Heat and Heat Transfer

Heat and Heat Transfer

1. Heat is a form of energy

   • Three states of matter
     • Liquid
     • Solid
     • Gas

2. Heat Capacity

   • The Heat Capacity (C) of an object is the amount of heat energy needed to change its temperature by 1K.
     • • SI unit – joule per kelvin (J K-1)
       • Heat needed to change temp = Heat capacity x change in temp (Q = C Δθ)

3. Specific Heat Capacity

   • The specific heat capacity (c) of a substance is the heat energy needed to change the temperature of one kilogram of that substance by one kelvin.
     • SI unit = joule per kilogram per kelvin (J kg-1 K-1).
       • Formula: Mass x Specific Heat Capacity x Change in temp

4. Latent Heat

   • The latent Heat (L) of an object is the heat energy needed to change its state without a change in temperature
     • SI unit is the joule (j)
       • The latent heat needed to change from a solid to a liquid is the latent heat of fusion
         • The latent heat needed to change from a liquid to a gas is called the latent heat of vaporisation.

5. Specific Latent Heat

   • The Specific Latent Heat (l) of a substance is the amount of energy needed to change the state of 1 kg of that substance without a change in temp
     • SI unit = joule per kilogram (K kg-1)
       • Q = ml

6. Methods of heat transfer

   • Conduction
     • Conduction is the movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule, without any overall movement of the substance.
       • U-Value
         • The U-Value of a structure is the amount of heat energy conducted per second through 1 m2 of that structure when a temperature difference of 1K is maintained between its ends.
           • Unit of U-Value: W m-2 K-1
             • Bad insulator: High U-Value
             • Good insulator: Low U-Value
   • Radiation
     • Radiation is the transfer of heat energy from one place to another in the form of electromagnetic waves.
       • Higher temps emit shorter waves and Darker = better radiator.
   • Convection
     • Convection is the movement of heat through a liquid or gas by means of circulating currents caused by the heat.
       • When the lower part of the fluid is heated, it expands and becomes less dense. It rises above the cooler fluid. This is called a convection current.

7. Storage Heaters

   • Bricks with a high specific heat capacity are heated overnight. During the day, the bricks slowly give out their heat thus heating the room. Because the bricks have a high specific heat capacity, they can absorb a lot of heat without increasing very much in temperature.

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