Title 16.Electric Charges and Fields-F.pdf ✅

1 | P a g e NEET-2022 Ultimate Crash Course PHYSICS Electric Charges and Fields
2 | P a g e
3 | P a g e POINTS TO REMEMBER 1. Charging an object by induction requires no contact with the body inducing charge. This is in contrast to charging by friction or by conduction which does require contact between the two objects. 2. A conductor can be charged by friction (if it is held by an insulated handle), conduction as well as by induction. 3. An insulator cannot be charged by conduction as it does not contain free electrons. It can be charged only by friction or can be polarized. 4. When an insulator is charged by rubbing (i.e., friction) only the area that is rubbed becomes charged and there is no tendency for the to move into other regions of the insulator. 5. When a charged insulator A touches an uncharged insulator B, the Charge remains on A and is not transferred to B. Charge is not shared between two insulators unless they are rubbed together so that most of their surfaces make close contact (charging by friction) 6. Charge can be shared between an a conductor but the charge will not flow off the insulator, rather it has to collected or picked up from the insulator's surface. (Charging an electroscope by contact with a charged insulator is not reliable . Further, a charged insulator cannot be discharged by touching at one point only. 7. Charge flows between conductors in contact giving them both a share of the same charge. If an uncharged conductor be made to touch a charged conductor, charge would flow to the uncharged conductor and both will indicate the presence of similar charge. Since the charged conductor shares its charge with the uncharged one, the amount of charge left on the charged conductor is evidently less than the original. 8. Coulomb used a torsion balance to perform his experiments. This device was imitated by Cavendish years later to study gravitation force. 9. Coulomb was able to show that the value of the exponent of r (in 2 F k q q / r = e 1 2 ) was 2 within an uncertainty of a few percent. Modern experiments have shown that the exponent is 2 parts in 1016 . Experiments by Williams, Faller and Hall in 1971 showed that the exponent of r in Coulomb’s law is (2 +) , where ( ) 16 2.7 3.1 10 ! −  =   10. Sometimes a net charge is the result not of electron transfer but of the transfer of either positively charged 'ions' (atoms missing or more electrons) or negatively charged ions (atoms with one or more extra electrons). 11. If electrons and protons were indestructible particles, charge conservation would be obvious. Experiments indicate, however that these particles are not indestructible. They, along with many other particles, may be created and destroyed. However, in all such experiments, there is never any creation or a destruction of net charge. 12. Approximate charge on some objects Single electron 19 10 C −  Aerosols and inkjet printers 15 10 C −  Person on insulating stand 6 10 C−  Charge transferred between storm cloud and Earth 2 4 10 10 C −  − 13. A glass rod charged by rubbing with silk possesses a charge of about ( ) 8 10nC 10 C− . Measuring the charge on the glass rod rubber with silk in coulombs is similar to measuring the thickness of this page in kilometers. 14. If the universe is considered as a whole, conservation of charge means that the net charge of the universe is constant, i.e., no physical process can result in an increase or decrease in the total amount of charge in the universe. However this law does not state that the number of electrons and protons in the universe is constant. , 15. The property which differentiates the two kinds of charges is called the polarity of the charge. 16. Any piece of matter or any particle is characterised by two independent but fundamental properties : mass and charge. 17. Since 7 2 9 e k 10 c 8.98742 10 − = =  , the coulomb is that charge which when placed one metre from an equal charge in vacuum, repels it with a force of 7 2 10 c − or 9 8.98742 10  newton’s or approximately 9 9 10 N  . 18. All charges observed in nature are equal to, or are multiples of the elementary charge, e. An elementary charge is always associated with some fixed mass. 'Electric charge is quantized' seems to be a fundamental law of nature. Until the present time, no one has found an explanation for this fact in terms of more fundamental concepts.
4 | P a g e 19. One coulomb is a very large amount of charge. Two particles, each with charge IC and separated a distance 1 m, exert a force of 9 9 10 N  on each other which is equivalent to the weight of about 15 million adult humans! The total charge of all the electrons in a copper penny is about 5 1.4 10 N  , which shows that we cannot disturb electric neutrality very much without using enormous forces. 20. A plastic comb can be charged by running it through dry hair, comb is a replacement for glass rod and the hair for the silk. 21. A material with high permittivity is one which reduces appreciably the force between two charges compared with the vacuum value. In fact, permittivity of a medium is a measure of its influence on the electrostatic force between two charges. Ironically, higher permittivity means lesser response of the medium to the electric force between two charges. 22. The use of word, insulator as a synonym for dielectric is based on Gray's demonstration [refer to (iii), Page 4]. The insulator isolates the conductor from the outside world by preventing the flow of charge onto or off the conducting body, analogous to the way a thermal insulator isolates a body from the outside world by preventing the flow of heat into or out of it (Insulator = Dielectric). 23. The Taser Stun gun, as used by some law enforcement agencies, works by generating a large electric charge separation and applying it to parts of the body, disrupting normal electrical signals and causing temporary incapacity. The stun gun needs to physically contact the body with its two electrodes and the shock can be delivered even through thick clothing. A long distance version of the laser works by firing barbed electrodes with trailing wire. 24. The electric eel (which can grow upto 6 feet in length and is actually a fish) acts electrically in a similar way to a Taser. More than 80% of the eel's body is tail, with its vital organs located behind its small head. It uses the electric field it creates for both locating prey and stunning them before eating. 25. Home purifiers use the electric force to reduce dust, bacteria and other particulates in the air. The electric force removes electrons from the pollutants, making them positively charged. These particles are attracted to negatively charged plates, where they stay until manually removed. When working properly, these purifiers can reduce the particulate level by more than 99%. 26. As 0 F q E F = , and E act in the same direction for a positive charge and in opposite directions for a negative charge. Thus, a positive charge (+q) moves in the same direction as E and a negative charge (-q) moves in a direction opposite to E as shown in figure. 27. The test charge ( ) 0 q is actually fictitious in nature as it is assumed to be infinitely small, i.e., q →0 28. When x = 0, from equation ( ) 3/2 2 2 0 1 4 qx E  R x = + , E = 0 . Does this surprise you. 29. For far off axial points, x >> R and as such R2 can be neglected as compared to x2. Thus, 2 2 0 1 4 e q q E k x x  = = ;Thus, for a far off point on the axial line, the loop behaves as a point charge at O. 30. When the point P lies very close to the disc, R >> x and as such 2 2 0 x R x → + . 0 2 E   = 31. If r is very large as compared to a, i.e., if the dipole is short, a 2 can be neglected as compared to r 2 and as such. 3 3 0 2 1 2 4 e p p E k r r  = = 32. E varies as 3 1/r in case of an electric dipole whereas in case of monopole, 2 1 E r 