# 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

• 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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