teaching heat at lc

7/30/2019 Teaching Heat at LC

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P H Y S I C A L S C I E N C E S M A G A Z I N E

I have just completed a MEd Science (physics) having studied only Biology and Chemistry previously. I found it qof the more useful aspects for me, as someone new to physics, was writing up the experiments, particularly the(Science, Technology and Society) sections. I learned a lot from this and use the material in class to make physhopefully more interesting for the students. The rest of this article is edited from experiment notes (without the dand method for experiments, which are described in most text books).

Latent Heat

The word latent was first used by Joseph Black (1728 1799), a French-born Scottish chemist noted for his funda

heat and specific heat as well as his discovery of carbon dioxide. Latent heat means hidden or concealed energy, inot show up on a thermometer. The latent heat (L) of a substance is the heat energy needed to change its stattemperature. Latent heat is energy used for loosening or breaking bonds between molecules and not for raising temfor latent heat is L. Its unit is the joule (J). The latent heat needed to change from a solid to a liquid is called the latlatent heat needed to change from a liquid to a gas is called the latent heat of vaporisation.

When a solid substance changes from the solid phase to the liquid phase, energy must be supplied in order to ovattractions between the constituent particles of the solid. This energy must be supplied externally, normally as heabout a change in temperature. In the case of changing from a solid to a liquid, energy is needed to increase the di

or molecules, pulling them apart and to reduce the number of bonds between neighbouring atoms or molecules soapart and become a liquid.

Teaching Heat at Leaving CertificateNoreen Gillies (Scoil Mhuire, Kanturk)


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Experiment 2 - To measure the specific latent heat of vaporisation of water

SOURCES OF ERROR REDUCING THE ERRORS

Obtaining dry steam The calculation assumes that only steam is added. Steam was allowed to isdelivery tube until the tube had warmed up. Liquid on the end of the tube

being inserted into the calorimeter. A steam trap was used to collect any litube is sloped so that condensate will run back into the steam generator. Th

be insulated to prevent condensation. Anti bumping granules could be addboiling smoothly.

Measuring temperature Use sensitive thermometer or temperature sensor. Mixture must be stirredtemperature. Use computer graph to record the highest temperature. Temp

least 20 C improves the accuracy by reducing the % error.Heat loss/gain from calorimeter Use cool water (10 degrees below room temp). Cold water allows steam to c

reduces heat loss, which improves accuracy. Equal temperatures above andthe heat losses and gains for the calorimeter balance. Use insulation/lagginsteam is passed into the water in the calorimeter the delivery tube should bsurface of the water to prevent evaporation of water from the surface.

Measurements of mass Use a sensitive balance. When removing the delivery tube ensure that wateris not being removed with it. The mass of the steam added is found by sub

Repeat The experiment should be repeated a number of times.


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A refrigerator works using latent heat. A volatile liquid is pumped through a system of closed pipes containingmaintains a higher pressure on one side of the valve than on the other. Because the pressure of the liquid is rethrough the valve it vaporises, and in so doing extracts the necessary latent heat from inside the cabinet. On th

the valve the vapour liquifies once again, and in so doing releases the latent heat of vaporisation. Keeping the Greenhouse Warm:

Placing large vats of water in greenhouses protects fruit from freezing; theheat liberated when the water freezes warms the air.

It is more effective to wrap a damp cloth around bottles or cartons of milk tokeep them cool, rather than placing them in a jar of water. The waterevaporating from the cloth around the milk takes heat energy from the milk,thereby cooling it. The Egyptians realized that water could be kept cool by

placing it in porous earthenware vessels. These containers allow sufficientliquid to escape and evaporate, and in so doing cool the water remaining.

The cold feeling one has after emerging from a bath or shower is due toevaporation which extracts latent heat from the body.

Perspiration acts as a temperature control mechanism in the human body.Evaporation of the sweat results in heat loss from the body surface.

It is more effective, to sponge the forehead of a patient with a high temperature with tepid water. Because of i

the tepid water will evaporate at a faster rate. It is then the latent heat absorbed from the body, which cools th


teaching heat at lc

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