Electrostatics 1-Shahjahan notes

Shahjahan notes:electrostatics
Electric Charge.: ELECTROSTATICS
(1) Definition : Charge is the property associated with matter due to which it produces and experiences electrical and magnetic effects.
(2) Origin of electric charge : It is known that every atom is electrically neutral, containing as many electrons as the number of protons
in the nucleus.
Charged particles can be created by disturbing neutrality of an atom. Loss of electrons gives positive charge (as then np > ne) and gain of
electrons gives negative charge (as then ne > np) to a particle. When an object is negatively charged it gains electrons and therefore its mass
increases negligibly. Similarly, on charging a body with positive electricity its mass decreases. Change in mass of object is equal to n × me.
Where, n is the number of electrons transferred and em is the mass of electron Kg31
101.9 −
×= .
(3) Type : There exists two types of charges in nature (i) Positive charge (ii) Negative charge
Charges with the same electrical sign repel each other, and charges with opposite electrical sign attract each other.
(4) Unit and dimensional formula : Rate of flow of electric charge is called electric current i.e.,
dt
dQ
i = ⇒ idtdQ = , hence S.I.
unit of charge is – Ampere × sec = coulomb (C), smaller S.I. units are mC, µC, nC
)101,101,101( 963
CnCCCCmC −−−
=== µ . C.G.S. unit of charge is – Stat coulomb or e.s.u. Electromagnetic unit of
charge is – ab coulomb coulombabcoulombstatC
10
1
1031 9
=×= . Dimensional formula [ ]ATQ =][
Note :  Benjamin Franklin was the first to assign positive and negative sign of charge.
 The existence of two type of charges was discovered by Dufog.
 Franklin (i.e., e.s.u. of charge) is the smallest unit of charge while faraday is largest (1 Faraday =
96500 C).
 The e.s.u. of charge is also called stat coulomb or Franklin (Fr) and is related to e.m.u. of charge through the relation
10
103
chargeofesu
chargeofemu
×=
(5) Point charge : A finite size body may behave like a point charge if it produces an inverse square electric field. For example an isolated
charged sphere behave like a point charge at very large distance as well as very small distance close to it’s surface.
(6) Properties of charge
(i) Charge is transferable : If a charged body is put in contact with an uncharged body, uncharged body becomes charged due to transfer
of electrons from one body to the other.
(ii) Charge is always associated with mass, i.e., charge can not exist without mass though mass can exist without charge.
(iii) Charge is conserved : Charge can neither be created nor be destroyed. e.g. In radioactive decay the uranium nucleus (charge
e92+= ) is converted into a thorium nucleus (charge e90+= ) and emits an α -particle (charge e2+= )
4
2
234
90
238
92 HeThU +→ . Thus the total charge is e92+ both before and after the decay.
Genius shahjahan notes
MassM′
Electron < Proton
Positively charged
+
M′ < M
Electron = Proton
Neutral
MassM
Negatively charged
M′′ > M
–
Electron > Proton
+ – – –+ +

2 Electrostatics
Shahjahan notes
(iv) Invariance of charge : The numerical value of an elementary charge is independent of velocity. It is proved by the fact that an atom is
neutral. The difference in masses on an electron and a proton suggests that electrons move much faster in an atom than protons. If the charges were
dependent on velocity, the neutrality of atoms would be violated.
(v) Charge produces electric field and magnetic field : A charged particle at rest produces only electric field in the space surrounding
it. However, if the charged particle is in unaccelerated motion it produces both electric and magnetic fields. And if the motion of charged particle
is accelerated it not only produces electric and magnetic fields but also radiates energy in the space surrounding the charge in the form of
electromagnetic waves.
(vi) Charge resides on the surface of conductor : Charge resides on the outer surface of a conductor because like charges repel and try
to get as far away as possible from one another and stay at the farthest distance from each other which is outer surface of the conductor. This is
why a solid and hollow conducting sphere of same outer radius will hold maximum equal charge and a soap bubble expands on charging.
(vii) Charge leaks from sharp points : In case of conducting body no doubt charge resides on its outer surface, if surface is uniform the
charge distributes uniformly on the surface and for irregular surface the distribution of charge, i.e., charge density is not uniform. It is maximum
where the radius of curvature is minimum and vice versa. i.e., ∝σ ( )/R1 . This is why charge leaks from sharp points.
(viii) Quantization of charge : When a physical quantity can have only discrete values rather than any value, the quantity is said to be
quantised. The smallest charge that can exist in nature is the charge of an electron. If the charge of an electron ( C19
106.1 −
×= ) is taken
as elementary unit i.e. quanta of charge the charge on any body will be some integral multiple of e i.e.,
neQ ±=
with ....3,2,1=n
Charge on a body can never be e
3
2
± , e2.17± or e5
10 −
± etc.
Note :  Recently it has been discovered that elementary particles such as proton or neutron are composed of quarks having charge
( )3/1± e and ( )3/2± e. However, as quarks do not exist in free state, the quanta of charge is still e.
 Quantization of charge implies that there is a maximum permissible magnitude of charge.
Comparison of Charge and Mass.
We are familiar with role of mass in gravitation, and we have just studied some features of electric charge. We can compare the two as
shown below
Charge Mass
(1) Electric charge can be positive, negative or zero. (1) Mass of a body is a positive quantity.
(2) Charge carried by a body does not depend upon velocity of the
body.
(2) Mass of a body increases with its velocity as
Shahjahan notes
= constant
E

andB

but no Radiation
+
0=v

E
+
≠ constant
E

,B

and Radiates energy
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+ +
+
+
+
+
+
+
+
+
+
+
+

22
0
/1 cv
m
m
−
= where c is velocity of light in vacuum,
m is the mass of the body moving with velocity v and 0m is
rest mass of the body.
(3) Charge is quantized. (3) The quantization of mass is yet to be established.
(4) Electric charge is always conserved. (4) Mass is not conserved as it can be changed into energy and
vice-versa.
(5) Force between charges can be attractive or repulsive, according
as charges are unlike or like charges.
(5) The gravitational force between two masses is always
attractive.
Methods of Charging.
A body can be charged by following methods :
(1) By friction : In friction when two bodies are rubbed together, electrons are transferred from one body to the other. As a result of this
one body becomes positively charged while the other negatively charged, e.g., when a glass rod is rubbed with silk, the rod becomes positively
charged while the silk negatively. However, ebonite on rubbing with wool becomes negatively charged making the wool positively charged.
Clouds also become charged by friction. In charging by friction in accordance with conservation of charge, both positive and negative charges in
equal amounts appear simultaneously due to transfer of electrons from one body to the other.
(2) By electrostatic induction : If a charged body is brought near an uncharged body, the charged body will attract opposite charge and
repel similar charge present in the uncharged body. As a result of this one side of neutral body (closer to charged body) becomes oppositely
charged while the other is similarly charged. This process is called electrostatic induction.
Note :  Inducting body neither gains nor loses charge.
 Induced charge can be lesser or equal to inducing charge (but never greater) and its maximum value is given by






−−=
K
QQ'
1
1 where Q is the inducing charge and K is the dielectric constant of the material of the uncharged
body. Dielectric constant of different media are shown below
Medium K
Vacuum / air
Water
Mica
Glass
Metal
1
80
6
5–10
∞
 Dielectric constant of an insulator can not be ∞
 For metals in electrostatics ∞=K and so ;QQ' −= i.e. in metals induced charge is equal and opposite to inducing
charge.
–
–
–
–
–
–
–
+ + + + +
+
+
+
+
+
+
+Q
–
–
–
–
–
–
– +
+
+
+
+
+ + + + +
+
+
+
+
+
+
+Q
Q′
⇒
–
–
–
–
–
–
–
+ + + + +
+
+
+
+
+
+
+Q
⇒ ⇒
–
–
–
–
–
–
–
–
–
–
–
–
–
–––

4 Electrostatics
Shahjahan notes
(3) Charging by conduction : Take two conductors, one charged and other uncharged. Bring the conductors in contact with each other.
The charge (whether ve− or ve+ ) under its own repulsion will spread over both the conductors. Thus the conductors will be charged with
the same sign. This is called as charging by conduction (through contact).
Note :  A truck carrying explosives has a metal chain touching the ground, to conduct away the charge produced by friction.
Electroscope.
It is a simple apparatus with which the presence of electric charge on a body is detected (see figure). When metal knob is touched with a
charged body, some charge is transferred to the gold leaves, which then diverges due to repulsion. The separation gives a rough idea of the amount
of charge on the body. If a charged body brought near a charged electroscope the leaves will further diverge. If the charge on body is similar to that
on electroscope and will usually converge if opposite. If the induction effect is strong enough leaves after converging may again diverge.
(1) Uncharged electroscope
(2) Charged electroscope
Concepts
 After earthing a positively charged conductor electrons flow from earth to conductor and if a negatively charged conductor is earthed
then electrons flows from conductor to earth.
 When a charged spherical conductor placed inside a hollow insulated conductor and connected if through a fine conducting wire the
Shahjahan notes
+Q
+
–
–
–
–
–
–
–
–
e–+
+
+
+
+
+
+
+
+
e–
D
+
++
+
+
+
+
+
+
+
+ +
(A)
+
+
+
+
+
+
+
+
+
+
+
+
+
C(C)
– –– – –
– – –
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
D(B)
– –– – –
– – –
–
+
–
–
+
+
+
+
+
–
–
–
–
–
C(B)
–
+
+
–
–
–
–
–
+
+
+
+
+
–
–
–
–
–
–
D(C)
– –– – –
– – –
–
Charging by conductionCharging by conduction
D
+
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
(A)
⇒ ⇒
Uncharged
+
+
+++
+
+
+
+
+ +
+
+
+
+
+
++
+
++
Bodies in contact
+
+
+
+
+
+
+++
+
+
+
+
+ +
+
+
+
+
+
+++
+
+
+
+
+
Both are positively charged
+
+
+
+
+
+++
+
+
+
+
+
Charged
+

charge will be completely transferred from the inner conductor to the outer conductor.
 Lightening-rods arrestors are made up of conductors with one of their ends earthed while the other sharp, and protects a building from
lightening either by neutralising or conducting the charge of the cloud to the ground.
 With rise in temperature dielectric constant of liquid decreases.
 Induction takes place only in bodies (either conducting or non-conducting) and not in particles.
 If X-rays are incident on a charged electroscope, due to ionisation of air by X-rays the electroscope will get discharged and hence its
leaves will collapse. However, if the electroscope is evacuated. X-rays will cause photoelectric effect with gold and so the leaves will
further diverge if it is positively charged (or uncharged) and will converge if it is negatively charged.
 If only one charge is available than by repeating the induction process, it can be used to obtain a charge many times greater than it’s
equilibrium. (High voltage generator)
Example: 1 A soap bubble is given negative charge. Its radius will [DCE 2000; RPMT 1997; CPMT 1997; MNR
1988]
(a) Increase (b) Decrease (c) Remain unchanged (d) Fluctuate
Solution: (a) Due to repulsive force.
Example: 2 Which of the following charge is not possible
(a) C18
106.1 −
× (b) C19
106.1 −
× (c) C20
106.1 −
× (d) None of these
Solution: (c) ,106.1 20
C−
× because this is
10
1
of electronic charge and hence not an integral multiple.
Example: 3 Five balls numbered 1 to 5 balls suspended using separate threads. Pair (1,2), (2,4) and (4,1) show electrostatic attraction, while
pair (2,3) and (4,5) show repulsion. Therefore ball 1 must be [NCERT 1980]
(a) Positively charged (b) Negatively charged (c) Neutral (d) Made of metal
Solution: (c) Since 1 does not enter the list of repulsion, it is just possible that it may not be having any charge. Moreover, since ball no. 1 is
being attracted by 2 and 4 both. So 2 and 4 must be similarly charged, but it is also given that 2 and 4 also attract each other.
So 2 and 4 are certainly oppositely charged.
Since 1 is attracting 2, either 1 or 2 must be neutral but since 2 is already in the list of balls repelling each other, it
necessarily has some charge, similarly 4 must have some charge. It means that though 1 is attracting 2 and 4 it does not have
any charge.
Example: 4 If the radius of a solid and hollow copper spheres are same which one can hold greater charge
[BHU 1999; KCET 1994; IIT-JEE 1974]
(a) Solid sphere (b) Hollow sphere
(c) Both will hold equal charge (d) None of these
Solution: (c) Charge resides on the surface of conductor, since both the sphere having similar surface area so they will hold equal charge.
Example: 5 Number of electrons in one coulomb of charge will be [RPET 2001; MP PMT/PET 1998]
(a) 29
1046.5 × (b) 18
1025.6 × (c) 19
106.1 × (d) 11
109 ×
Solution: (b) By using
e
Q
nneQ =⇒=
18
19
1025.6
106.1
1
×=
×
=⇒ −
n
Example: 6 The current produced in wire when 107
electron/sec are flowing in it [CPMT 1994]
(a) 1.6 × 10–26
amp (b) 1.6 × 1012
amp (c) 1.6 × 1026
amp (d) 1.6 × 10–12
amp
Solution: (d) amp
t
ne
t
Q
i 12197
106.1106.110 −−
×=××===
+ +
+
+
+
++
+
+
+
Examples based on properties of
charge
Examples based on properties of
charge

6 Electrostatics
Shahjahan notes
Example: 7 A table-tennis ball which has been covered with a conducting paint is suspended by a silk thread so that it hangs between two
metal plates. One plate is earthed. When the other plate is connected to a high voltage generator, the ball
(a) Is attracted to the high voltage plate and stays there
(b) Hangs without moving
(c) Swings backward and forward hitting each plate in turn
(d) None of these
Solution: (c) The table tennis ball when slightly displaced say towards the positive plate gets attracted towards the positive plate due to induced
negative charge on its near surface.
The ball touches the positive plate and itself gets positively charged by the process of
conduction from the plate connected to high voltage generator. On getting positively
charged it is repelled by the positive plate and therefore the ball touches the other plate
(earthed), which has negative charge due to induction. On touching this plate, the
positive charge of the ball gets neutralized and in turn the ball shares negative charge of
the earthed plate and is again repelled from this plate also, and this process is repeated
again and again.
Here it should be understood that since the positive plate is connected to high voltage generator, its potential and hence its
charge will always remain same, as soon as this plate gives some of its charge to ball, excess charge flows from generator to
the plate, and an equal negative charge is always induced on the other plate.
In 1 gm of a solid, there are 5 × 1021
atoms. If one electron is removed from everyone of 0.01% atoms of the solid, the
charge gained by the solid is (given that electronic charge is 1.6 × 10–19
C)
(a) + 0.08 C (b) + 0.8 C (c) – 0.08 C (d) – 0.8 C
Solution: (a) To calculate charge, we will apply formula Q = ne for this, we must have number of electrons. Here, number of electrons
%01.=n of 5 × 1021
i.e.
100
01.105 21
××
=n 421
10105 −
××= = 5 × 1017
So Q = 5 × 1017
× 1.6 × 10–19
= 8 × 10–2
= 0.08 C
Since electrons have been removed, charge will be positive i.e. Q = + 0.08 C
Coulomb’s Law.
If two stationary and point charges 1Q and 2Q are kept at a distance r, then it is found that force of attraction
or repulsion between them is
2
21
r
QQ
F ∝ i.e.,
2
21
r
QkQ
F = ; (k = Proportionality constant)
(1) Dependence of k : Constant k depends upon system of units and medium between the two charges.
(i) Effect of units
(a) In C.G.S. for air ,1=k 2
21
r
QQ
F = Dyne
(b) In S.I. for air 2
2
9
0
109
4
1
C
mN
k
−
×==
πε
, 2
21
0
.
4
1
r
QQ
F
πε
= Newton (1 Newton = 105
Dyne)
Shahjahan notes
Tricky example: 1
Q2
Q1
r
+
+
+
+
+
+
+
+
+
–
–
–
–
–+
+
+
+ +

Note :  =0ε Absolute permittivity of air or free space =
2
2
12
1085.8
mN
C
−
× −






=
m
Farad
. It’s Dimension is
][ 243
ATML−
 0ε Relates with absolute magnetic permeability ( 0µ ) and velocity of light (c) according to the following relation
00
1
εµ
=c
(ii) Effect of medium
(a) When a dielectric medium is completely filled in between charges rearrangement of the charges inside the dielectric medium takes
place and the force between the same two charges decreases by a factor of K known as dielectric constant or specific inductive capacity (SIC)
of the medium, K is also called relative permittivity εr of the medium (relative means with respect to free space).
Hence in the presence of medium 2
21
0
.
4
1
r
QQ
KK
F
F air
m
πε
==
Here εεεε == rK 00 (permittivity of medium)
(b) If a dielectric medium (dielectric constant K, thickness t) is partially filled between the charges then effective air separation between
the charges becomes )( Kttr +−
Hence force 2
21
0 )(4
1
Kttr
QQ
F
+−
=
πε
(2) Vector form of coulomb’s law : Vector form of Coulomb’s law is ,ˆ.. 122
21
123
21
12 r
r
qq
Kr
r
qq
KF == where 12
ˆr is the
unit vector from first charge to second charge along the line joining the two charges.
(3) A comparative study of fundamental forces of nature
S.No. Force Nature and formula Range Relative
strength
(i) Force of gravitation
between two masses
Attractive F = Gm1m2/r2
, obey’s
Newton’s third law of motion, it’s a
conservative force
Long range (between planets and
between electron and proton)
1
(ii) Electromagnetic force (for
stationary and moving
charges)
Attractive as well as repulsive, obey’s
Newton’s third law of motion, it’s a
conservative force
Long (upto few kelometers) 37
10
(iii) Nuclear force (between
nucleons)
Exact expression is not known till
date. However in some cases empirical
formula 0/
0
rr
eU can be utilized
for nuclear potential energy 0U and
0r are constant.
Short (of the order of nuclear size
10–15
m)
1039
(strongest)
(iv) Weak force (for processes
like β decay)
Formula not known Short (upto 10–15
m) 1024
Note :  Coulombs law is not valid for moving charges because moving charges produces magnetic field also.
 Coulombs law is valid at a distance greater than .10 15
m−
Q2
Q1
r
K
Q2
Q1
K
r

8 Electrostatics
Shahjahan notes
 A charge 1Q exert some force on a second charge 2Q . If third charge 3Q is brought near, the force of 1Q exerted
on 2Q remains unchanged.
 Ratio of gravitational force and electrostatic force between (i) Two electrons is 10–43
/1. (ii) Two protons is 10–36
/1 (iii)
One proton and one electron 10–39
/1.
 Decreasing order to fundamental forces nalGravitatioWeakneticElectromagNuclear FFFF >>>
(4) Principle of superposition : According to the principle of super position, total force acting on a given charge due to number of
charges is the vector sum of the individual forces acting on that charge due to all the charges.
Consider number of charge 1Q , 2Q , 3Q …are applying force on a charge Q
Net force on Q will be
nnnet FFFFF

++++= −121 ..........
Concepts
 Two point charges separated by a distance r in vacuum and a force F acting between them. After filling a dielectric medium having
dielectric constant K completely between the charges, force between them decreases. To maintain the force as before separation between
them changes to Kr . This distance known as effective air separation.
Example: 8 Two point charges Cµ3+ and Cµ8+ repel each other with a force of 40 N. If a charge of Cµ5− is added to each of
them, then the force between them will become [SCRA 1998]
(a) N10− (b) N10+ (c) N20+ (d) N20−
Solution: (a) Initially
2
12
1083
r
kF
−
××
×= and Finally
2
12
1032
r
k'F
−
××
−= so
4
1
−=
F
'F
⇒ N'F 10−=
Example: 9 Two small balls having equal positive charge Q (coulomb) on each are suspended by two insulated string of equal length L meter,
from a hook fixed to a stand. The whole set up is taken in satellite into space where there is no gravity (state of weight less
ness). Then the angle between the string and tension in the string is
[IIT-JEE 1986]
(a) 2
2
0 )2(
.
4
1
,180
L
Qo
πε
(b) 2
2
0
.
4
1
,90
L
Q
πε

(c) 2
2
0 2
.
4
1
,180
L
Q
πε

(d) 2
0 4
.
4
1
,180
L
QLo
πε
Solution: (a) In case to weight less ness following situation arises
So angle 
180=θ and force
( )2
2
0 2
.
4
1
L
Q
F
πε
=
Shahjahan notes
r1
r2
r3
Q
Q1
Q2
Q3
Qn – 1
Qn
Examples based on Coulomb’s+Q – Q
LL
+Q+Q 180o
L
+Q
L
+Q

Example: 10 Two point charges 1 Cµ & Cµ5 are separated by a certain distance. What will be ratio of forces acting on these two
[CPMT 1979]
(a) 5:1 (b) 1:5 (c) 1:1 (d) 0
Solution: (c) Both the charges will experience same force so ratio is 1:1
Example: 11 Two charges of Cµ40 and Cµ20− are placed at a certain distance apart. They are touched and kept at the same distance.
The ratio of the initial to the final force between them is [MP PMT 2001]
(a) 1:8 (b) 1:4 (c) 1 : 8 (d) 1 : 1
Solution: (a) Since only magnitude of charges are changes that’s why 21qqF ∝ ⇒
1
8
1010
2040
21
21
2
1
=
×
×
==
q'q'
qq
F
F
Example: 12 A total charge Q is broken in two parts 1Q and 2Q and they are placed at a distance R from each other. The maximum force
of repulsion between them will occur, when [MP PET 1990]
(a)
R
Q
QQ
R
Q
Q −== 12 , (b)
3
2
,
4
12
Q
QQ
Q
Q −== (c)
4
3
,
4
12
Q
Q
Q
Q == (d)
2
,
2
21
Q
Q
Q
Q ==
Solution: (d) Force between charges 1Q and 2Q
( )
2
11
2
21
R
QQQ
k
R
QQ
kF
−
==
For F to be maximum, 0
1
=
dQ
dF
i.e.,
( ) 02
2
11
1
=







 −
R
QQQ
k
dQ
d
or
2
,02 11
Q
QQQ ==−
Hence
2
21
Q
QQ ==
Example: 13 The force between two charges 0.06m apart is 5 N. If each charge is moved towards the other by 0.01m, then the force between
them will become [SCRA 1994]
(a) 7.20 N (b) 11.25 N (c) 22.50 N (d) 45.00 N
Solution: (b) Initial separation between the charges = 0.06m
Final separation between the charges = 0.04m
Since 2
1
r
F ∝ ⇒
2
1
2
2
1






=
r
r
F
F
⇒
9
4
06.0
04.05
2
2
=





=
F
⇒ NF 25.112 =
Example: 14 Two charges equal in magnitude and opposite in polarity are placed at a certain distance apart and force acting between them is F.
If 75% charge of one is transferred to another, then the force between the charges becomes
(a)
16
F
(b)
16
9F
(c) F (d) F
16
15
Solution: (a)
Initially
2
2
r
Q
kF = Finally
16
4
.
2
2
F
r
Q
k
'F =






=
Example: 15 Three equal charges each +Q, placed at the corners of on equilateral triangle of side a what will be the force on any charge








=
04
1
πε
k [RPET 2000]
(a)
2
2
a
kQ
(b)
2
2
2
a
kQ
(c)
2
2
2
a
kQ
(d)
2
2
3
a
kQ
r
– Q+Q
A B
r
– Q/4+Q/4
A B

10 Electrostatics
Shahjahan notes
Solution: (d) Suppose net force is to be calculated on the charge which is kept at A. Two charges kept at B and C are applying force on that
particular charge, with direction as shown in the figure.
Since
2
2
a
Q
kFFF cb ===
So, 60cos222
CBCBnet FFFFF ++=
2
2
3
3
a
kQ
FFnet ==
Example: 16 Equal charges Q are placed at the four corners A, B, C, D of a square of length a. The magnitude of the force on the charge at B
will be [MP PMT 1994]
(a) 2
0
2
4
3
a
Q
πε
(b) 2
0
2
4
4
a
Q
πε
(c) 2
0
2
42
211
a
Q
πε






 +
(d)
2
0
2
42
1
2
a
Q
πε







+
Solution: (c) After following the guidelines mentioned above
DCADACnet FFFFFF ++=+= 22
Since
2
2
a
kQ
FF CA == and
2
2
)2(a
kQ
FD =






+=+=
2
1
2
2
2
2
2
2
2
2
2
a
kQ
a
kQ
a
kQ
Fnet 






 +
=
2
221
4 2
0
2
a
Q
πε
Example: 17 Two equal charges are separated by a distance d. A third charge placed on a perpendicular bisector at x distance, will experience
maximum coulomb force when [MP PMT 2002]
(a)
2
d
x = (b)
2
d
x = (c)
22
d
x = (d)
32
d
x =
Solution: (c) Suppose third charge is similar to Q and it is q
So net force on it Fnet = 2F cosθ
Where








+
=
4
.
4
1
2
20 d
x
Qq
F
πε and
4
cos
2
2 d
x
x
+
=θ
∴
2/3
2
2
0
2/1
2
2
2
20
4
4
2
44
.
4
1
2








+
=








+
×








+
×=
d
x
Qqx
d
x
x
d
x
Qq
Fnet
πε
πε
for Fnet to be maximum 0=
dx
dFnet
i.e. 0
4
4
2
2/3
2
2
0
=
























+
d
x
Qqx
dx
d
πε
or 0
4
3
4
2/5
2
22
2/3
2
2
=
















+−







+
−−
d
xx
d
x i.e.
22
d
x ±=
Example: 18 ABC is a right angle triangle in which AB = 3 cm, BC = 4 cm and
2
π
=∠ABC . The three charges +15, +12 and – 20 e.s.u. are
placed respectively on A, B and C. The force acting on B is
(a) 125 dynes (b) 35 dynes (c) 25 dynes (d) Zero
Shahjahan notes
60o
60o
A
B C
+Q+Q
+Q
FB
FC
A
D C
B
FC
FAC
FA
FD
+Q+Q
+Q
θ
B C
QQ
FF
θ
q
x
θ θ
2
d
4/
22
dx + 4/
22
dx +
A
FC
F
3 cm
4 cm
+15 esu
– 20 esu+12 esu
B C
22
CAnet FFF +=

Solution: (c) Net force on B 22
CAnet FFF +=
( )
dyneFA 20
3
1215
2
=
×
=
( )
dyneFC 15
4
2012
2
=
×
=
dyneFnet 25=
Example: 19 Five point charges each of value +Q are placed on five vertices of a regular hexagon of side L. What is the magnitude of the force
on a point charge of value – q placed at the centre of the hexagon [IIT-JEE 1992]
(a)
2
2
L
Q
k (b)
2
2
4 L
Q
k
(c) Zero (d) Information is insufficient
Solution: (a) Four charges cancels the effect of each other, so the net force on the charge placed at centre due to remaining fifth charge is
2
2
L
Q
kF =
Example: 20 Two small, identical spheres having +Q and – Q charge are kept at a certain distance. F force acts between the two. If in the
middle of two spheres, another similar sphere having +Q charge is kept, then it experience a force in magnitude and
direction as [MP PET 1996]
(a) Zero having no direction (b) 8F towards +Q charge
(c) 8F towards – Q charge (d) 4F towards +Q charge
Solution: (c) Initially, force between A and C
2
2
r
Q
kF =
When a similar sphere B having charge +Q is kept at the mid point of line joining
A and C, then Net force on B is
CAnet FFF +=
( ) ( )
F
r
kQ
r
kQ
r
Q
k 88
22 2
2
2
2
2
2
==+= . (Direction is
shown in figure)
Two equal spheres are identically charged with q units of electricity separately. When they are placed at a distance 3R
from centre-to-centre where R is the radius of either sphere the force of repulsion between them is
(a)
2
2
0
.
4
1
R
q
πε
(b)
2
2
0 9
.
4
1
R
q
πε
(c)
2
2
0 4
.
4
1
R
q
πε
(d) None of these
Solution: (a) Generally students give the answer
2
2
0 )3(4
1
R
q
πε
but it is not true. Since the charges are not uniformly distributed,
they cannot be treated as point charges and so we cannot apply coulombs law which is a law for point charges. The actual
distribution is shown in the figure above.
Electrical Field.
++
+
+
+
+
+
+
+
+
+
++
+ +
+
+
+
+
+
+
+
+
Tricky example: 2
r
A C
– Q+Q
B
r/2r/2
+Q
FA
FC
L
+Q+Q
+Q
+Q+Q
– Q

12 Electrostatics
Shahjahan notes
A positive charge or a negative charge is said to create its field around itself. If a charge 1Q exerts a force on charge 2Q placed near
it, it may be stated that since 2Q is in the field of 1Q , it experiences some force, or it may also be said that since charge 1Q is inside the
field of 2Q , it experience some force. Thus space around a charge in which another charged particle experiences a force is said to have
electrical field in it.
(1) Electric field intensity )(E

: The electric field intensity at any point is defined as the force experienced by a unit positive charge
placed at that point.
0q
F
E


=
Where 00 →q so that presence of this charge may not affect the source charge Q and its electric field is not changed, therefore
expression for electric field intensity can be better written as
0
0q q
F
LimE
0


→
=
(2) Unit and Dimensional formula : It’s S.I. unit –
metercoulomb
Joule
meter
volt
coulomb
Newton
×
== and
C.G.S. unit – Dyne/stat coulomb.
Dimension : [ E ] =[ 13 −−
AMLT ]
(3) Direction of electric field : Electric field (intensity) E

is a vector quantity. Electric field due to a positive charge is always away
from the charge and that due to a negative charge is always towards the charge
(4) Relation between electric force and electric field : In an electric field E

a charge (Q) experiences a force QEF = . If charge is
positive then force is directed in the direction of field while if charge is negative force acts on it in the opposite direction of field
(5) Super position of electric field (electric field at a point due to various charges) : The resultant electric field at any point is equal to
the vector sum of electric fields at that point due to various charges.
...321 +++= EEEE

The magnitude of the resultant of two electric fields is given by
θcos2 21
2
2
2
1 EEEEE ++= and the direction is given by
θ
θ
α
cos
sin
tan
21
2
EE
E
+
=
(6) Electric field due to continuous distribution of charge : A system of closely spaced electric
charges forms a continuous charge distribution
Continuous charge distribution
Linear charge distribution Surface charge distribution Volume charge distribution
In this distribution charge distributed on a
line.
For example : charge on a wire, charge on a
In this distribution charge distributed on the
surface.
For example : Charge on a conducting
In this distribution charge distributed in the
whole volume of the body.
For example : Non conducting charged
Shahjahan notes +
+
+
++
+
+
+
+
R
Q
Spherical shall
+
+
+
++
+
+
+
+
R
+
+ +
+
+
+
Q
Non conducting
+
+
+
+
++
+
+
+
R
Circular charged
Q
(q0
)
F

P
+Q
E2
E1
E
θ
α
+Q E

– Q
E

+Q
E
F – Q
E

ring etc. Relevant parameter is λ which is
called linear charge density i.e.,
length
charge
=λ
R
Q
π
λ
2
=
sphere, charge on a sheet etc. Relevant
parameter is σ which is called surface
charge density i.e.,
area
charge
=σ
2
4 R
Q
π
σ =
sphere. Relevant parameter is ρ which is
called volume charge density i.e.,
volume
charge
=ρ
3
3
4
R
Q
π
ρ =
To find the field of a continuous charge distribution, we divide the charge into infinitesimal charge elements. Each infinitesimal charge
element is then considered, as a point charge and electric field dE is determined due to this charge at given point. The Net field at the given
point is the summation of fields of all the elements. i.e., ∫= dEE
Electric Potential.
(1) Definition : Potential at a point in a field is defined as the amount of work done in bringing a unit positive test charge, from infinity to
that point along any arbitrary path (infinity is point of zero potential). Electric potential is a scalar quantity, it is denoted by V;
0q
W
V =
(2) Unit and dimensional formula : S. I. unit – volt
Coulomb
Joule
= C.G.S. unit – Stat volt (e.s.u.); 1 volt
300
1
= Stat volt Dimension –
][][ 132 −−
= ATMLV
(3) Types of electric potential : According to the nature of charge potential is of two types
(i) Positive potential : Due to positive charge. (ii) Negative potential : Due to negative charge.
(4) Potential of a system of point charges : Consider P is a point at which net electric potential is to be determined due to several
charges. So net potential at P
( )
...
4
4
3
3
2
2
1
1
+
−
+++=
r
Q
k
r
Q
k
r
Q
k
r
Q
kV
In general ∑=
=
X
i i
i
r
kQ
V
1
Note :  At the centre of two equal and opposite charge V = 0 but 0≠E
 At the centre of the line joining two equal and similar charge 0,0 =≠ EV
(5) Electric potential due to a continuous charge distribution : The potential due to a continuous charge distribution is the sum of
potentials of all the infinitesimal charge elements in which the distribution may be divided i.e., ∫∫ ==
r0
dQ
dVV
πε4
,
(6) Graphical representation of potential : When we move from a positive charge towards an equal negative charge along the line
joining the two then initially potential decreases in magnitude and at centre become zero, but this potential is throughout positive because when
we are nearer to positive charge, overall potential must be positive. When we move from centre towards the negative charge then though
potential remain always negative but increases in magnitude fig. (A). As one move from one charge to other when both charges are like, the
potential first decreases, at centre become minimum and then increases Fig. (B).
r1
r2
r3
P
– Q1
r4
– Q2
+ Q3
– Q4
+ q – q
V
x
Y
XO
(A)
v
x
Y
X
O
(B)
+ q + q

14 Electrostatics
Shahjahan notes
(7) Potential difference : In an electric field potential difference between two points A and B is defined as equal to the amount of work
done (by external agent) in moving a unit positive charge from point A to point B.
i.e.,
0q
W
VV AB =− in general VQW ∆= . ; =∆V Potential difference through which charge Q moves.
Electric Field and Potential Due to Various Charge Distribution.
(1) Point charge : Electric field and potential at point P due to a point charge Q is
r
r
Q
kE ˆor 2
==

2
r
Q
kE 





=
04
1
πε
k ,
r
Q
kV =
Note :  Electric field intensity and electric potential due to a point charge q, at a distance t1 + t2 where t1 is thickness of medium of
dielectric constant K1 and t2 is thickness of medium of dielectric constant K2 are :
2
2210 )(4
1
KtKt
Q
E
1 +
=
πε
;
)22104
1
KtK(t
Q
V
1 +
=
πε
(2) Line charge
(i) Straight conductor : Electric field and potential due to a charged straight conducting wire of length l and charge density λ
(a) Electric field : )sin(sin βα
λ
+=
r
k
Ex and )cos(cos αβ
λ
−=
r
k
Ey
If α = β; α
λ
sin
2
r
k
Ex = and Ey = 0
If l → ∞ i.e. α = β =
2
π
;
r
k
Ex
λ2
= and Ey = 0 so
r
Enet
02πε
λ
=
If α = 0,
2
π
β = ;
r
k
EE yx
λ
== |||| so
r
k
EEE yxnet
λ222
=+=
(b) Potential :








++
−+
=
1
1
log
2 22
22
0 lr
lr
V e
πε
λ
for infinitely long conductor crV e +
−
= log
2 0πε
λ
Shahjahan notes
r
Q P
+
+
+
+
l
+ β
α P
r
Ey
Ex

Electrostatics 1-Shahjahan notes

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