Wednesday, February 15, 2012

Explanation of convection currents - How heat is transferred through liquids

Convection currents in liquids

  When a vessel containing a liquid is heated at the bottom a current of hot liquid moves upwards and its place is taken by a cold current moving downwards. Unlike conduction, where heat is passed on from one section of the substance to another as described in the previous post ( transmission of heat energy explained), the heat is here actually carried from one place to another in the liquid by the movement of the liquid itself. This phenomenon is called convection. The same process occurs when a gas is heated.

Experiment to demonstrate convection currents in water

Convection currents in water
  Convection currents in water may be shown by filling a large spherical flask with water and dropping a single large crystal of potassium permanganate ( KMnO4) to the bottom of it through a length of glass tubing. A finger is placed over the end of the tube, which is then removed, together with the colored water it contains.

  This method of introducing the crystal ensures getting it in the centre and also prevents it from coloring the water before it is required. On heating the bottom of the flask with a very small gas flame an upward current of colored water will ascend from the place where heat is applied. This colored stream reaches the top and spreads out.

After a short time it circulates down the sides of the flask, showing that a convection current has been set up.

Explanation of convection currents

  When a portion of liquid near the bottom of a vessel is heated it expands. Since its mass remains unaltered, it becomes less dense ( you remember that Density = mass / volume ), and therefore rises. Thus a warm convection current moves upwards; for the same reason a cork rises in water or a hydrogen-filled balloon rises in air. In effect, convection is an application of Archimedes' principle. For further illustration please read Balloons and floating bodies.

  If, on the other hand, some liquid in a vessel is heated at the top, the liquid there expands and remains floating on the denser liquid beneath. No convection current is set up, and the only way in which heat can travel downwards under these conditions is by conduction.

Convection in air - Ventilation by convection

Windmill pattern
Ventilation by convection

  The air convection current rising from an electric lamp may be shown with the aid of a small windmill.

A suitable windmill may be cut with scissors from thin card or aluminium foil to the pattern on the image above. The vanes are slightly bent and the mill, pivoted on a piece of bent wire, is held over the top of an electric lamp. When the lamp is switched on the windmill rotates in the upward hot air current. A device similar to this is often used to produce a glimmering effect in domestic electric heaters of a type which are disguised to resemble glowing coal fires.

  During the eighteenth century coal-mines were ventilated by sinking two shafts to the workings, known as the upcast and downcast shafts respectively. A fire was lit at the bottom of the upcast shaft, which caused the air in it to become heated and rise. Fresh air entered the downcast shaft and passed through the passages of the mine workings before it, in turn, became heated and passed out through the upcast shaft.

In this way a constant flow of fresh air was maintained through the mine. The image above shows a laboratory model to illustrate this method of ventilation. It consists of two wide glass tubes projecting from the top of a rectangular wooden box with a removable glass front.

A short piece of candle is lit at the base of one of the tubes. When a piece of smouldering brown paper is held over the top of the other tube, the direction of the convection currents will be rendered visible by the passage of smoke through the box.

The domestic hot water supply system

Composition of the domestic hot water supply system
  The domestic hot water supply system is composed of a boiler, a hot water storage tank and a cold supply tank interrelated by pipes arranged as shown in the image above.

When the system is working a convection current of hot water from the boiler rises up the flow pipe A while cold water descends to the boiler through the return pipe B, where it becomes heated in turn. In this way a circulation is set up, with the result that the hot water storage tank slowly becomes filled with hot water from the top downwards.

It is important to notice that the flow pipe A leaves the boiler at the top and enters the top of the hot tank, while the return pipe B connects the bottom of the hot tank to the bottom of the boiler.

  Hot water for use in kitchen and bathroom is taken from a pipe leading from the top of the hot tank. When hot water is run off an equal volume of water from the cold supply tank enters the hot storage tank at the bottom through the pipe C. The whole system is thus kept continually full of water and no air can enter. The water-level in the cold tank is maintained by a supply from the mains which enters through a ball-cock.
  An expansion pipe rises from the top of the hot tank and is bent twice at right angles so that its end is over the cold tank. This is a safety precaution; if the fire is allowed to burn so fiercely that the water boils, steam and hot water are discharged harmlessly into the cold tank and no damage results.

Circulation by convection

Circulation by convection
  This image shows a glass model which demonstrates the convection currents described above. originally, the water in the lower flask is colored with blue ink, while the tubes and upper vessel are full of colorless water.

When the flask is steadily heated with a Bunsen-burner a hot convection current of colored water rises up the bent tube. After a short time a visible layer of hot colored water collects at the top of the upper vessel. Eventually this layer increases in depth until the upper vessel is entirely filled with hot colored water.

Finally, warm colored water will be seen descending the straight tube. I hope that was fun:). Thanks for visiting and I hope you come back soon.