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Why is latent heat significant for XMM? Next page

     Latent heat is concerned with changes of state of matter (the change from water to water vapour is an example) and of the energies involved in these transformations. It plays a very important role in our everyday lives particularly in the weather and how vast amounts of energy are transported from place to place.
    As far as XMM is concerned it is important not so much from the aspect of the energies involved but from the changes of state themselves, especially as these can occur more readily in the lower pressure and lower temperature regimes which can be found in space.

What is latent heat and why is it so called?

    When you heat up a substance, let us take water as an example, the substance absorbs the energy and its temperature rises. The amount by which the temperature rises is proportional to the amount of heat put in and the mass of the water. The constant of proportionality is called the specific heat. When heat is extracted from a substance the converse happens. That is to say, the heat is accounted for by the observed change in temperature. Sometimes, however, no change in temperature is observed and so the heat somehow becomes hidden or dormant. This is what we call latent heat from the Latin word "lateo" which means "to hide". The same thing can occur when you heat up a solid.

When does heat become latent?

    Although no change in temperature is observed when this happens, other changes do occur which are far more dramatic. The substance changes its state by undergoing a phase change. The different states of matter that are most familiar to us all are solids, liquids and gases. Thus a phase change is between any of these states of matter, for example from a liquid to a gas or from a liquid to solid or from a solid to a liquid. Some phase changes cut out the "middle man" such as solid to gas directly. This is called sublimation. and we will discuss it later.

    So, in the example above, when we were putting heat into the water and the temperature was not rising the liquid was beginning to boil thus changing the state of the water into gas (steam). Conversely we can extract heat from steam and the temperature will drop until some point where we will be able to extract heat without any further drop in temperature. This occurs when the steam condenses back into water and we recover the latent heat apparently lost when we made the steam. When the condensation process is complete and there is only the water, further extraction of heat again results in a decrease in temperature.

Is latent heat a way of storing energy?

     Because we can hide heat in this way which is recoverable at some later stage and because heat is energy then, yes, this is a way of storing energy in much the same way as storing electrical energy in a battery, or gravitational energy in a water reservoir in the hills, or chemical energy in fireworks, or nuclear energy in radioactive elements.

How much energy is being stored?

    The units for latent heat of fusion and latent heat of vaporisation are Jkg-1 , so the amount of energy depends on the mass. But per unit mass the amounts of stored energy is very significant. To illustrate this do the following calculation.

    You are given a kg of very cold ice at a temperature of -100o C and a 3 kW heater (about the power of a fast electric kettle). What are the approximate times in minutes to :

  • raise the temperature of the ice to its melting point
  • completely melt the ice
  • raise the temperature of the water to its boiling point
  • completely boil the water

(Ice specific heat: 2100 Jkg-1 K-1 Specific latent heat 3.4 x 105 Jkg-1 )

(Water specific heat 4190 Jkg-1 K-1 Specific latent heat 2.3 x 106 Jkg-1)

Click here for the answer.

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The University of Birmingham 

Physics and Astronomy Department, The University of Birmingham