Science universe: Physics articles

How liquids evaporate    It is a matter of common knowledge that some water in an open vessel eventually dries up through evaporation. Liquids vary in the rate at which they evaporate at ordinary temperatures. They are said to be volatile. On the other hand, lubricating oil and mercury never seem to evaporate, however they are allowed to stand.   When discussing the process of evaporating, we used the kinetic energy to explain how molecules escape from a liquid.   If a liquid is heated, the energy which goes into it becomes mechanical energy in the molecules. More and more of the molecules gain enough kinetic energy to enable them to escape from the attraction of their neighbours and jump right out of the liquid. A rise in temperature is, therefore, accompained by an increase in the rate of evaporation.   Saturated vapour pressure (s.v.p.)    Let us suppose that some liquid is poured into a bottle which is then corked up. Owing to ...

Ice crystal. Change of volume on solidification    When water freezes to form ice expansion occurs and the ice takes up a bigger volume than the water. For this reason, water pipes are liable to brust during very cold weather, although the leaks do not occur until a thaw sets in.   Sometimes the expansion of a substance on solidification serves a useful purpose. Molten type-metal, for example, expands very slightly when it solidifies, and so takes up a sharp impression of the mould. Paraffin wax experiment.    Most substances, however, contract in volume when they solidify. Paraffin wax is a typical example. When some molten paraffin wax is allowed to become solid in a test-tube the shrinkage in volume is shown by a deep cleft in the surface ( as you can see in the image ).    Effect of pressure on melting point. Regelation   If a substance expands on solidifying, then the application of pressure lowers the mel...

Cooling by evaporation experiment Latent heat graph Latent heat of vaporization   Most of my students did not completely understand latent heat. So I will try to keep it as simple as possible.   When a kettle is put on a boil the temperature of the water steadily rises until it reaches 100 °C. At this temperature it starts to boil, that is to say bubbles of vapour form at the bottom and rise to the surface, where they burst and escape as steam.   Once the water has begun to boil, the temperature remains constant at 100 °C. But at the same time, heat is being steadily absorbed by the water from the gas flame or heating element. This heat, which is going into the water but not increasing its temperature, is the energy needed to convert the water from the liquid state to the vapour state.   Experiment shows that 2260000 J are required to convert 1 kg of water at its boiling-point to steam at the same temperature. This is known as the spec...

Conversion of potential energy into internal energy    In accordance with the principle he was trying to establish, Joule came to the conclusion that the water at the bottom of a waterfall ought to be slightly warmer than that at the top. Waterfall   At the top of the fall the water possesses ( yes, 5 s :) potential energy, which becomes converted to kinetic energy as it descends. Part of this kinetic energy becomes transformed into internal molecular energy when the motion of the water is arrested at the bottom.   Joule decided to carry out a test on a particular waterfall in Switzerland. Although the expected temperature rise was exceedingly small, he was able to detect it with a very sensitive thermometer and found the result to agree closely with the calculated value. This man was very clever.   Experiment to measure the number of Joules equivalent to a calorie by the shot-tube method. Heat by falling shot tube    This exper...

Competitor theories of heat energy    Our present thought regarding the nature of heat began to be put on a firm basis only from about the middle of the nineteenth century. Before then, discussion on the nature of heat had been going on among men of science for more than 250 years. It is interesting to trace the path of investigations which led to the development of our modern viewpoint.   During the seventeenth century the foremost thinkers of the day regarded heat as a form of motion . For example, it was known that a nail became hot when it was hammered into a piece of wood. Robert Boyle explained this by saying that the blows of the hammer set the molecules of the nail into violent vibration, and therefore they become hot. This view, of course, comes very close to the present-day impression of internal energy . But while the seventeenth-century scientists understood the meaning of motion and force , it did not occur to them to combine the two and think ...

Composition of radiant heat    Both conduction and convection are ways of carrying heat from one place to another which require the presence of a material substance , either solid, liquid or gas. Greenhouse    There is a third process of heat transmission which does not require a material medium. This is called radiation , and is the same means by which energy travels from the sun across the empty space beyond earth's atmosphere.     Radiant heat consists of invisible electromagnetic waves which are able to pass through a vacuum. These waves are partly reflected and partly absorbed by objects on which they fall. The part which is absorbed becomes converted into heat . Radiant heat which has passed through a vacuum can easily felt by holding the hand near to a vacuum-filled electric lamp when the current is switched on. The detection of radiant heat. The thermopile    Radiant heat may be detected by converting the heat energy into elect...

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 ( KMnO 4 ) 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.  ...

How does heat energy travel through a metal? Transmission-of-heat-energy    If a steel poker is pushed into the fire and left there for a time the handle becomes warm. Heat travels through the metal by a process called conduction .   This process is complex. It differs between metals and non-metal, and only a brief explanation can be attempt here.   When a metal is heated the free electrons which it contains begin to move faster, i.e., their kinetic energy increases. The hot electrons then drift towards the cooler parts of the metal and at the same time there is a drift of slower-moving ( cooler) electrons in the reverse direction.   To a much less extent, heat energy is transmitted through a metal by vibrations of the atoms themselves which pass on energy from one to the other in the form of waves . These waves are of very high frequency and are transmitted in tiny energy packets called "phonons".   In non-metals which have no ...