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Electric Charge

What is an electric charge?

The three most common elementary particles are electrons, protons and neutrons.

With the advancement in technology, we have discovered hundreds of elementary particles. Many of them are rare and of no concern to us in the present course.

Because of their mass these particles attract each other by gravitational forces. Thus, an electron attracts another electron, placed 1 cm away, with a gravitational force.

However, an electron is found to repel another electron at 1 cm with a force of 2.3 × 10 – 24 N. This extra force is called the electric force. The electric force is very large as compared to the gravitational force. The electrons must have some additional property, apart from their mass, which is responsible for the electric force. We call this property charge.

Like mass, electric charge is an intrinsic property of protons and electrons

Two types of charge have been discovered positive and negative.

A proton has a positive charge, and an electron has a negative charge. A neutron has no net electric charge.

Regarding charge the following points are worth noting:

  1. Like charges repel each other and unlike charges attract each other.
  2. Charge is a scalar and can be of two types positive or negative.
  3. Charge is quantized. The quantum of charge is e. The charge on any body will be some integral
    multiple of e, i.e. q = ± ne

Charge on any body can never be e/3, 1.5e, etc.

Apart from charge, energy, angular momentum and mass are also quantized.

The quantum of energy is hν and that of angular momentum is h/2π

Quantum of mass is not known till date.

The protons and neutrons are combination of other entities called quarks.

Quarks have charges ± e/3 and ± 2e/3. However, isolated quarks have not been observed. So, quantum of charge is still e.

Questions Based on Electric Charge

Q.1. Is attraction a true test of electrification?

Answer No

Q.2. Is repulsion a true test of electrification?

Answer Yes

Q.3. How many electrons are there in one coulomb of negative charge?

Answer 6.25 x 1018 electrons

Q.4. If we comb our hair on a dry day and bring the comb near small pieces of paper, the comb attracts the pieces, why?

Answer This is an example of frictional electricity and induction. When we comb our hair, it gets positively charged by rubbing. When the comb is brought near the pieces of paper some of the electrons accumulate at the edge of the paper piece which is closer to the comb. At the farther end of the piece there is deficiency of electrons and hence, positive charge appears there. Such a redistribution of charge in a material, due to presence of a nearby charged body is called induction. The comb exerts larger attraction on the negative charges of the paper piece as compared to the repulsion on the positive charge. This is because the negative charges are closer to the comb. Hence, there is a net attraction between the comb and the paper piece.

Q.5. Does the attraction between the comb and the piece of papers last for longer period of time?

Answer No, because the comb loses its net charge after some time. The excess charge of the comb transfers to earth through our body after some time.

Q.6. Can two similarly charged bodies attract each other?

Answer Yes, when the charge on one body (q1) is much greater than that on the other (q2) and they are close enough to each other so that force of attraction between q1 and induced charge on the other exceeds the force of repulsion between q1 and q2 . However, two similar point charges can never attract each other because no induction will take place here.

Q.7. Does in charging the mass of a body change?

Answer Yes, as charging a body means addition or removal of electrons and electron has a mass.

Q.8. Why a third hole in a socket provided for grounding?

Answer All electric appliances may end with some charge due to faulty connections. In such a situation charge will be accumulated on the appliance. When the user touches the appliance, he may get a shock. By providing the third hole for grounding all accumulated charge is discharged to the ground and the appliance is safe.

Q.9. Why does a phonograph record attract dust particles just after it is cleaned?

Answer When a phonograph record is cleaned, it develops a charge on its surface due to rubbing. This charge attracts the neutral charge particle due to induction.

Q.10. What is the total charge, in coulombs, of all the electrons in three gram mole of hydrogen atom?

Answer 2.88 x 105

Q.11. The charge on a proton is +1.6 × 10 − 19 C and that on an electron is −1⋅6 × 10 − 19 C. Does it mean that the electron has a charge 3⋅2 × 10 − 19 C less than the charge of a proton ?

Answer It is just a convention. Just because we write an electron’s charge as −1.6×10−19, it does not make the electron negative or its charge to be less than that of a proton. We could have called charge on a proton to be −1.6×10−19C and the charge on an electron to be 1.6×10−19C. It does not make any difference in physics.

Q.12. When a charged comb is brought near a small piece of paper, it attracts the piece. Does the paper become charged when the comb is brought near it ?

Answer No, The net charge on the paper remains zero.

Q.13. The number of electrons in an insulator is of the same order as the number of electrons in a conductor. What is then the basic difference between a conductor and an insulator ?

Answer The outer electrons of an atom or molecule in a conductor are only weakly bound to it and are free to move throughout the body of the material. On the other hand, in insulators, the electrons are tightly bound to their respective atoms and cannot leave their parent atoms and move through a long distance.

Q.14. Is the total charge of the universe constant?

Answer Yes

Q.15. Is the total positive charge of the universe constant?

Answer No

Q.16. Is the total negative charge of the universe constant?

Answer No

Q.17. Is the total negative charge of the universe constant?

Answer No

Q.18. Estimate the number of electrons in 100 g of water. How much is the total negative charge on these electrons ?

Answer 3.35 × 1025, 5.35 × 106 C


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