Examville.com - Electric Charge

ELECTRIC CHARGE Important concepts to remember 1. A body is said to be neutral if it has the same number of electrons and protons under normal conditions. 2. A body is said to be charged if it has either excess or deficit of electrons. If the body has excess of electrons from normal due share, it has negative charge. If the body has deficit of electrons from normal due share, it has positive charge. 3. Since a charged body has a deficiency or excess of a given number of electrons, it means that the charge on a body is whole-number multiple of the charge on an electron or proton i.e. Charge on a body, q = ±ne Where, e = charge on an electron or proton = ± 1.6 x 10-19C, and n = number of electrons or protons = 1, 2, 3 … 4. When we charge a body, no electrons are created or destroyed. Electrons are just transferred from one body to another. It means charge is conserved (Law of conservation of charge). 5. Electric charges have the following important properties: (1). Like charges repel each other and unlike charges attract each other. (2). Electric charge is quantized i.e. charge on a body is a whole number multiple of the charge on an electron or proton.

(3). Electric charge is always conserved. (4). Charges are scalar. (5). The force between charged bodies varies as the inverse square of their separation i.e. the electric force between charges obeys inverse square law. 6. The branch of physics which deals with charges at rest is called electrostatics. 7. The force between charges at rest is called electrostatic force. 8. The electrostatic force is a central force i.e. it acts along the line joining the center of the charges. It is also known as conservative force. 9. The electrostatic force (F) between two point charges q1 and q2 separated by a distance r is given by F α q1q2 / r2 F = k (q1q2/r2) where k is a constant. The value of constant k depends on the medium in which the charges are situated. This particular property of the medium which determines the value of k is known as absolute permittivity of the medium. Absolute permittivity is represented by Є. The value of k =1/4ΠЄ So, F = (1/4ΠЄ) (q1q2/r2)

10. The permittivity of free space (vacuum) is denoted by Є0. The value of Є0 is determined experimentally and is found to be Є0=8.854 x 10-12C2N-1m-2 11. The absolute permittivity of air at standard temperature and pressure is 1.0005Є0 ≈ Є0.For all practical purposes we can say that permittivity of air to be Є0. 12. The electrostatic force (F) between two point charges q1 and q2 separated by a distance r in air is given by F = (1/4ΠЄ0) (q1q2/r2) The force will be in Newton if all other quantities are expressed in their S.I units. 13. If the charges have spherical symmetry, the separation r is the distance between their centers. 14. Relative permittivity Єr of a medium is Єr= (Absolute permittivity of medium Є)/ (Permittivity of free space (Є0)) Єr = Є/Є0 Or

Є = ЄrЄ0

Єr is a number, being the ratio of two permittivities. 15. The electrostatic force (F) between two point charges q1, q2 separated by a distance r in a medium of relative permittivity Єr is F = (1/4ΠЄ0Єr) (q1q2/r2) 16. Relative permittivity of medium Єr = Fair / Fmedium Where F air = the force between two charges (q1, q2) having separation r in the air. And F medium = force between same point charges having same separation in the medium.

17. Suppose the force between two charges in air is (Fair). If air is replaced by a medium of relative permittivity Єr, then F medium = F air/Єr = Fair/k Sometimes Єr is denoted by symbol k. 18. The value of Єr of all media (other than vacuum or air) is greater than 1. 19. The main difference between the gravitational and electrostatic forces is that whereas gravitational force is always attractive, the electrostatic force may be either attractive or repulsive. It is because mass is of one kind whereas charges are of two kinds (negative or positive). ******************************************