Version 4 Surface Tension

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If due measures are taken steel needles and safety-razor blades may float on water surface. This is easier when they are slightly oiled. Flies, mosquitoes, and other insects can walk on the surface of water without even getting their feet wet.

The reason for such strange phenomena is surface ten­sion, that is, the tendency of a liquid surface to act like a stretched elastic membrane. When water comes in contact with air, the molecules at the surface are attracted more strongly to the water beneath than to the air above. The result is that the surface molecules become more tightly joined together than those in the interior of the liquid. If an object (such as a fly's foot, a steel needle, or a safety-razor blade) is not too heavy and is not easily wet by water, it merely dents this surface layer without breaking through. (The student is unlikely to know that the verb "to dent" means to make a minute slight hollow).

Some liquids have greater surface tension than others. Alcohol, for instance, has a considerably lower tension than water. Temperature also affects surface tension.

Quiz

for the third-year students, 5^th term (EP, ES)

Prepared by A.V.Kosobokova

Version 5 Electric Meter

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How would you measure electricity? You can weigh coal. You can count apples. You can measure milk. But it is quite different with electricity because you cannot even see it. Electricity does not weigh anything. How do you measure electricity?

Well, that was not an easy question to answer, even for the scientists who tried all kinds of ways to find a suitable arrangement. But the problem was finally solved and if you want to see how, look at your electric meter.

It does more than just measure current. It multiplies current times voltage which is not an easy thing to do when you remember that the voltage is changing from 127 volts positive to 127 volts negative and back again 50 times a sec­ond. The multiplication of current times voltage gives watts, which is a measure of the electric power.

Having the watts all figured out at any instant, the me­ter multiplies those by the length of time they are being used. This gives an answer in watt-hours, which is a measure of electric energy.

Quiz

for the third-year students, 5^th term (EP, ES)

Prepared by A.V.Kosobokova

Version 6 Galvanometer

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The most important measuring instrument is the galvano­meter. It is used to detect and measure small electric cur­rents. For the sake of simplicity it may be thought of as a d.c. motor which can rotate only part of a turn because it has no commutator. It has a very low resistance.

The current to be measured passes through a coil which is wound around a soft-iron armature turned between the poles of a permanent magnet. A pointer attached to the coil measures the rotation of the coil. A.c. cannot be used because the armature would no sooner start to rotate in one direction than the reversal of the current would start it rotating in the opposite direction. Hence it would remain stationary.

In all of the experiments in which we use an ammeter, its connection in the circuit is always in series. This is nec­essary because all the current to be measured has to pass through the ammeter. If we attempted to use a galvanometer instead of an ammeter in order to measure current, the gal­vanometer would be probably damaged.There are two reasons why we cannot use the galvano­meter directly in series.

Quiz

for the third-year students, 5^th term (EP, ES)

Prepared by A.V.Kosobokova