SITEMAP   School-college Physics Notes: Thermal energy 4.4 Explaining Latent Heat

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Thermal energy & particle theory: 4.4 Introduction to latent heat and thermal energy changes when a material undergoes a physical state change

Doc Brown's Physics exam study revision notes

4.4 Explaining latent heat and thermal energy changes of physical state g <=> l <=> s

• The energy needed to change the physical sate of substance at constant temperature is called the latent heat.

• There are two latent heat values:

• (i) The numerical energy store transfer at the temperature corresponding to melting or freezing.

• The latent heat of fusion (melting) for melting (solid ==> liquid), thermal energy added to the system.

• The internal energy is increased to a point where the interparticle forces are weakened enough for the solid lattice of particles breaks down to form a liquid.

• The latent heat of freezing (solidifying) (liquid to solid), thermal energy removed from the system.

• The internal energy is decreased to a point where the interparticle forces are strong enough for liquid particles to come together and form a solid.

• These two latent heats are numerically the same for state changes involving solid <=> liquid

• (ii) The numerical energy store transfer at the temperature corresponding to boiling or condensing.

• The latent heat of vaporization (boiling/evaporation) (liquid ==> gas), thermal energy added to the system.

• The internal energy is increased to a point where the interparticle forces are weakened enough for liquid particles to escape to form a gas.

• The latent heat of condensation (gas/vapour ==> liquid), thermal energy added to the system.

• The internal energy is decreased to a point where the interparticle forces are strong enough for gas particles to come together and form a liquid.

• Again, these two latent heats are numerically the same for the state changes involving liquid <=> gas/vapour.

• A historical curiosity - latent heat ('hidden' heat), which was unexplained until the particle theory of matter was developed and inter-particle bonding understood.

• Changes of state can be represented as a temperature - time graphs (in parts 4.5 and 4.6)

• Part 4.5 Heating curve - increasing temperature due to the addition of thermal energy, increasing the internal energy of the system.

• Part 4.6 Cooling curve - decreasing temperature due to the removal of thermal energy, decreasing the internal energy of the system.

• BUT, the graphs are not simple 'curves', there are horizontal sections that need explaining using the concept of latent heat in parts 4.5 and 4.6.

Keywords, phrases and learning objectives for particle models explaining latent heat

Be able to define, use and explain the latent heat energy changes when a substance changes physical state including the latent heat of fusion/melting, latent heat of vaporization/boiling (vaporization, vapourisation, vaporisation), latent heat of condensation, latent heat of melting/freezing

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