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Electrostatics is the branch of Physics that deals with charges at rest and their effects such as forces, fields and potentials. Electric charge – The property of matter due to which it shows electric and magnetic effects.
Charge may be fundamental property or may be acquired. Electrons and protons have fundamental charges, other bodies acquire charge. Charge is scalar quantity SI unit of charge is coulomb (C) Dimension of charge [AT] Other units of Charge cgs unit - esu or stat coulomb or franklin 1C = 3 x 10^9 esu
emu of charge 10C = 1 emu
Practical unit: 1 Faraday = 96500 C
Ampere Hour 1Ahr = 3600C Smallest unit of charge - franklin Methods of charging:
1. Charging by friction : when two objects are rubbed against each other energy is provided to remove electrons from one object to another this method of charging is known as charging by friction. ● Electrons are transferred from a body of lower work function to a body of higher work function.
Cause of charging by friction – Charging by friction is due to actual transfer of electrons from one body to another. Positive Charge Negative charge Glass rod Silk Wool Plastic rod Dry hair Comb
● The body which loses electrons acquires positive charge and the body which gains electrons acquires negative charge.
With transfer of charge there occurs transfers of mass as well. Charging an Insulator and a Conductor: Conductor Insulator
● Charging by conduction : When a charged body is brought in contact with an uncharged body some of the charge is transferred from the charged body to the uncharged body, this is called charging by conduction.
Both the bodies acquire the same kind of charge.
If both the bodies brought in contact were charged then their final nature depends on the sum of two charges.
For identical bodies charge gets equally distributed between them.
When a charged body is brought near an uncharged body. Opposite charge gets induced at the near end and a similar charge is induced at the far end. The force of attraction overweighs the force of repulsion. As a result a neutral body gets attracted towards a charged body. Q. Why is repulsion considered a sure test of presence of charge? Ans – Attraction is possible between a neutral body and a charged body but repulsion is possible only between two likely charged bodies, hence repulsion is considered a sure test of presence of charge. Q. How can you charge a metal sphere positively without touching it?
We Place an uncharged metallic sphere on an insulating metal stand. We bring a negatively charged rod close to the metallic sphere. The free electrons in the sphere are repelled to the farther end. The near end becomes positively charged. We connect the sphere to the ground by a conducting wire. The electrons flow to the ground while the positive charges at the near end remain there due to the attractive force of the negative charges on the rod. We disconnect the sphere from the ground. The positive charge continues to be held at the near end.
Finally we remove the electrified rod. The positive charge will spread uniformly over the sphere.
Consider two metallic spheres A and B placed on insulating stands. Let both spheres be in contact as shown in Fig. What happens when (i) a positively charged glass rod is brought near sphere A taking care that the rod does not touch the sphere. (ii) spheres A and B are separated keeping the positively charged glass rod near the sphere A. (iii) the glass rod is removed away from the spheres.
Gold-leaf electroscope It is an instrument which is used to detect the presence of charge. It consists of a vertical metal rod placed in a box, with two thin gold leaves attached to its bottom end. Case 1: Uncharged Electroscope
Charged object touches the metal knob Charged object is brought near the metal knob Charge flows on to the leaves and they diverge.
Opposite charges appear on the knob and similar charges appear on the leaves and hence they diverge.
There will be no divergence if the body is uncharged. The degree of divergence is an indicator of the amount of charge.
- If the motion of a charged particle is accelerated, it not only produces electric and magnetic fields but also radiates energy. Quarks Quarks are the subatomic particles which are constituents of protons and neutrons. Up Quark: +2/3e Down Quark: -1/3e Proton : uud Neutron: udd
Comparison between charge and mass. Charge Mass Can be positive negative or zero only positive Quantised Quantisation not established. Invariant of speed Relativistic Conserved Mass converts to energy Charge can’t exist without mass but mass may exist without charge. Specific Charge - ratio of charge and mass = q/m. On Increasing the speed of a charge its specific charge______? [mass increases, q/m decreases] Q. Why can we ignore quantization of charge for macroscopic objects? Ans - The magnitude of charge on an electron is too small to be compared with the magnitude of charge involved in the case of electrically charged large objects. Problems For Practice:
- When an ebonite rod is rubbed with fur, the ebonite rod is found to have a negative charge of 3. x 10–8^ C. How much mass is transferred from the fur to ebonite? Mass of electron = 9.1 x 10–31^ kg. [Ans - 1.82 x 10–19^ kg.]
- If 10^9 electrons move out of a body to another body every second, how much time is required to get a total charge of 1 C on the other body?
- How much positive and negative charge is there in 180g of water?
- A system containing six charges + 1 μC, + 2 μC, – 3 μC, + 4 μC, –5 μC and 6 μC. The total charge of the system is (1) 5 μC (2) –5 μC (3) Zero (4) 2 μC
- The number of electrons present in 2 coulomb of charge are:
(1) 1.25 × 10^17 (2) 1.25 × 10^19 (3) 6.25 × 10^18 (4) 6.25 × 10^19
- When a soap Bubble is charged its size _________?[Increases]
- The figure shows five pairs of plates: A, B, and D are charged plastic plates and C is an electrically neutral copper plate. The electrostatic forces between the pairs of plates are shown for three of the pairs. For the remaining two pairs, do the plates repel or attract each other?
[Ans: C and D attract; B and D attract]
- A glass rod is rubbed with silk cloth. The glass rod acquires a charge of +19.2 x 10-19C. (a) Find the number of electrons lost by a glass rod? [Ans: 12] (b) Find the negative charge acquired by the silk. [- 19.2 x 10-19C] (c) Is there a transfer of mass from glass to silk?[Yes: 1.08 x 10-29^ kg] (me = 9 x 10-31kg)
- If A is rubbed with B, then A becomes positively charged and B becomes negatively charged. If A is rubbed against C, then A becomes negatively charged. What will happen if B is rubbed against C. [Ans - C positive and B negative]
- Object A, B, and C are three identical, insulated, spherical conductors. Originally A and B have charges of +3 mC whereas C has a charge of -6mC. A and C are touched and moved apart. Object B and C are touched and moved apart. (1) If A and B are now held near each other they will: (a) attract (b) repel (c) have no effect on each other. (1) If A and C are now held near each other they will: (a) attract (b) repel (c) have no effect on each other.
- Which of the following charge is possible
a. 1. 6 × 10−18C b. 8 × 10−
c. 1. 6 × 10−20C d. 1C
- How many electrons must be removed from a piece of metal so as to leave it with a positive charge of 3.2 × 10−17^ coulomb? [Ans - 200]
- A copper penny has a mass of 32 g. Being electrically neutral, it contains equal amounts of positive and negative charges. What is the magnitude of these charges inμC. A copper atom has a positive nuclear charge of 3 × 10−26^ C. Atomic weight of copper is 64g/mole and Avogadro’s number is 6 × 10 –26^ atoms/mole. [Ans - 9μC]
- Two identical metallic spherical shells A and B having charges +4Q and -10Q are kept a certain distance apart. A third identical uncharged sphere C is first placed in contact with sphere A and then with sphere B, then spheres A and B are brought in contact and then separated. Find the charge on the spheres A and B.
