Specific Latent Heat

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  • Specific Latent Heat
    • Defining SLH
      • The energy required to change the phase per unit mass while at a constant temperature
      • L = E/m
        • E is energy transferred
        • m is the mass
      • When the substance changes from solid to liquid phase, this is the SLH of fusion
      • When the substance changes from liquid to gas phase, this is the SLH of vaporisation
    • SLH of fusion
      • When a substance is at its melting point, energy is required to change phase to a liquid
      • The energy transferred increases the internal energy, without increasing the temperature of the substance
      • A circuit can be used to determine SLH of fusion
        • Ammeter connected in series
        • Voltmeter connected in parallel with a heater
        • Variable resistor used to control current
      • A thermometer is used to ensure the ice is at its melting point. The ice should just be starting to melt when the heater is switched on
      • The energy transferred is current x p.d. x time
      • The mass is the mass of ice melted, calculating by subtracting the mass of an empty beaker from a beaker with melted ice in it
    • SLH of vaporisation
      • Energy required for SLH of vaporisation is usually considerably more than SLH of fusion
      • To determine SLH of vaporisation, a heater can be used with a condenser to collect and measure the mass of liquid that changes phase
      • SLH of vaporisation can be found using L = (IVt)/m
      • m is the mass of water that changes phase during heating
    • Combining SLH and SHC
      • Considering both SHC and SLH, you can determine the total energy required to heat and then change the phase of a substance
      • Heating a solid to a melting point (E = m x c of solid x temp. change)
      • Melting solid at constant temperature (E = m x SLH(f))
      • Heating liquid to its boiling boint (E = m x c of liquid x temp. change)
      • Boiling the liquid at constant temperature (E = m x SLH(v))
      • The total energy is determined by adding up all the energy transferred in each section

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