On the beach

beachMaybe you are lying on the beach reading this, or if not dreaming about lying on the beach.  We enjoy lying on the beach, or next to a swimming pool, in part because it involves doing nothing and in part because of the heat transfer.  Heat transfer is transfer of energy from a high to a lower temperature zone.  It can occur in four ways: conduction, free convection, forced convection and radiation; and all of them occur on the beach on a hot day.

Conduction occurs as a flow of kinetic energy from one molecule to the next by direct contact.  When you are lying on the beach it occurs between you and the surface that you are lying on.  When you first lie down on hot sand, then the energy flows from the hot sand to your cooler body by conduction.

Free or natural convection is heat transfer carried by a rising current of fluid due to buoyancy effects created by the hotter fluid being less dense.  This tends to happen above your warm body after you have been lying in the sun for a while.  It also happens above the hot sand and you can sometimes see a heat haze caused by the rising hot air that has a lower density and thus different refractive index compared to the surrounding air.

Forced convection also involves heat transfer by a moving current of fluid but in this case the flow is caused by an external source.  So if there is breeze across the beach then you will be cooled by forced convection as you lie on the beach.

Radiation consists of electromagnetic waves in the infrared spectrum travelling away from a source in all directions.  This is the heat from the sun that makes it so pleasant to lie on the beach on a sunny day.

Ok, shut your eyes and go back to sleep.  The heat transfer lesson is over – though some of you might want to think about whether that breeze is really forced convection since it is probably caused by natural convection on a climatic scale.

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