Electrostatics Flashcards
SI unit of charge
DIMENSION
Coulomb, scalar quantity
[AT]
1 m C
1 u C
1 n C
1 p C
10^-3 C
10^-6 C
10^-9 C
10^-12 C
Properties of Charge
- scalar quantity
- is transferrable
- is associated with mass
- charge is conserved
- charge is quantised
- addictive nature of electric charge
- like charges repel and unlike charges attracts
quantisation of charge
is the property by virtue of which all free charges are integral multiple of a basic fundamental unit of charge represented by electrons
Q = +_ ne
charge of electrons
1.6 X 10^-19 C
Additive nature of electric charge
the total electric charge on a body is equal to algebraic sum of all the charges located at different points
Ques: is a charge of 5.8X10^-18 C possible?
q = +_n e
5.6X10^-18 c = n * 1.6X10^-19c
36.25 : not possible
point charges
if the size of charged bodies are very small as compared to the distance b/w them, we treat them as point charges.
Coulomb’s Law
the electrostatic force of attraction or repulsion between two static point charges is directly proportional to the product of magnitude of 2 charges and inversely proportional to the square of distance between them
value of k depends on
- nature of medium separating the charges
- system of units
value of k in free space
k = 1/4piEo
9x10^9 Nm^2C^-2
[ ML^3 T^-4 A^-2]
Eo value in freespace
8.85 X10^-12 C^2N^-1m^-2
[M^-1 L^-3T^4A^2]
relative permeability
Er = Em/ Eo
Fo/Fm = Er
QUES: the electrostatic forces of 2 charges kept at certain distance in vacuum is F. If these are immersed in liquid, the force becomes F/4 provided separated becomes same. Find relative permeability of medium
using Fo/Fm = Er
F/
F/4 == 4
Absolute permittivity of free space
the degree upto which the electric charge influenced the air/vaccum
Definition of Coulomb
1C is the charge that when placed at a distance f 1m from another charge of same magnitude in vacuum experiences an electrostatic force of attraction or repulsion of magnitude 9X10^9N
features of coulomb’s law
- valid for only stationary point
- an experimental law
- coulomb’s force is a central force
- coulomb’s force b/w 2 charge particles is mutual force
- it is conservative force
- it is attractive for unlike charge and repulsive for like charge
properties of electric field intensity
- vector quantity. direction is same as the force experienced by +ve charge
- Electric field due to +ve charge is radially outward while due to negative charge it is radially inward
- unit is N/C
DIMENSION: MLT^-3A^-1 - FORCE is in the same direction on +ve charge and in opposite direction on a negative charge
- it obeys the superposition principle
properties of electric field lines
- are continuous curves. they start from +vely charged body and end at a -vely charged body
- EFL do not form continuous loo[s
- Tangent to the EFL at any point gives the direction of electric field at that point
- no two electric field lines can intersect each other
- EFL is parallel straight and equispaced in uniform electric field
- No. of field lines give intensity of electric field
7.the EFL are always normal to the surface of a conductor, both while starting or ending on the conductor - EFL contract longitudinally, on account of attraction between unlike charges
- the EFL exerts a lateral pressure on account of repulsion b/w like charges
Why under electrostatic condition there is no EFL
bcz if E is not =O then e- inside a conductor will experience a force and move producing current which is not electrostatic
Dipole moment
is to measure strength of electric dipole
total charge and force on dipole
zero
Si unit and DIMENSION of DIPOLE MOMENT
Cm
[LAT]
lEaxl is how many times lEeql
lEaxl = 2 lEeql
compare E ax and E eq
- electric field on the axial is opposite to equatorial
- for a small dipole at same distance. the magnitude of EF on axial is twice that on the equatorial line
when E is increasing along P (non uniform EF)
- net force is along E and P
- net torque is 0
when E is decreasing along P (non uniform EF)
- net force on dipole is opposite to P
- net torque is 0
QUES) How comb attracts tiny particles when charged
comb acquires charge thru rubbing. the charged comb induces dipole moment in the direction of the field.
as a result EF due to the comb is not uniform. there acts a net force and paper moves.
Gauss’s Law
the surface integral of electric field intensity thru any closed surface in vacuum is equal to 1/Eo times the total charge enclosed within the surface
net charge within closed surface area
0
Electric field due to an infinitely long straight wire
λ/2piEor
if λ> 0
electric field at every point is radially OUT
if λ<0
electric field at every point is radially IN
Electric field due to infinite plane sheet of charge
σ/2Eo
Electric field due to a thick sheet
σ/Eo
ELECTRIC FIELD inside a spherical shell
0
electric field on surface (r=R)
σ/Eo
electric field when r>R
R^2 σ/r^2Eo
ELECTRIC FIELD inside a sphere
rρ /3Eo
