Page 1 of 7
CHAPTER-1 Insulators
ELECTRIC CHARGES AND FIELDS • Substances which does not allow passage
of charges.
Electrostatics - study of forces, fields and • Eg: plastic, rubber etc.
potentials due to charges at rest. • The charge transferred to an insulator
Examples for static electricity are stays at the same place.
• spark or hearing a crackle when we take Grounding or Earthing
off our synthetic clothes or sweater, • The process of sharing charges with earth.
particularly in dry weather • Earthing provides a safety measure for
• Sensation of an electric shock while electrical circuits and appliances.
opening the door of a car or holding the Methods of charging a body
iron bar of a bus after sliding from our Rubbing (charging by friction)
seat. • When two bodies are rubbed electrons
• Lightning are transferred from material with lower
• A comb rubbed with hair attracts small work function to material with higher
pieces of paper etc. work function.
Electric Charge • Work function – energy required to
• Electric charge is the physical remove an electron from a metal surface.
property of matter that causes it to • Body gains electrons- negatively charged
experience a force when placed in • Body which loses electron – positively
an electromagnetic field. charged.
• The two types of charges are positive and Effect on the mass of a body due to rubbing
negative (Named by Benjamin Franklin) • Positively charged body – mass decreases
• Like charges repels and unlike charges • Negatively charged body – mass increases
attracts. Conduction ( by direct contact)
• When amber rubbed with wool or silk • When a charged body is brought in to
cloth attracts light objects – discovered by
contact with an uncharged conductor,
Thales.
charge flows from the charged body to
• Electroscope – device for charge
the uncharged body.
detection
• This is used to charge a conductor.
• It is a scalar quantity .
Induction – without direct contact
• SI unit of electric charge- coulomb (C)
• When a charged body is brought near to
• Charge of a proton is positive
an uncharged conductor (without
(1.602192 × 10-19 C)
touching), that end of the uncharged
• Charge of an electron is negative conductor which is near to the charged
(-1.602192 × 10-19 C) body gets oppositely charged and the
• Matter with equal number of electrons farther end is charged with the same type
and protons are electrically neutral. of charge.
• Matter with excess number of electrons – Charging a metal sphere positively without
negatively charged touching it
• Matter with excess protons – positively
charged.
Conductors
• Substances which allow passage of
charges .
• Eg : Metals, human body etc
• The charge transferred to a conductor is
distributed over the entire surface of the
conductor.
, Page 2 of 7
Charging of two spheres •Solution
n= 1022 , e = 1.6 × 10-19 C, q = ne= 1.6x103 C
charge of comb = -1.6x103 C
Problem 3
• If a body gives out 109 electrons every
second, how much time is required to get
a total charge of 1C from it?
• Solution
Number of electrons in 1s = 109
Charge in 1s = ne = 109x1.6 X 10 -19
= 1.6x10 -10C
Time to get 1 C charge
= 1/(1.6x10 -10C) = 6.25 x 109 s = 198.18
years
Coulomb’s law
Point charges • The force of attraction or repulsion
• If the sizes of charged bodies are very between two stationary electric charges is
small as compared to the distances directly proportional to the product of the
between them, we treat them as point charges and inversely proportional to the
charges. square of the distance between them.
• All the charge content of the body is
assumed to be concentrated at one point
in space.
• Force between two stationary charges is
Properties of electric charges
• Charges are additive –total charge of 1 q1q2
F=
system is the sum of all charges. 4πε 0ε r r 2
Q = q1+q2+q3+ ….. • Where ε 0 -permittivity of free space, ε r -
• Charges are quantized- charge of a body
relative permittivity.
in the universe is integer multiple of a
basic charge (e). ε
• Relative permittivity is given by , ε r =
Q = ne, n- integer, e =1.6 X 10 -19 C. ε0
• The quantisation of charge was first • ε - Permittivity of the medium.
suggested by the experimental laws of •
Also ε 0 = 8.854x10-12 C2N-1m-2
electrolysis discovered by Faraday. 1
•
• It was experimentally demonstrated by Thus = 9 × 109
4πε 0
Millikan.
• Charges are conserved – the total charge Definition of coulomb
of an isolated system is a constant. • When q1 = q2 = 1 C, r = 1 m , F = 9 × 109 N
Problem 1 • 1 C is the charge that when placed at a
• How many electronic charges form 1 C of distance of 1 m from another charge of
charge? the same magnitude in vacuum
• Solution experiences an electrical force of
q=ne, n= ?, e = 1.6 × 10-19 C, repulsion of magnitude 9 × 109 N.
n= q/e = 6.25 x 1018 Coulomb’s law in vector form
Problem 2
• A comb drawn through person’s hair
causes 1022 electrons to leave the
person’s hair and stick to the comb.
Calculate the charge carried by the comb.
CHAPTER-1 Insulators
ELECTRIC CHARGES AND FIELDS • Substances which does not allow passage
of charges.
Electrostatics - study of forces, fields and • Eg: plastic, rubber etc.
potentials due to charges at rest. • The charge transferred to an insulator
Examples for static electricity are stays at the same place.
• spark or hearing a crackle when we take Grounding or Earthing
off our synthetic clothes or sweater, • The process of sharing charges with earth.
particularly in dry weather • Earthing provides a safety measure for
• Sensation of an electric shock while electrical circuits and appliances.
opening the door of a car or holding the Methods of charging a body
iron bar of a bus after sliding from our Rubbing (charging by friction)
seat. • When two bodies are rubbed electrons
• Lightning are transferred from material with lower
• A comb rubbed with hair attracts small work function to material with higher
pieces of paper etc. work function.
Electric Charge • Work function – energy required to
• Electric charge is the physical remove an electron from a metal surface.
property of matter that causes it to • Body gains electrons- negatively charged
experience a force when placed in • Body which loses electron – positively
an electromagnetic field. charged.
• The two types of charges are positive and Effect on the mass of a body due to rubbing
negative (Named by Benjamin Franklin) • Positively charged body – mass decreases
• Like charges repels and unlike charges • Negatively charged body – mass increases
attracts. Conduction ( by direct contact)
• When amber rubbed with wool or silk • When a charged body is brought in to
cloth attracts light objects – discovered by
contact with an uncharged conductor,
Thales.
charge flows from the charged body to
• Electroscope – device for charge
the uncharged body.
detection
• This is used to charge a conductor.
• It is a scalar quantity .
Induction – without direct contact
• SI unit of electric charge- coulomb (C)
• When a charged body is brought near to
• Charge of a proton is positive
an uncharged conductor (without
(1.602192 × 10-19 C)
touching), that end of the uncharged
• Charge of an electron is negative conductor which is near to the charged
(-1.602192 × 10-19 C) body gets oppositely charged and the
• Matter with equal number of electrons farther end is charged with the same type
and protons are electrically neutral. of charge.
• Matter with excess number of electrons – Charging a metal sphere positively without
negatively charged touching it
• Matter with excess protons – positively
charged.
Conductors
• Substances which allow passage of
charges .
• Eg : Metals, human body etc
• The charge transferred to a conductor is
distributed over the entire surface of the
conductor.
, Page 2 of 7
Charging of two spheres •Solution
n= 1022 , e = 1.6 × 10-19 C, q = ne= 1.6x103 C
charge of comb = -1.6x103 C
Problem 3
• If a body gives out 109 electrons every
second, how much time is required to get
a total charge of 1C from it?
• Solution
Number of electrons in 1s = 109
Charge in 1s = ne = 109x1.6 X 10 -19
= 1.6x10 -10C
Time to get 1 C charge
= 1/(1.6x10 -10C) = 6.25 x 109 s = 198.18
years
Coulomb’s law
Point charges • The force of attraction or repulsion
• If the sizes of charged bodies are very between two stationary electric charges is
small as compared to the distances directly proportional to the product of the
between them, we treat them as point charges and inversely proportional to the
charges. square of the distance between them.
• All the charge content of the body is
assumed to be concentrated at one point
in space.
• Force between two stationary charges is
Properties of electric charges
• Charges are additive –total charge of 1 q1q2
F=
system is the sum of all charges. 4πε 0ε r r 2
Q = q1+q2+q3+ ….. • Where ε 0 -permittivity of free space, ε r -
• Charges are quantized- charge of a body
relative permittivity.
in the universe is integer multiple of a
basic charge (e). ε
• Relative permittivity is given by , ε r =
Q = ne, n- integer, e =1.6 X 10 -19 C. ε0
• The quantisation of charge was first • ε - Permittivity of the medium.
suggested by the experimental laws of •
Also ε 0 = 8.854x10-12 C2N-1m-2
electrolysis discovered by Faraday. 1
•
• It was experimentally demonstrated by Thus = 9 × 109
4πε 0
Millikan.
• Charges are conserved – the total charge Definition of coulomb
of an isolated system is a constant. • When q1 = q2 = 1 C, r = 1 m , F = 9 × 109 N
Problem 1 • 1 C is the charge that when placed at a
• How many electronic charges form 1 C of distance of 1 m from another charge of
charge? the same magnitude in vacuum
• Solution experiences an electrical force of
q=ne, n= ?, e = 1.6 × 10-19 C, repulsion of magnitude 9 × 109 N.
n= q/e = 6.25 x 1018 Coulomb’s law in vector form
Problem 2
• A comb drawn through person’s hair
causes 1022 electrons to leave the
person’s hair and stick to the comb.
Calculate the charge carried by the comb.
