Page 3 - 12-phy-12 ELECTROSTATICS

12. Electrostatics eLearn.Punjab

Now we investigate the quantitative nature of these A lightning flash
forces. The first measurement of the force between
electric charges was made in 1784 AD by Charles
Coulomb, a French military engineer. On the basis
of these measurements, he deduced a law known as
Coulomb's law. It states that

The force between two point charges is directly
proportional to the product of the magnitudes
of charges and inversely proportional to the
square of the distance between them.

It is mathematically expressed as

F ∝ q1q2 or F =k qr1q22 ........(12.1)
r2

where F is the magnitude of the mutual force that acts on each
of the two point charges q1, q2 and r is the distance between
them. The force F always acts along the line joining the two
point charges (Fig. 12.1),
k is the constant of proportionality. Its value depends upon
the nature of medium between the two charges and system
of units in which F, q and r are measured.

Fig. 12.1 : (a) Repulsive forces between like
charges and (b) attractive forces between
unlike charges

If the medium between the two point charges is
free space and the system of units is SI, then k is
represented as

k = 1 ........(12.2) Animation 12.2 : Charges
4πε 0 Source and Credit

where f0 is an electrical constant, known as

permittivity of free space. In SI units, its value is
8.85 X 10-12 C2N-1m-2 Substituting the value of f0
the constant

k = 1/4rf0 = 9 X 109 Nm2C-2

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