SITEMAP   School-college Physics Notes: Thermal energy 4.11 Particle models

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Thermal energy: 4.11 Using kinetic particle theory to explain the factors that affect the rate of evaporation of a liquid and rate of condensation of a gas or vapour (vapor)

Doc Brown's Physics exam study revision notes

4.11 Factors that affect the rate of evaporation and condensation

(KE is shorthand for kinetic energy)

• Condensation occurs when a gas/vapour is cooled sufficiently to a low enough temperature to allow the attractive forces to be strong enough to attract the particles together as a liquid. This can only happen if the kinetic energy of the particles is low enough (the lower the temperature the smaller the kinetic energy).

• Water vapour in the air condenses out on cold surfaces in the winter eg window condensation, invisible steam from a boiling kettle condenses out into clouds of tiny droplets of water, which technically isn't steam! and rain drops form in the higher cooler regions of the atmosphere.

• Factors affecting the rate of condensation

• The cooler the gas, the faster it condenses - more lower KE particles can be attracted together.

• The lower the temperature of the surface the gas is in contact with.

• The lower the airflow over the surface, this keeps the concentration of the condensing gas as high as possible.

• When a vapour/gas is condensed the latent heat of vaporisation must be removed to cool the particles down sufficiently for condensation to take place.

• Because of this, being scalded by steam is worse than be scalded by boiling hot water.

• Both involve transfer of thermal energy due to the heat capacity of liquid water.

• BUT, water vapour must be first condensed, so initially you are scalded by the release of the latent heat of vaporisation = the 'latent heat of condensation'.

• Evaporation is when the highest kinetic energy particles of a liquid escape from the surface ie can overcome the attractive forces of the bulk of particles.

• The greater the KE of a liquid surface particle, the greater the chance to escape and become a gas particle. Evaporation can take place at any temperature between a substance's melting point and boiling point.

• As the highest KE particles escape, leaving the slower lower KE particles, the bulk of the liquid will cool, so a cooling effect accompanies the evaporation of a liquid.

• The cooling effect of sweating is due to evaporation of water from your skin and the thermal energy absorbed from your body in the process.

• Factors affecting the rate of evaporation

• The higher the liquid temperature, the faster the rate of evaporation - more particles with enough kinetic energy to escape from the surface (graph above).

• Reminder of particle model of evaporation

• The greater the surface area, the faster the evaporation - more area, more chance of evaporation.

• The greater the airflow over the surface of the faster the evaporation rate - the air can become saturated with the vapour of the liquid, so it is more readily replaced if the already evaporated liquid is swept away by air flowing over the surface.

• Efficient drying of washing is a good example of these three factors - you need:

• a warm sunny day (higher temperature) to increase the KE of the water molecules,

• the washing well spread out on the line to surface the evaporation area

• and a nice breeze sweeping the evaporated water away!

• When water evaporates the latent heat of vaporisation is absorbed by the water molecules giving a cooling effect.

This is why sweating cools your body, especially if you in a cooling breeze on a hot summer's day.

Keywords, phrases and learning objectives for particle models and thermal energy

Be able to use the kinetic particle theory to explain factors affecting rate of evaporation i.e. change of liquid state to a gas or vapour/vapor.

Be ale to describe that factors that affect the rate of condensation of a gas/vapour to a liquid.

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