# UPSC Physics Miscellaneous Miscellaneous : Physics

## Miscellaneous : Physics

Category : UPSC

Miscellaneous : Physics

1.           Units and Measurement

• Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit.
• The units for the fundamental or base quantities are called fundamental or base units. The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base units and derived units, is known as the system of units.
• In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS (or British) system and the MKS system were in use extensively till recently.
• The base units for length, mass and time in these systems were as follows :
• In CGS system they were centimetre, gram and second respectively.
• In FPS system they were foot, pound and second respectively.
• In MKS system they were metre, kilogram and second respectively.
• The system of units which is at present internationally accepted for measurement is the Systeme Internationale d. Unites (French for International System of Units), abbreviated as SI. The SI, with standard scheme of symbols, units and abbreviations, was developed and recommended by General Conference on Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work. Because SI units used decimal system, conversions within the system are quite simple and convenient.
• In SI, there are seven base units besides these there are two more units.
• SI Base Quantities and Units :
• Length - metre (m): The metre is the length of the path travelled by light in vacuum during a time interval of $1/299,792,458$ of a second. (1983)
• Mass - kilogram (kg): The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889)
• Time - Second (s): The second is the duration of $9,192,631,770$periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. (1967)
• Electric current - ampere (A): The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to $2\times {{10}^{-7}}$ newton per metre of length. (1948)
• Thermo Dynamic Temperature - kelvin (K) : The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967)
• Amount of Substance - mole (mol) : The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in $0.012$kilogram of carbon-12. (1971)
• Luminous intensity - candela (cd) : The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540\times {{10}^{12}}$ hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. (1979)
• Mass is a basic property of matter. It does not depend on the temperature, pressure or location of the object in space. The SI unit of mass is kilogram (kg). The prototypes of the International standard kilogram supplied by the International Bureau of Weights and Measures (BIPM) are available in many other laboratories of different countries. In India, this is available at the National Physical Laboratory (NPL), New Delhi,
• The masses of the objects, we come across in the universe, vary over a very wide range. These may vary from tiny mass of the order of ${{10}^{30}}$ kg of an electron to the huge mass of about ${{10}^{55}}$ kg of the known universe.

2.           Work and Energy

• Two conditions need to be satisfied for work to be done :
• a force should act on an object, and
• the object must be displaced. If any one of the above conditions does not exist, work is not done.
• When the force and the displacement are in the same direction, the work done by the force is taken as positive. If force, (F) is applied in the opposite direction the work done by the force is taken as negative and denoted by the minus sign.
• Kinetic energy is the energy possessed by an object due to its motion. The kinetic energy of an object increases with its speed. Kinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.
• The energy gets stored due to the work done on the object. The energy transferred to an object is stored as potential energy if it is not used to cause a change in the velocity or speed of the object.
• We stretch a rubber band. The energy transferred to the band is its potential energy. We do work while winding the key of a toy car. The energy transferred to the spring inside is stored as potential energy. The potential energy possessed by the object is the energy present in it by virtue of its position or configuration.
• An object increases its energy when raised through a height. This is because work is done on it against gravity while it is being raised. The energy present in such an object is the gravitational potential energy. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
• The form of energy can be changed from one form to another. What happens to the total energy of a system during or after the process? Whenever energy gets transformed, the total energy remains unchanged. This is the law of conservation of energy. According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation remains the same. The law of conservation of energy is valid in all situations and for all kinds of transformations.
• The unit joule is too small and hence is inconvenient to express large quantities of energy. We use a bigger unit of energy called kilowatt hour (kW h).
• Let us say we have a machine that uses 1000 J of energy every second. If this machine is used continuously for one hour, it will consume 1 kW h of energy. Thus, 1 kW h is the energy used in one hour at the rate of 1000 J ${{s}^{-i}}$ (or 1 kW).
• 1 kW h $=3.6\times {{10}^{6}}$ J.
• The energy used in households, industries and commercial establishments are usually expressed in kilowatt hour. For example, electrical energy used during a month is expressed in terms of 'units'. Here, 1 'unit' means 1 kilowatt hour.
• Thus, the meaning of work in physics is different from its usage in everyday language, No work is done if :
• the displacement is zero as seen in the example above. A weightlifter holding a 150 kg mass steadily on his shoulder for 30 s does no work on the load during this time.
• the force is zero. A block moving on a smooth horizontal table is not acted upon by a horizontal force (since there is no friction), but may undergo a large displacement.
• the force and displacement are mutually perpendicular. This is so since, for $\theta =\pi /2$rad $(=90{}^\circ )$, cos $(\pi /2)$ = 0. For the block moving on a smooth horizontal table, the gravitational force mg does no work since it acts at right angles to the displacement. If we assume that the moon's orbits around the earth is perfectly circular then the earth's gravitational force does no work. The moon's instantaneous displacement is tangential while the earth's force is radially inwards and $\theta =\pi /2$.
• The SI unit of these is joule (J), named after the famous British physicist James Prescott Joule (1811-1869).
• Power, like work and energy, is a scalar quantity. Its dimensions are $[ML2{{T}^{-3}}]$. In the SI, its unit is called a watt (W). The watt is 1 J ${{s}^{-1}}$. The unit of power is named after James Watt, one of the innovators of the steam engine in the eighteenth century.
• There is another unit of power, namely the horse-power (hp)
• 1 hp $=\text{ }746$W
• We encounter the unit watt when we buy electrical goods such as bulbs, heaters and refrigerators. A 100 watt bulb which is on for 10 hours uses 1 kilowatt hour (kWh) of energy.
• 100 (watt) x 10 (hour) $=3.6\times {{10}^{6}}$ J
• Our electricity bills carry the energy consumption in units of kWh. Note that kWh is a unit of energy and not of power.

3.           Semiconductor Electronics

• Devices in which a controlled flow of electrons can be obtained are the basic building blocks of all the electronic circuits.
• Before the discovery of transistor in 1948, such devices were mostly vacuum tubes (also called valves) like the vacuum diode which has two electrodes, viz., anode (often called plate) and cathode; triode which has three electrodes - cathode, plate and grid; tetrode and pentode (respectively with 4 and 5 electrodes).
• In a vacuum tube, the electrons are supplied by a heated cathode and the controlled flow of these electrons in vacuum is obtained by varying the voltage between its different electrodes. Vacuum is required in the inter-electrode space; otherwise the moving electrons may lose their energy on collision with the air molecules in their path.
• In these devices the electrons can flow only from the cathode to the anode (i.e., only in one direction). Therefore, such devices are generally referred to as valves.
• These vacuum tube devices are bulky, consume high power, operate generally at high voltages $\left( \sim 100\text{ }V \right)$and have limited life and low reliability. The seed of the development of modem solid-state semiconductor electronics goes back to 1930's when it was realized that some solidstate semiconductors and their junctions offer the possibility of controlling the number and the direction of flow of charge carriers through them. Simple excitations like light, heat or small applied voltage can change the number of mobile charges in a semiconductor.
• Note that the supply and flow of charge carriers in the semiconductor devices are within the solid itself, while in the earlier vacuum tubes/valves, the mobile electrons were obtained from a heated cathode and they were made to flow in an evacuated space or vacuum.
• No external heating or large evacuated space is required by the semiconductor devices. They are small in size, consume low power, operate at low voltages and have long life and high reliability. Even the Cathode Ray Tubes (CRT) used in television and computer monitors which work on the principle of vacuum tubes are being replaced by Liquid Crystal Display (LCD) monitors with supporting solid state electronics.
• Much before the full implications of the semiconductor devices was formally understood, a naturally occurring crystal of galena (Lead sulphide, PbS) with a metal point contact attached to it was used as detector of radio waves.
• Metals : They possess very low resistivity (or high conductivity).
• Semiconductors : They have resistivity or conductivity intermediate to metals and insulators.
• Insulators : They have high resistivity (or low conductivity).
• Most of the currently available semiconductor devices are based on elemental semi-conductors Si or Ge and compound inorganic semiconductors. However, after 1990, a few semiconductor devices using organic semiconductors and semiconducting polymers have been developed signalling the birth of a futuristic technology of polymerelectronics and molecular-electronics.
• LEDs have the following advantages over conventional incandescent low power lamps :
• Low operational voltage and less power.
• Fast action and no warm-up time required.
• The bandwidth of emitted light is 100 A to 500 A or in other words it is nearly (but not exactly) monochromatic.
• Long life and ruggedness.
• Fast on-off switching capability.

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