Electrostatic Potential and Capacitance 3 Flashcards
define
i) capacitor
ii) capacitance
i) Capacitor is an arrrangement of 2 parallel conductors for storing a large amount of charge and hence a large amount of electrical energy in a small place.
ii) capacitance of a conductor is the ability to store charge on it
how do you charge a capacitor
When we close the conductor,
-ve charge flows from the -ve terminal of the cell to plate B
The -ve charge on plate B induces a +ve charge on plate A, ie, the free electrons from plate A leave due to repulsion from plate B and reach the+ve terminal of the cell.
This process continues till the potential difference across the plates equal to the emf of the cell
Each plate gets a a charge q that is directly proportional to the potential diff across the plates.
what does capacitance depend upon
Q=CV
C=Q/V where C=> Capacitance
Capcitance depends upon the geometrical factors : size, shape, seperation and medium b/w 2 plates
Unit: Farad( couloumb per volt)
distinguish b/w polar and non polar molecules
Polar molecules:
Molecules where the centres of -ve and +ve charge remain seperated even in the absence of an external electric field
such molecules have a permeanent electric dipole
eg: h20, hcl
Non polar molecules:
Molecules where the centres of -ve and +ve charge coincide
such molecules do not have permanent dipole moment
eg: h2, o2
explain the behaviout of a conductor in an external electric field
When a conductor is placed in an electric field, the external field moves the free charge carriers in the direction opposite to the electric field and hennce electric field Eind ( e induced) is created in the direction opp to that of external field
This process continues till the 2 electric field cancel each other and net electric field in the conductor becomes 0.
Enet= Eind-Eo=0
explain the behaviour of dielectric in the presence of external electric field
In the presence of a dielectric, the external electric field induces a dipole moment by rearrangeing the molecules, hence there will be a net charge on the surface of the dielectric which produces a field that opposes the existing field.
The opposing field so induced does not exactly cancel the external field only reduces it.
Enet= Eo- Eind >0
explain the behaviour of non polar molecules in the presence of an external electric field
In the absence of an electric field, the dipole moment of each non polar molecule is 0
WHen placed in an external field, the -ve and +ve charges aof non polar molecules are aligned in the opp direction and hence develop an induced dipole monent
the displacement of the charges stop when the external force is balanced by the restoring force.
The directionof induced dipole moment is in the direction of the applied field and proportional to the field strength.
explain the behaviour of polar molecules in the presence of an external electric field
In the absence of external field, the molecules are oriented randomly, tand due to thermal agitation the total dipole moment is 0
In the presence of external field, the individual dipole field tend to align w/ the electric field and decelop a net dipole moment in the direction of external field.
define polarisation and explain
Both polar and non polar dielectrics develop a net dipole moment in the presence of an external field. it is called polarisation of dielectric
it is also defined as the dipole moment per unit volume.
Polarisation= dipole moment/ volume
. For
linear isotropic dielectrics,
P= χe .E (2.37)
where χe
is a constant characteristic of the dielectric and is known as the
electric susceptibility of the dielectric medium
what happens when a dielectric is placed in an external field
When a dielectric is placed in an external field, all the molecules get polarised anywhere inside the material
The +ve charge of one dipole is close the -ve charge of other dipole. They will cancel each other and volume charge density in the interior of the slab is 0.
However there is anet uncancelled +ve charge in the right side and -ve charge on the left side of the slab. This gives rise an induced electric field which is opposite to the external field. So field inside the slab decreases.
explain capacitors in series
The reciprocal of equivalent capacitance is equal to the sum of recpirocal of individual capacitance
The equivalent capacitance is smaller than any individual capacitance.
charge on each capacitor is same
the potential difference across any capacitor is inversely prop to the capacitance
explain capacitors in parallel
The equivalence capacitance is the sum of capacitance of individual capacitor
the equivalent capacitance is greater than any individual capacitance
the potential diff across each capacitor is same
The charge of capacitor is directly prop to potential diff across capacitor
energy stored in a capacitor
the chraging of a capacitor requires some energy which is drawn from a batttery.
this energy of charged capacitor is the work done to charge a capacitor.
effect of a dielectric when a battery is disconnected from capacitor
i) Charge on the capacitor plate remains the same b/c battery is removed bfr attaching the dielectric
ii)The field is reduced
E= Eo/K
iii)Potential
The potential is also reduced as field decreases
Eod=Vo
V=KVo
iv) Capacitance
Capacitance increases when the dielectric is placed
C= Q/V
v)Energy
Energy decreases.
E=1/2CV^2
effect of a dielectric when a battery remains connected
i)Charge changes as the capacitance increases, charge icnreases
ii)Potential remains the same as the battery is still connected
iii) Electric field is also a constant
iv) Capacitance increases
C=KCo
vi)Energy increases
