JEE Class 12 Study Rankers Physics Revision Notes offer concise, comprehensive summaries of key concepts, equations, and solved examples crucial for acing the Joint Entrance Examination (JEE). Tailored for Class 12 students, these notes streamline revision, providing a structured approach to master...
Revision Notes
Electric Charge
¾¾ Electric Charge is one of the property of a matter due to which, it experiences a force when placed in an
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electromagnetic field.
¾¾ Point Charge is an accumulation of electric charges at a point, without spatial extent.
¾¾ Electrons are the smallest and lightest fundamental particles in an atom having negative charge as these are
surrounded by invisible force field known as electrostatic field.
¾¾ Protons are larger and heavier than electrons with positive electrical charge which is similar in strength as
electrostatic field in an electron with opposite polarity.
¾¾ Two electrons or two protons will tend to repel each other as they have negative and positive electrical charges
respectively.
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¾¾ The electron and proton will get attracted towards each other due to their unlike charges.
P F F e–
¾¾ The charge present on the electron is equal and opposite to charge on the proton.
Charge of on a proton = + 1·6 × 10–19 C
and Charge of on an electron = – 1·6 × 10–19 C
Electrostatic Charge
¾¾ Electrostatic charge means the charge is at rest.
¾¾ Electrostatic charge is a fundamental quantity like length, mass, and time.
¾¾ Charge on body is expressed as q = ± ne
¾¾ The magnitude of charge is independent of the speed of the particle.
¾¾ Based on the flow of charge across them, materials are divided as :
• Conductors - allow electric charge to flow freely - metals
• Semi-conductors - behave as conductor or insulator - silicon
• Insulators - do not allow electric charge to flow freely - rubber, wood, plastic, etc.
¾¾ Net charge is given by :
• Charging by friction -charging insulators
• Charging by conduction -charging metals and other conductors
• Charging by induction -charging metals and other conductors.
Charging by Induction
¾¾ Charging by induction means charging without contact.
¾¾ On rubbing a glass rod and silk cloth piece together, glass rod gets positively charged whereas silk cloth gets
negatively charged.
Silk
Glass
,2 ] Study Rankers Revision Notes Oswaal CBSE Chapterwise & Topicwise Revision Notes, physics, Class – XII
¾¾ If a plastic rod is rubbed with wool, it becomes negatively charged.
¾¾ If a negatively charged rod is brought near neutral metal with insulator mounting, it repels free electrons and
attracts positive charges on metal.
Neutral (sphere) A negative rod nearby
separates (+)s & (–)s.
¾¾ If far end is connected to Earth by a wire, electrons will flow towards ground while positive charges are kept
captive by the rod.
Electrons
flow to the
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Earth via wire
Ground
With rod nearby the negative With rod away, the
end may be discharged. (+)s distribute evenly
Properties of Electric Charge
Addition of charges
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¾¾ If a system contains three point charges q1, q2 and q3, then the total charge of the system will be the algebraic
addition of q1, q2 and q3, i.e., charges will add up.
q = q1 + q 2 + q 3
Conservation of charges
¾¾ Electric charge is always conserved. It is the sum of positive and negative charges present in an isolated system,
which remains constant.
¾¾ Charge cannot be created and destroyed in a process, but only in positive-negative pairs.
Quantization of charges
¾¾ Electric charge is always quantized i.e., electric charge is always an integral multiple of charge ‘e’.
¾¾ Net charge qnet of an object having Ne electrons, Np protons and Nn neutrons is :
qnet = – eNe + eNp + 0Nn = e(Np – Ne) = ne
¾¾ Neutron (n) : m = 1.675 × 10–27 kg; q = 0
¾¾ Proton (p) : m = 1.673 × 10–27 kg; q = +1.6 × 10–19 C
¾¾ Electron (e): m = 9.11 × 10–31 kg; q = –1.6 × 10–19 C
Coulomb’s Law
¾¾ The force of attraction or repulsion between two point charges q1 and q2 separated by a distance r is directly
proportional to product of magnitude of charges and inversely proportional to square of distance between
charges, written as :
| q || q2 | 1 | q1 || q2 |
F=k 1 =
r2 4πε0 r2
where,
F = Force of attraction/repulsion between charges q1 & q2.
q1, q2= Magnitudes of charge 1 and charge 2
r = Distance between charges q1, q2
k = Constant whose value depends on medium where charges are kept.
1
k=
4 πε 0
1
As e = ke0, k =
4 πK ' ε0
e0 = permittivity of vacuum = 8.854 × 10–12 F/m
K' = relative permittivity of medium or dielectric constant.
¾¾ For vacuum, relative permittivity, K' = 1,
¾¾ As e = e0, force of attraction/repulsion among two electric charges q1, q2 placed in vacuum and medium is :
, Study Rankers Revision Notes
Oswaal CBSE Chapterwise & Topicwise Revision Notes, physics, Class – XII [ 3
1 . q1 q 2
F = (vacuum)
4 πε0 r 2
1 . q1q2
F= (medium)
2
4 πε0εr r
¾¾ The unit coulomb (C) is derived from the SI unit ampere (A) of the electric current.
dq dq
¾¾ Current is the rate at which charge moves past a point or through a region, i = , hence 1 C=(1 A) × (1 s).
dt dt
¾¾ The vector form of Coulomb force with r 12 = unit vector from q1 to q2 is given as :
1 . q1q2
1 q1q2
r F12
F12 = r 12 and F21 = r 21
4 πε0 r 2 4 πε0 r 2 q 2
r12
F21 = − F12
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Þ q1
F21
Principle of Superposition
¾¾ The force on any charge due to a number of other charges at rest is the vector sum of all the forces on that charge
due to the other charges, taken one at a time.
¾¾ The individual forces are unaffected due to presence of other charges.
y a F23
- + F13
q2 q3
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a
2a
q1
+ x
¾¾ force exerted by q1 on q3 = F 13
¾¾ force exerted by q2 on q3 = F 23
¾¾ net force exerted on q3 is vector sum of F 13 and F 23
Electric field
¾¾ The space around a charge up to which its electric force can be experienced
is called electric field.
¾¾ If a test charge q0 is placed at a point where electric field is E, then force on
the test charge is F = q0E
¾¾ The electric field strength due to a point source charge ‘q’ at an observation
point ‘A’ at a distance ‘r’ from the source charge is given by
1 .1 →
E= r
4 πε0 r 3
1 1
or E = ·
4 πε 0 r 2
¾¾ The unit of electric field is N/C
¾¾ Electric field inside the cavity of a charged conductor is zero.
¾¾ If a charged/uncharged conductor is placed in an external field, the field in conductor
is zero.
¾¾ In case of charged conductor, electric field is independent of the shape of conductor.
Electric field lines
¾¾ Electric field lines are imaginary lines that extend from positive charge towards
negative charge.
¾¾ Direction of electric field lines around positive charge is imagined by positive test
charge q0 located at points around source charge.
¾¾ Electric field has same direction as force on positive test charge.
¾¾ Electric field lines linked with negative charge are directed inward described by
force on positive test charge q0.
¾¾ Field lines never cross each other.
¾¾ Strength of field is encoded in density of field lines.
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