Equation of Convection and Types of Convection, Sea Breeze, Land Breeze (For CBSE, ICSE, IAS, NET, NRA 2022)

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Equation and Types of Convection

Equation of Convection

Equation of Convection
  • When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along, this heat transfer is called convection.
  • The fluid above a hot surface expands, becomes less dense, and rises.
  • At the molecular level molecules expand upon introduction of thermal energy.
  • If temperature of the given fluid mass increases, the volume of the fluid must increase by same factor.
  • This effect on the fluid causes displacement. As the immediate hot air rises, it pushes denser, colder air down.
  • This series of events represents how convection currents are formed. The equation for convection rates is calculated as follows:

Where,

  • Q = heat transferred per unit time;
  • = convective heat transfer coefficient;
  • A = heat-transfer area of the surface;
  • = temperature of the surface;
  • = temperature of the fluid.

Types of Convection

Two types of convection

  • Natural convection
  • Forced convection

Natural convection

  • When convection takes place because of buoyant force as there is a difference in densities caused by the difference in temperatures it is known as natural convection.
  • Example: oceanic winds.

Forced convection

  • When external sources such as fans and pumps are used for creating induced convection, it is called as forced convection.
  • Examples: Using water heaters or geysers for instant heating of water and using a fan on a hot summer day.
  • Forced convection is related to Newton՚s law of cooling,

Where,

  • = the rate at which heat is transferred
  • h = convection heat-transfer coefficient
  • A = exposed surface area
  • T = temperature of the immersed object
  • temperature of the fluid which is under convection

The value of the h depends on:

  • Density
  • Viscosity
  • Thermal conductivity
  • Specific heat capacity

Convection Examples

Sea Breeze

  • This occurs during the day. The sun heats up both the sea surface and land.
  • As the sea has a greater heating capacity, it absorbs much of the sun՚s energy but gets warmed up much slower than the land.
  • As a conclusion, the temperature above the land rises and heats the air in the atmosphere above it.
  • Warm air is less dense, and hence, it expands, creating a low-pressure area over the land near the coast.
  • There is relatively high pressure over the sea. The difference in air pressure causes the air to flow from sea to land.
  • The sudden gush of wind felt due to this is known as the sea breeze.

Land Breeze

  • This occurs during the night when the situation reverses.
  • If the sun sets, the land, and sea start cooling down.
  • The land quickly loses heat when compared to water due to the differences in heat capacity.
  • The temperature of the sea is relatively higher, which creates low air pressure there. This sets up a flow of cool breeze offshore, known as the land breeze.

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