Category Science

Static electricity is electricity that doesn’t move. It is an electric charger held on an object, caused by the gain or loss of electrons. Lightning is static electricity suddenly discharging between clouds or between clouds and the ground. Static electricity builds up with friction, when two different materials rub together.

A plastic rule can be given an electric charge, called static electricity, by rubbing it against a T-shirt or a jumper. Static electricity makes small pieces of tissue paper jump and stick to the ruler.

  All objects are made of atoms. Each atom has equal numbers of electrons and protons. Electrons have a negative charge; protons have a positive charge. These charges balance each-other exactly to make objects neutral (unchanged). But friction, like rubbing a balloon on one’s jumper, makes electrons rub-off from the jumper on to the balloon. This charges the balloon with static electricity.

 It now has more electrons than protons, so it is negatively charged. And the jumper with more protons than electrons is positively charged. A balloon that has been charged by friction will attract small pieces of paper.

  The word electricity comes from the Greek word electron, which means ‘amber’. Amber is petrified tree resin. Basically electricity arises due to valance electrons i.e., the electrons revolving in the outermost orbit of an atom. When these electrons are transferred from one body to another due to friction caused by rubbing, then it is called frictional or static electricity.

The transferred electrons are spread over the surface and remain at rest and hence called static electricity. But when the valance electrons moves through a conductor due to an external electric field, as in the case of a wire and bulb connected to a cell, then it is called current electricity. Under normal conditions, everybody is electrically neutral. Now when a glass rod is rubbed with a silk cloth, the glass rod loses some electrons and the slim cloth acquires these charges.

 The glass rod is said to be positively charged and the silk cloth negatively charged. Here the electrons in the glass rod are held less tightly than those in silk cloth. So the electron transfer is from rod to silk cloth. Thus the total number of charges is conserved.

 Similarly when a plastic pen is rubbed with woolen cloth, the plastic pen acquires electrons from the woolen cloth. This is because the electrons in the plastic pen are more tightly bound than those in woolen cloth. This is the origin of static electricity and it leads to the fundamental law ‘Like charges repel and unlike charges attract’.

Not all electrical appliances draw more current in direct proportion with the reduction in voltage and hence the answer given last week is not complete. Broadly, electrical appliances used in homes can be classified into two groups – appliances such as electric irons and electrical heaters which convert electrical energy into heat energy.  Inca descent bulbs also come under this group. The second category includes those which use electrical motors, in which electrical energy is converted into mechanical (rotational) energy.

            In the first group, the current drawn is proportional to the square root of the voltage. Hence, when the voltage is 90 per cent of the rated voltage, the current drawn will be 95 percent of the current drawn at rated voltage, that is when the voltage is lower, the current also will be lower, though not in direct proportion.

                In the case of appliances using motors, the capacity marked on the name plate (usually in kW or W) is the power available on the shaft for conversion into mechanical power. In such appliances, the current drawn is inversely proportional to the applied voltage, that is, when the applied voltage is lower; the current drawn will be proportionally higher, with the mechanical power remaining constant.

            Motors operating at low voltages burn out because they tend to draw unduly large currents which cannot be carried by the wires wound inside them.

            In the case of fluorescent lamps, their characteristics vary with input voltage, that is, the exact changes depend on the type of circuit used, and so manufacturers’ technical data may be consulted.

Two basically different methods are used for describing the severity of an earthquake. One is Intensity and the other is Magnitude. Intensity is an estimation of the earthquake’s effect on people, damages inflicted to structures and the changes caused to the earth’s surface etc. In 1905, an Italian Seismologist Giuseppe Mercalli devised an Intensity Scale, based on evidences such as human reactions, damages to structures, fissures in the earth’s surface, landslides, floods etc. This Scale had 10 divisions – Roman Numbers I to X – and was known as the Mercalli Scale. In 1931, two Seismologists, Wood and Newman modified this Scale and extended it to 12 divisions, i.e. up to XII. After this, the Mercalli Scale came to be known as Modified Mercalli Scale or simply, the MM Scale.

Of far more importance and of greater scientific value is to describe the severity of earthquakes by Magnitude. Magnitude is related to the amount of strain energy released at the focus or epicenter of the earthquake, as recorded by the Seismographs. This is where the Richter scale comes into relevance. American Physicist and Seismologist, Charles Francis Richter (1900-1985), while with the Carnegie Institute, USA, embarked on the project for developing a suitable scale to describe the degree of intensity of earthquakes in terms of Magnitude, by numerical values.

The culmination of his efforts was the evolvement of a Scale in 1935. This was called the Richter scale, after his name.  In this endeavour, Richter was supported by his collaborator, Professor Beno Guternerg of the California Institute of Technology. The Richter scale is an open-ended numerical Scale that describes an earthquake independently of its effects on people, buildings or other objects. It begins with Zero, in which the greatest wave-amplitude registered on a seismograph at a distance of 100 kms. From the epicenter of the earthquake does not exceed one Micron (one thousandth of a millimetre).

Water boils at 100 degree Celsius and 1 atm. pressure (i.e., pressure at sea level). Variation in boiling point and expansion of liquids are due to the presence of various other substances. When a low boiling liquid is present along with water, the mixture will boil at even lower temperatures. But only the low boiling liquid will be evaporated. In case of milk, during heating, fatty organic substances present in the milk separate out and float on the surface due to their weight. Those fatty matters cover the surface like a blanket, and prevent any loss of water or any other substance by evaporation. This increases the pressure beneath the layer and leads to expansion.

Milk consists of mostly water and some fats, proteins, lactose and minerals. Milk fat is a mixture of glycerides of fatty acids with a density less than that of the milk serum. The solid fat is dispersed in the serum in the form of small globules. When the milk is heated up, these fat globules raise to the top and at a temperature around their melting point, about  C form a layer of skin on the hot milk.

The steam bubbles that form within the milk get trapped by this skin and accumulate under it. The grow and coalesce and build up a pressure that eventually raises the skin and makes some of the milk spill over, stirring breaks the skin, releases the pressure and prevents spilling over. 

Milk is a white opaque fluid in which fat is present as an emulsion, protein and some mineral matter in colloidal suspension and lactose with some minerals and soluble protein in true solution.

Milk expands when it foams. This expansion results in increase of volume. The volume increases due to foaming as a result of trapping of air in the liquid. This takes place when the milk is whipped or agitated. At the time of foaming air is trapped as bubbles, surrounded by thin layers of protein with fat interspersed to act as a stabilizer. The stability of foam depends on the surface tension of the liquid. Water does not ready foam unless on emulsifying agent is present, since it has higher surface tension, whereas milk readily foams since it s an emulsion by nature and also has low surface tension.

The surface tension of milk is related to proteins, fat, phospholipids and few fatty acids present in it.  When the milk is heated up to its boiling point, it gets agitated and its density also decreases, which result in foaming. This can be prevented by pouring some water on its foaming surface, which helps to increase the surface tension to some extent. 

In the beginning of heating, the low temperature at the top layer helps for the formation of a scum. This tenacious layer is formed by coalescence of fat globules, which are held apart originally in the unheated milk, by the surrounding protein molecules.

When the milk is heated these protein molecules get separated from the fat globules, resulting in coalescence of freed fat globule, which in turn results in the formation of scum.

Scum formation can be prevented by beating or stirring the milk while heating it or by covering the milk pan. Use of milk boiler will prevent the formation of scum. When allowed to cool, the drying out of the top region of the milk helps again for the formation of scum. The scum contains a small amount of coagulated proteins, minerals and fat globules.

A micro wave oven cooks not with the heat but with the radiation similar to radar waves. The heat in an ordinary oven first hits the outside of the food and works its way inward. But microwave radiation goes through the food, bounces off the floor or wall of the oven and goes through the food again.

The radiation also changes its polarity or its positive-negative direction several billion times a second.

The rapidly oscillating microwave radiation acts on the water in food because of a special property of water. Water molecules also have polarities, one made of oxygen atom, which is negative and two atoms which are positive, each water molecule has a positive and negative end.

Every water molecule responds to the reversal of micro wave field by reversing itself, twisting back and forth billions of times a second.

As the twisting water molecules rub against other molecules, they generate friction, which causes the food to heat up and cook rapidly. 

The principle of a pressure cooker is cooking under increased pressure. It is well known that food gets cooked fast at high temperatures. Generally we cook food in water kept in open vessels. In these vessels, when the water is heated to 100 degrees Centigrade it begins to boil, becomes steam and escapes. Thus there is no possibility of heating the water beyond 100 degrees in open vessels. Hence it takes a lot of time to cook the food.

From physics, we know the boiling point increases with increase in pressure. Hence in pressure cookers, the steam is not allowed to escape but enclosed within the vessel. As more water is converted into gaseous steam, the pressure increases which in a feedback mechanism increases the boiling point to well beyond 100 degrees enabling fast cooking.  Normally the temperature reaches about 120 degrees inside the pressure cooker.

The fundamental equation in physics that relates pressure (P), volume (V) and temperature (T) is given by Boyle’s law

            P V = k T where k is constant.

According to the equation, if V is kept constant as in a pressure cooker and P or T is increased, the other parameter increases. In the cooker, both of them increase to enable fast cooking.

  In order that the pressure does not reach very high values so as to cause an explosion, a weight and safety valve are provided to let out the excess steam. Also the body of the cooker is made of an alloy which can withstand high pressures.