Heat Transfer

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Physics (and general science) courses study the various methods of heat transfer from one medium to another. It must be remembered that it is heat that moves from a “hotter” object to a colder object. This is one of the laws of thermodynamics. The studies in schools are concerned with demonstrating the mechanisms of heat transfer and possibly applications to reduce the impact of these mechanisms.

Background

Heat is transferred in three ways: Conduction, Convection and Radiation. There are various demonstrations available to show these, although the opportunity for class practicals are limited. As well as the mechanisms of heat transfer, some studies show the influence of material properties on the rate of heat exchange. The investigation of Specific Heat Capacity and “latent heats” of vaporisation and freezing are often excluded from modern courses but, as often occurs, may be re-introduced.
Technicians should be aware of these.


Conduction

This is the prime method of heat transfer in the solid phase of a substance.
Energy is passed from each adjacent molecule to the next (of lower energy) throughout the solid. Some materials are able to transfer the heat easier than others. A value may be associated with the material at a given temperature and is called the “thermal conductivity” of that substance. This is given either the letter U, k or the greek letter lambda and has units Wm-1K-1
Classically the experiments used to illustrate the mechanism were “Searle's Bar” (for good conductors) and Lee's Disc (for insulators) and their derivatives. However modern materials are used to illustrate the principles without a quantitative method. This is usually achieved by using rods of different materials connected to a heat source. This equipment comes in various guises:

  • Edser's apparatus,
  • Ingenhousz's apparatus,
  • conducting cross (or wheel) and
  • conducting bars utilising thermochromic indicators.

The “modern experiment” of “huddling penguins” in which test tubes are grouped together with an elastic band may be used. Heat loss from a cental tube is less than that from the outside. A detailed analysis of this is too complex for the age group at which is aimed as there is more than one process ocurring.
A conducting cross has also been described as a variant of the conduction apparatus. Ball bearings are held in place with wax and can be heard to drop as the different bars conduct heat to the ball bearing "indecies" on different metals. Wax may also be used in other apparatus to fix an index. A rule can be used to ensure equidistant settings and a hair drier used to melt the wax. Do not use excessive quantities of wax as it may interfere with the motion of the index.

One obvious demonstration is to have samples, especially of floor coverings, that may be passed around a class. Glass, metal, carpet,marble, rubber etc. will all feel "hot" or "cold" despite being at the same (room temperature)


Convection

This is the prime method of heat transfer in a fluid.
As a fluid is heated the density of the warmed region, normally, decreases. The warm area rises into the colder bulk of the fluid and is replaced by cooler, denser surrounding fluid. Various demonstrations are available to illustrate this.

  • The convection tube (a glass tube formed into a rectangular shape with a “T” connector for filling at the top) is probably the easiest demonstration. This tube is filled with water and either aluminium powder or a small quantity of potassium permanganate crystals are introduced. When heat is applied at one limb of the equipment (via a bunsen burner) the circulation of the fluid is observed. The apparatus should be cleaned asap after the demonstration to avoid staining.
  • An alternative method is to use a large beaker and introduce the potassium permanganate via a glass tube (to deliver without distributing through the body of the beaker). The permanganate should be to one side of the beaker where heat can be applied.
  • The bi-directional nature can be shown with coloured ice and hot water placed on the surface and released from a weighted sack at the base repectively. Water soluble blue and red dies are best.
  • The other demonstration frequently asked for for this is the “ventilation apparatus” often called the “glass chimneys”. This apparatus illustrates how mine shafts were once ventilated. A candle is lit under one glass tube “chimney” if smoke is introduced to the other chimney it is drawn down into the base and then out again with the rising air column over the candle.
  • One other popular demonstration is the teabag. A cylinder type teabag is emptied and the tube placed on a heat mat. The teabag is lit and it creates its own convection current to disperse the sooty remains.
  • A Convection snake may also be made and placed over heat source.

Although expensive and difficult to set up, Schlieren imaging techniques may also be deployed to illustrate convection currents. However an OHP used as a light source can act as a "shadowgraph" light source to illustrate air based convection especially over bunsens and chimneys.

For a video illustrating one Schlieren technique see you tube clip

Radiation

This is the only method of heat transfer where no medium is able to allow other processes to be used. A hot object will radiate energy according to the Black body laws. The nature of the surface will influence how efficient this heat transfer is. See Leslie's Cube. Alternatives can be shown by using heating up/ cooling flasks.
For radiation a set of black and silver (baked bean) cans which are held equi-distant from a bunsen. Data logging equipment maay used to take temperatures and draw graphs. The use of cans has been found to be more sucessful than test tubes. Be careful on the selection of paints as the emmissivity may give unexpected results.
Two sheets of metalmay be used also, one painted matt black, the other silver. These are mounted in wood so they stand upright with drawing pins are fixed to the metal sheets with Vaseline, equal distances from the base. These are stood either side of a bunsen and the pins slide down.
For added interest, take the temperature using an infra red temperature gun. (See [1]) Can also be used with Leslies cube. As well as all of the above, one suggestion is to do an ‘insulation challenge.’
Students insulate one beaker with material (bubble wrap, cotton wool, foil, material, rubber bands, tape etc. – plus a lid). Measured amounts of hot water are put in the beaker and a control – a normal beaker of the same size and temperatures are recorded as the water cools. Data loggers can be used. Graphs drawn etc


Parabolic reflectors can be used to light a match with a suitable infra-red (heat) source. Using a thermochromic sheet in the path will illustrate the transfer mechanism too.

For a suitable small class, a sheet of metal polished on one side and rough on the other can be heated in a bunsen flame. When held in close proximity to a hand the rougher side can be felt to emit more heat than the shiny side.


Introductory /summarising demos.

A good introductory demonstration may be fashioned out of a balloon or greaseproof paper water bomb and candle. The balloon is partially filled with water then blown up. If the lit candle is placed underneath and in contact with the balloon it does not pop as the water conducts the heat away from the membrane. The water bomb can also be used in a similar way to show the same effect and may even be able to get water to boil in the bag. See also Heating_water_in_a_paper_kettle
Or construction details: (External link for instructions to make) One other demonstration involving heat transfer is where two large metal balls are struck together with a sheet of paper between them. Inspecting the paper shows a burn hole that is produced as the energy is transferred to the paper. It is more convincing to smell the paper as the small size is of the discolouration is not always obvious. The same effect can be achieved by placing the paper on an anvil or similar and striking with a ball-peined hammer. Safety glasses must be worn as there is a chance the hammer may shatter.

--D.B.Ferguson 15:22, 9 November 2008 (UTC) Additional material suggested from postings to Kath Kynaston added by--D.B.Ferguson 21:45, 22 November 2008 (UTC) Back to Physics
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