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My mother used to leave the lights on in the kitchen after washing the floor, saying that it would dry faster.

Does this really happen, or is it just a superstition? If true, how substantial is the effect?

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In principle yes, but the effect is usually marginal. It also depends on how powerful your lights are compared to the size of the kitchen (a 1000 Watt flood light in a home kitchen will probably have a noticeable effect on the speed of drying).

  • Bascially, the floor dries through evaporation, i.e. the water on the floor goes into the gaseous phase ('becomes vapour') as long as the air in the kitchen is not saturated with water.
  • In other words, water continues to evaporate until there is no water left or until the equilibirum vapour pressure of water in the kitchen's air has been reached.
  • The equilibrium vapour pressure on the other hand depends on the air's temperature. Higher air temperature means higher equilibrium vapour pressure (the air can 'hold' more water).
  • Adding additional sources of heat, such as leaving the lights on, increases the temperature and thus increases the 'water capacity' of the air

The question essentially boils down to how much the temperature of the air increases in the kitchen by leaving the lights on. This however not only depends on the power of the lights (one can assume that all power is converted to heat in the end) but also on the size of the kitchen (how much air needs to be heated up) and the thermal isolation of the kitchen (how much heat goes e.g. through the window).

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    $\begingroup$ I would think that the light would heat the floor (or water) before it heats the air. $\endgroup$ – LDC3 Nov 1 '14 at 19:43
  • $\begingroup$ yes, good point, and the water in turn would heat the region of air immediately above the water $\endgroup$ – Andre Holzner Nov 1 '14 at 20:47
  • $\begingroup$ although traditional incandescent light bulbs and fluorescent tube lights have an efficiency far below 50% (according to en.wikipedia.org/wiki/Fluorescent_lamp#Luminous_efficacy), so more energy is converted to heat than to light for these. $\endgroup$ – Andre Holzner Nov 5 '14 at 11:07
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In addition to the points mentioned by Andre Holtzner, there is also a non-thermal effect due to photons hitting the water molecules and kicking them out of the water directly. This process plays an important role in the vaporation of water due to irradiation by sunlight. You can think of the photons being at a temperature that is much higher than the ambient temperature. The energy of these photons will eventually thermalize to the much lower ambient temperature, raising the ambient temperature a bit. But when a photon hits a molecule, the kick it gets is similar to the kick it would get if it were in thermal contact with a heat source at an extremely high temperature, the difference is that the rate at which molecules are hit by photons is quite low. However, one collision can be enough to launch a molecule out of the water.

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