Page 74 - Physics 10_Float

ELECTROSTATICS

charged particles. Point to ponder
Coulomb's Law: The force of attraction or repulsion between Why leaves of charged
electroscope collapse if we
two point charges is directly proportional to the product of
touch its disk with a metal rod
the magnitude of charges and inversely proportional to the but they do not collapse if we
q q
square of the distance between them. Therefore, touch the disk with a rubber
F q
q
F 
11 ﻮ 2
........ (13.1) rod?
1
1
F
F 
r r ﻮ 2 ........ (13.2)
q 2
Combining Eqs. (13.1) and (13.2), we get
q q
F = k 1 2 2 q
r ........ (13.3) 1 F r
Eq. (13.3) is known as Coulomb’s law. Fig.13.10 (a) Attraction
where F is the force between the two charges and is called between opposite charges
the Coulomb force, q and q are the magnitudes of two
1 2 F
charges and ‘r’ is the distance between the two charges
(Fig.13.10). k is the constant of proportionality. F q q 2
1 r
The value of k depends upon the medium between the two
Fig.13.10 (b) Repulsion
charges.
between similar charges
If the medium between the two charges is air, then the value
2 -2
9
of k in SI units will be 9 ×10 N m C .
Coulomb's law is true only for point charges whose sizes are Point to ponder
very small as compared to the distance between them. On a dry day if we walk in a
Example 13.1: Two bodies are oppositely charged with carpeted room and then touch
500 C and 100 C charge. Find the force between the two µ µ some conductor we will get a
small electric shock! Can we
charges if the distance between them in air is 0.5m. tell why does it happen?
-6
Solution: Given that, r = 0.5 m, q = 500 µC = 500 × 10 C ,
1
-6
q = 100 µC = 100 × 10 C
2
Substituting these values in Eq. (13.3), we have
-2
-6
-6
9
q q 9 x 10 N m 2 C x 500 x 10 C x 100 x 10 C
F = k = 2
1
2
r 2 (0.5 m) For your information
In SI, the unit of charge is
F = 1800 N coulomb (C). It is equal to the
18
charge of 6.25 x 10 electrons.
This is very big unit. Usually,
13.5 ELECTRIC FIELD AND ELECTRIC FIELDINTENSITY charge is measured in micro
coulomb. One micro coulomb
According to Coulomb's law, if a unit positive charge q (call it -6
0 is equal to 10 C.
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