Contents
Physics Topics such as mechanics, thermodynamics, and electromagnetism are fundamental to many other scientific fields.
What are the Units of Electrical Quantities? And What are the Main Factors that Affect Resistance?
Unit of electric charge: The SI unit of charge is coulomb (C). The amount of charge that deposits 0.001118 g of sil-ver on the cathode by electrolysing silver nitrate solution is called 1 coulomb.
CGS or Gaussian unit of electric charge is esu of charge or stat- coulomb (statC).
1 C = 3 × 109 esu charge or simply, esu By the way, the charge of an electron,
e = 1.6 × 10-19 C = 4.8 × 10-10 esu
Unit of Current: The unit of current in SI is ampere (A). A current of 1A is fairly large. So smaller units are generally used; milliampere (1 mA = 10-3 A) and microampere (1 µA = 10-6 A).
CGS unit of current is esu of current or statampere (stat A).
1 A = 3 × 109 esu current or simply, esu
Another unit of current is emu of current or abamp.
1 abamp = 1 emu current = c × 1 esu current
= 3 × 1010 esu current = 10 A
Here, velocity of light in vacuum = c = 3 × 1010 cm ᐧ s-1
Unit of potential difference: The unit of electrical potential or potential difference in SI is volt (V). For very low and very high potential difference millivolt (1 mV = 10-3 V) and kilovolt (1 kV = 103V) are used respectively.
CGS unit of potential is esu of potential or statvolt (statV).
1 V = \(\frac{1}{300}\) esu potential
Another unit of potential difference is emu of potential, or abvolt.
1 abvolt = 1 emu potential = \(\frac{1}{3 \times 10^{10}}\) esu potential
= 10-8V
Unit of resistance: The unit of resistance in SI is ohm (Ω). As Ω resistance is considerably low, comparatively bigger units are often required. The units commonly used are kiloohm (1 Ω = 103 Ω) and megaohm (1 MΩ = 106Ω).
The CGS or Gaussian unit of resistance is esu of resistance or statohm (stat Ω).
1Ω = \(\frac{1 \mathrm{~V}}{1 \mathrm{~A}}\) = \(\frac{\frac{1}{300}}{3 \times 10^9}\) esu resistance
= \(\frac{1}{9}\) × 10-11 esu resistance
= 1.1 × 10-12 esu resistance
Another unit of resistance is emu of resistance or abohm.
1 abohm = 1 emu resistance \(=\frac{1 \text { emu potential }}{1 \text { emu current }}\)
= \(\frac{10^{-8} \mathrm{~V}}{10 \mathrm{~A}}\) = 10-9Ω
or, 1Ω = 109 emu resistance
International Definitions of Coulomb, Ampere, Volt and Ohm
International coulomb: The coulomb is the quantity of electricity carried in 1 s by a current of 1 A.
International ampere: 1 A is defined as the constant current which will produce an attractive force of 2 × 10-7 N per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed 1 m apart in vacuum.
International volt: 1 V is \(\frac{1}{1.01830}\) of the emf of a stan-dard Weston cadmium cell at 20 °C.
International ohm: 1Ω is the resistance of a column of mercury of length 106.3 cm , cross sectional area 1 mm2 and mass 14.4521 g kept at the melting point of ice (0°C).
In SI, ampere is a fundamental unit. All other units of electricity are derived units. Other units can be derived if the definition of ampere is known. For example,
coulomb = ampere × second; volt \(=\frac{\text { joule }}{\text { coulomb }}\) ;
ohm \(=\frac{\text { volt }}{\text { ampere }}\)
Effect Of Different Factors On Resistance
Dimension Of the conductor: The resistance offered by a conductor to the flow of current though it is more if
- the conductor is thin, and/or
- the conductor is long.
Material of the conductor: Resistance depends on the nature of the material of the conductor. Current flows easily through the metals like silver, copper, aluminium, etc. i.e., resis-tance of these metals is very low. These are called good conduc-tors. Almost all the metals are good conductors. Carbon, although a non metal, is a good conductor of electricity. Graph-ite, gas-carbon, charcoal (under high pressure) which are the allotropes of carbon are also good conductors.
Air, wood, rubber, plastic, ebonite, cloth etc.—all these non metallic substances are bad conductors of electricity and called insulators, which allow almost no current to pass through them.
It is to be noted that there are a few types of substances other than metals which may be called good conductors of electricity; examples-electrolyte, gas under low pressure, semiconductor, etc. Ordinary water is an electrolyte and so it conducts electricity, but pure water is not.
Temperature of the conductor: The resistance of the metallic conductors increases with the rise of temperature. Plat-inum resistance thermometer is constructed depending on this principle. On the other hand, when the temperature reaches close to OK, the resistance of some metallic conductors vanishes. This phenomenon is called superconductivity.
For example, the resistivity of mercury absolutely disappears below about 4.2 K. Currents created in a superconducting ring persists for several years without diminution. The phenomenon is of vast potential importance in technology. The explanation is given by the so-called BCS theory offered in 1957.
It is to be noted that the resistance of electrolyte, carbon, glass, gas maintained at a low pressure, semiconductors etc. decreases with increase of temperature.
Other factors: The following incidents happen in some special types of conductors.
i) If light is incident on selenium, its resistance decreases. Its resistance decreases further with the increase of the inten-sity of the incident light.
ii) If bismuth is placed in a magnetic field, its resistance increases. Its resistance increases further with the increase of the intensity of the magnetic field.
iii) Air bubbles exist in the pores of charcoal. As air is an insu-lator, the resistance of charcoal is high. If pressure is applied on charcoal, the bubbles within its pores will evap-orate and the particles of charcoal will come in close con-tact with each other. So, resistance of carbon decreases.
These effects have been utilised in many important practi-cal applications.
Superconductors: There are some metals or compounds whose resistance becomes zero at certain low temperature. These substances are named superconductors and that very temperature is called critical temperature. Above critical tem-perature the resistance-temperature graph of any superconduc-tor is just like that of other common metals. But at critical temperature the resistance of the superconductor suddenly comes to zero. Dutch physicist Heike Kamerlingh Onnes (1911 AD) has discovered this phenomenon. He showed that below 4.2 K temperature, mercury is a superconductor. Superconductivity can be found in different substances, e.g., tin, aluminium, various metallic alloys, highly doped semiconductors etc.