heat loss by conduction

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

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## Pay Back Time

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

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## U-Values

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.

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## Solar heating panel

• The metal plate is matte black as this is the best absorber of radiationThe 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.
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