CHAPTER-21

Electric Charges

CHAPTER-21 Electric Charges

Topics to be covered: Types of electric charge Forces among two charges

(Coulomb’s law) Charge quantization Charge conservation

Ch 21-2 Electric Charges

Electric Charges: Intrinsic property of fundamental particles

Fundamental particles: electron, proton and neutron with negative, positive and zero charges respectively.

Objects : electrically neutral (containing positive and negative charges in equal number)

Electron charge =1.602x 10-19 Coulomb (C) Charged Object: Objects having net excessive

charge of specific type Negatively or positively Charged Objects:

objects containing excessive electrons or missing electrons respectively.

Ch 21-2 Electric Charges

Charging an object: Placing a charge on an object

Charging a glass rod in rubbing with silk: glass rod losses electrons- glas rod positively charged

Charging a plastic rod in rubbing with fur : plastic rod gain electrons- plastic rod negatively charged

silk

glass rod

silk

glass rod

-- --

++

++

Glass rod repels glass rod.

Similar charges repels each other

Plastic rod

attracts glass rod.

Opposite charges attracts each other

Ch 21-2 Electric Charges

Conductors: Materials through which charges can move freely

Insulators (Non-Conductors): Materials through which charges cannot move

Semiconductors: Materials having properties between conductors and insulators

Superconductors: Perfect conductors allowing charges to move without any hindrance.

Ch 21-3 Conductors and Insulators

Coulomb Law: The electrostatic force of attraction or repulsion F between two charges is:

F= kq1q2/r2

Direction of F: similar charges

(Repulsion): away from each

other. opposite charges

(Attraction): towards each other

Ch 21-4 Coulomb Law

Ch 21-4 Coulomb Law Zero Force Position

between three charges along a line:

Pair of opposite charges: it is on the side of the charges , closer to weaker charge

Pair of similar charges: it is between the charges and closer to weaker charge

Ch 21-5, 6 Charge Quantization and Charge Conservation

Charge Quantization: Objects can contain the charges which are integer multiple of elementary charge e=1.602x 10-19 C i.e charge q= ne where n is a positive or negative integer number.

Charge Conservation: Net electric charge of an isolated system is conserved and cannot change

Ch 21- 6 Charge ConservationConservation of ChargeConsider a glass rod and a piece of silk cloth (both uncharged) shown in the upper figure. If we rub the glass rod with the silk cloth we know that positive charge appears on the rod (see lower figure). At the same time an equal amount of negative charge appears on the silk cloth, so that the net rod-cloth charge is actually zero. This suggests that rubbing does not create charge but only transfers it from one body to the other, thus upsetting the electrical neutrality of each body. Charge conservation can be summarized as follows: In any process the charge at the beginning equals the charge at the end of the process.

silk

glass rod

silk

glass rod

-- --

++

++

Net charge before = Net charge after

Suggested Problems Chapter 21