CLASS 12 PHYSICS CHAPTER 1 PHYSICS BEST NOTES ELECTRIC FIELD AND CHARGES

1




Parishram (2024)
FORMULA SHEET_(Electrostatics)

CHARGE

➢ Quantization: Charge is always in the form of an integral multiple of electronic charge and never its fraction.
q = ±ne where n is an integer and e = 1.6 × 10–19 Coulomb
= 1.6 × 10–19 C.
Charge on an electron/proton is the minimum charge.

e e 2e
➢ Particles, known as quarks, are predicted theoretically by Gell-mann. Quarks contain charges  ,  ,  .
2 3 3
Nobel prize of physics declared in 2004 refers to quarks.

➢ Millikan's oil drop experiment showed the discrete nature of charge. Charge cannot be fractional multiple of e.
Quarks now throw a challenge.

➢ Charge on an electron is –ve. e = –1.6 × 10–19 C.
Charge on a proton is +ve. e = +1.6 × 10–19 C.
Total charge = ± ne.
➢ A particle/body is positively charged because it loses electrons or it has shortage of electrons.
➢ A particle is negatively charged because it gains electrons or it has excess of electron.
➢ Conservation: The total net charge of an isolated physical system always remains
constant. Charge can neither be created nor destroyed. It can be transferred from one body to another.

Coulomb’s Inverse Square Law

1 q1q2
➢ F= . where F denotes the force between two charges q1 and q2 separated by a distance r in free space.
40 r 2
0 is a constant known as permittivity of free space. Free space is vacuum and may be deemed to be air practically.
1 newton-metre2 9 N-m
2
➢ = 9  109 = 9  10
40 coulomb2 C2
➢ If free space is replaced by a medium, then 0 is separated by (0K) or (0r) where K is known as dielectric
constant or relative permittivity or specific inductive capacity (S.I.C.) or dielectric coefficient of the
medium/material/matter. Thus,
1 q1q2 1 qq 1 qq
F= . 2 = . 1 22 = . 1 22
4 r 40 K r 40r r
 
➢ K= or  r = .
0 0
K = 1 for vacuum (or air), K =  for conductor/metal.
➢ 0 = 8.85 × 10–12 C2 N–1 m–2.

, 2




➢ Vector form of the law (q1 and q2 are like charges)
1 q1q2 1 q1q2
(i) F12 = r =
3 21
(r1 − r2 )
40 r21 40 r1 − r2 3
q1q2 r12 1 q1q2
(ii) F21 = = (r2 − r1 )
40 r123 40 r2 − r1 3
➢ If r̂12 is a unit vector pointing from q1 to q2, then
1 q1q2
(i) F12 = rˆ = force on q1 by q2.
2 21
40 r21
When q1q2 > 0 for like charges.
1 q1q2
(ii) F21 = rˆ12 = force on q2 by q1.
40 r122
When q1q2 > 0 for like charges.

Intensity/strength of electric field

➢ Intensity at a point is numerically equal to the force acting on a unit positive charge placed at the point.
➢ It is a vector quantity.
➢ The units of intensity E are NC–1, volt/metre.
➢ The dimensions of E are [MLT–1 A–1].
➢ Intensity due to a charge q at distance r
1 q
(i) E = . 2.
40 r
It acts in the direction in which a +ve charge moves.
1 q
(ii) E = . 2 , if point is in the medium.
40 K r
➢ Potential (V) and intensity (E)
dV
(i) E = − when potential varies with respect to distance.
dr
potential difference V
(ii) E = = when potential difference is constant.
distance r
(iii) Potential at a point distance r from charge q.
1 q
V= . in free space
40 r
1 q
(iv) V = . in medium
40 K r
(v) Potential is a scalar quantity
E.d r = dV
➢ From positively charged surface, E acts outwards at right angles . . along outward drawn normal.
➢ Intensity is equal to flux (number of electric lines of force) crossing unit normal area.
flux ()
E=
area ( s )