Vidyarthi Academy

Home NCERT Solutions Chapter Notes Test Papers Contact Us

CBSE NCERT NOTES CLASS 12 PHYSICS CHAPTER 2

ELECTROSTATICS POTENTIAL AND CAPACITANCE

Electric potential energy

Potential energy difference

Electrostatic potential

Potential difference

Potential due to a point charge

Potential due to a system of charges

Electric potential due to a spherical shell

Electric potential of a uniformly charged sphere

Potential due to an electric dipole

Potential due to electric dipole on the dipole axis

Potential due to electric dipole in the equatorial plane

Equipotential surfaces

Relation between electric field and potential

Potential energy of a system of charges

Potential energy of a system of charges in an external field - Two charges

Potential energy of a dipole in an external field

Electrostatics of conductors

Inside a conductor, electrostatic field is zero

At the surface of a charged conductor, electrostatic field must be normal to the surface at every point.

The interior of a conductor can have no excess charge in the static situation.

Electrostatic shielding

Electric field at the surface of a charged conductor

Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface.

Dielectrics and polarisation

Polarisation in non-polar dielectric

Polarisation in polar dielectric

Factors affecting polarisation

Definition of polarisation

How does the polarised dielectric modify the original external field inside it?

Capacitors and capacitance

Effect of dielectric on capacitance

Capacitors in series

Parallel Plate capacitor

Dielectric constant

Capacitors in parallel

Energy stored in a capacitor

Capacitance of coaxial cylindrical conductors

Capacitance of concentric spherical conductors

Van de Graaff generator

CBSE NCERT NOTES CLASS 12 PHYSICS CHAPTER 2 ELECTROSTATICS POTENTIAL AND CAPACITANCE

Electric Potential Energy

Potential Energy of charge q at a point (in the presence of field due to any charge configuration) is the work done by the external force (equal and opposite to the electric force) in bringing the charge q from infinity to that point without accelerating,

We can define electric potential energy difference between two points as the work required to be done by an external force in moving (without accelerating) charge q from one point to another in electric field of any arbitrary charge configuration.

Consider an electrostatic field E due to a charge Q placed at the origin. Now, bring a test charge q from a point R to a point P against the repulsive force on it due to the charge Q. This will happen if Q and q are either positive or both negative.

Vidyarthi Academy

Let us take both Q, q > 0. Following conditions are assumed,

  1. The test charge q is so small that it does not disturb the original configuration.

  2. Second, in bringing the charge q from R to P, the applied external force Fext is just enough to counter the repulsive electric force FE (i.e, Fext= –FE). This means there is no net force on or acceleration of the charge q when it is brought from R to P, i.e., it is brought with infinitesimally slow constant speed.

Work done by the external force is the negative of the work done by the electric force, and gets fully stored in the form of potential energy of the charge q.

If the external force is removed on reaching P, the electric force will take the charge away from Q, and the stored energy (potential energy) at P is used to provide kinetic energy to the charge q in such a way that the sum of the kinetic and potential energies is constant.

Thus, work done by external force in moving a charge q from R to P is

WRP=RPFext.dr=RPFE.dr

At every point in electric field, a particle with charge q possesses a certain electrostatic potential energy, this work done increases its potential energy by an amount equal to potential energy difference between points R and P.

Thus, potential energy difference will be,

Since this displacement is in an opposite direction to the electric force, the work done by electric field is negative, i.e., –WRP.

  1. Work done by an electrostatic field in moving a charge from one point to another depends only on the initial and the final points and is independent of the path taken to go from one point to the other. This is the fundamental characteristic of a conservative force.

    Mathematically,

    E.dl=0


  2. We normally take electrostatic potential energy zero at infinity, i.e., if R is at infinity,

      W∞P = UP – U

Electrostatic Potential

Electrostatic Potential at any point is the work done in bringing a unit +ve charge from infinity to that point.

Potential Difference between two points (say R and P) is the work done in bringing a unit positive charge from point R to P

= VP  VR =UP  URq,

Potential due to a point Charge

Vidyarthi Academy

Consider a point charge Q at the origin. For Q > 0, the work done against the repulsive force on the test charge is positive. Choose a convenient path along the radial direction from infinity to the point P.

At some intermediate point P′ on the path, the electrostatic force on a unit positive charge is

Q4πεor'2r̂'

where r̂' is the unit vector along OP′.

Work done against this force from r′ to r′ + ∆r′ is

ΔW=- Q4πεor'2Δr'

The negative sign appears because for ∆r′ < 0, therefore ∆W is positive.

Total work done (W) by the external force can be obtained by integrating from r′ = ∞ to r′ = r,

W=-r Q4πεor'2dr'

This, by definition is the potential at P due to the charge Q or

V(r)= Q4πεor

We see that, potential is inversely proportional to r and electric field is inversely proportional to square of r.

V 1r

And

E 1r2. 

The graph of V and E with respect to distance r can be plotted as shown in the figure.

Vidyarthi Academy

Potential due to a System of Charges

Vidyarthi Academy

Consider a system of charges q1, q2,…, qn at disances r1P, r2P,…, rnP from a point P. The potential V1 at P due to the charge q1 is

V1= q14πεor1P

 where r1P is the distance between q1  and P.

Similarly, the potential V2 due to q2 and V3 due to q3 are given by

V2= q24πεor2P

V3= q34πεor3P

where r2P and r3P are the distances of P from charges q2 and q3, respectively; and so on for the potential due to other charges.

Since potential is related to the work done by the field, electrostatic potential also follows the superposition principle. By the superposition principle, the potential V at P due to the total charge configuration is the algebraic sum of the potentials due to the individual charges

V= 14πεoq1r1P+q2r2P+ . +qnrnP

Electric potential due to a Spherical shell

The plot of r vs V will be

Vidyarthi Academy

Electric potential of a uniformly charged sphere of radius a

Vidyarthi Academy

The electric field at B outside the sphere is, as if the entire charge is concentrated at the centre of the sphere,

E=Q4πεor2r̂

Therefore,

VB=Vr- V

The electric potential at D inside the sphere is given by

VD=Vr- V

We can plot the graph of V wrt r as follows,

Vidyarthi Academy

Potential due to an Electric Dipole

Vidyarthi Academy

Take the origin at the centre of the dipole. Since potential is related to the work done by the field, electrostatic potential also follows the superposition principle. Thus, the potential due to the dipole is the sum of potentials due to the charges q and –q

Now electric potential at P due to charge +q

V+q =kqr1

And

 V-q =-kqr2

And electric potential at P due to charge +q

V=V+q+V-q=kq1r1-1r2 

where r1 and r2 are the distances of the point P from q and –q, respectively.

Now, by geometry, we can see,

r12=r2+a2-2 ar  cos θ

and     r22=r2+a2+2 ar  cos θ

That is,

r12=r21+a2r2-2arcos θ 

And

r22=r21+a2r2+2arcos θ

If r is much greater than a ( r >> a ), we may ignore terms of higher order in ar

Using the Binomial theorem and retaining terms up to the first order in ar; we get,

1r1≅ 1r1-2ar cos θ-12

And

 1r2≅ 1r1+2ar cos θ-12

Thus

V=kq1r1-1r2

where r̂ is the unit vector along OP.

Equation is approximately true only for distances large compared to the size of the dipole, so that higher order terms in ar are negligible.

For a point dipole p at the origin,

Potential on the dipole axis (i.e. for, θ = 0, π) is given by,

V=k pr2

(Positive sign for θ = 0, negative sign for θ = π.)

The potential due to dipole in the equatorial plane (θ = π2) is zero.

  1. The potential due to a dipole depends not just on r but also on the angle between the position vector r and the dipole moment vector, p.

  2. The electric dipole potential falls off, at large distance, as 1r2, not as 1r, which is characteristic of the potential due to a single charge.

Equipotential Surfaces

An equipotential surface is a surface with a constant value of potential at all points on the surface.

For a single charge q, the potential is given by

V(r) = q4πεor

This shows that V is a constant if r is constant. Thus, equipotential surfaces of a single point charge are concentric spherical surfaces centred at the charge and electric field lines are radial, starting from the charge if q > 0.

Vidyarthi Academy

Vidyarthi Academy

Vidyarthi Academy Vidyarthi Academy

Relation between Electric Field and Potential

Vidyarthi Academy

Consider two closely spaced equipotential surfaces A and B with potential values V and V + δV, where δV is the change in V in the direction of the electric field E.

Let P be a point on the surface B. δl is the perpendicular distance of the surface A from P.

Imagine that a unit positive charge is moved along this perpendicular from the surface B to surface A against the electric field. The work done in this process is E δl.

This work equals the potential difference VA –VB. Thus,

   E= -δVδl

Since δV is negative,

So, we can write,

 E = -δVδl= +δVδl

Important conclusions

  1. Electric field is in the direction in which the potential decreases steepest.

  2. Its magnitude is given by the change in the magnitude of potential per unit displacement normal to the equipotential surface at the point.

Potential energy of a system of charges

Consider a group of charges q1, q2, q3, …, qn. Suppose, first the charge q1 is brought from infinity to the point r1. There is no external field against which work needs to be done, so work done in bringing q1 from infinity to r1 is zero. This charge produces a potential in space given by

V1= q14πεor1P

Work done in bringing charge q2 from infinity to the point r2 is q2 times the potential at r2 due to q1, that is,

Work done on q2

W= V1×q2= q1q24πεor12

where r12 is the distance between points 1 and 2.

Since electrostatic force is conservative, this work gets stored in the form of potential energy of the system. Thus, the potential energy of a system of two charges q1 and q2 is

U= q1q24πεor12

The charges q1 and q2 produce a potential, at point P, which is given by

V1,2= 14πεoq1r1P+q2r2P

Work done in bringing q3 from infinity to the point with position vector, r3 is,

Work done on q3

=q3×V1,2= 14πεoq1q3r13+q2q3r23

The total work done in assembling the charges at the given locations is,

U =14πεoq1q2r12+q1q3r13+q2q3r23

Potential energy of a system of charges in an external field – Two Charges

Consider two charges q1 and q2 with position vectors, r1 and r2 respectively, in an external field.

First, we calculate the work done in bringing the charge q1 from infinity to a point with position vector r1.

Work done in this step = q1 V(r1).

Next, we consider the work done in bringing q2 to to a point with position vector r2.

In this step, work is done not only against the external field E but also against the field due to q1.

Work done on q2 against the external field = q2 V(r2)

and work done on q2 against the field due to q1

= q1q24πεor12

where r12 is the distance between q1 and q2.

Therefore, work done in bringing q2 to r2

= q2V(r2)+q1q24πεor12

Thus, Potential energy of the system = the total work done in assembling the configuration

U = q1 V(r1)+q2V(r2)+q1q24πεor12

Potential energy of a dipole in an external field

Consider a dipole with charges q1 = +q and q2 = –q placed in a uniform electric field E.

Vidyarthi Academy

In a uniform electric field, the dipole experiences no net force; but experiences a torque τ given by,

τ = p×E,

which will tend to rotate it (unless p is parallel or antiparallel to E).

Suppose an external torque τext is applied in such a manner that it just neutralises this torque and rotates it in the plane of paper from angle θ0 to angle θ1 at an infinitesimal small angular speed and without angular acceleration.

The amount of work done by the external torque will be given by

W=θoθ1τextθ dθ

We can then associate potential energy U(θ) with an inclination θ of the dipole.

Similar to other potential energies, there is a freedom in choosing the angle where the potential energy U is taken to be zero. A natural choice is to take θ0 = π2.

Uθ=pE cosπ2- cosθ

Electrostatics of conductors

Conductors contain mobile charge carriers. In metallic conductors, these charge carriers are electrons. In a metal, the outer (valence) electrons part away from their atoms and are free to move. These electrons are free within the metal but not free to leave the metal.

In an external electric field, they drift against the direction of the field. The positive ions made up of the nuclei and the bound electrons remain held in their fixed positions. In electrolytic conductors, the charge carriers are both positive and negative ions.

  1. Inside a conductor, electrostatic field is zero

    In the static situation, the free charges are so distributed on the surface so that the electric field is zero everywhere inside.

    • Electrostatic field is zero inside a conductor.

  2. At the surface of a charged conductor, electrostatic field must be normal to the surface at every point.

    If E were not normal to the surface, it would have some non-zero component along the surface. Free charges on the surface of the conductor would then experience force and move. In the static situation, therefore, E should have no tangential component (that is it should be normal to the surface).

    1. The interior of a conductor can have no excess charge in the static situation.

      A neutral conductor has equal amounts of positive and negative charges in every small volume or surface element. When the conductor is charged, the excess charge can reside only on the surface in the static situation. This follows from the Gauss’s law.

      Consider any arbitrary volume element v inside a conductor. On the closed surface S bounding the volume element v, electrostatic field is zero. Thus the total electric flux through S is zero. Hence, by Gauss’s law, there is no net charge enclosed by S.


    2. Electrostatic potential is constant throughout the volume of the conductor and has the same value (as inside) on its surface.

      This follows from results 1 and 2 above. Since E = 0 inside the conductor and has no tangential component on the surface, no work is done in moving a small test charge within the conductor and on its surface. That is, there is no potential difference between any two points inside or on the surface of the conductor.


  3. Electric field at the surface of a charged conductor

Vidyarthi Academy

To derive the result, choose a pill box (a short cylinder) as the Gaussian surface about any point P on the surface. The pill box is partly inside and partly outside the surface of the conductor. It has a small area of cross section δS and negligible height.

Just inside the surface, the electrostatic field is zero; just outside, the field is normal to the surface with magnitude E.

Thus, the contribution to the total flux through the pill box comes only from the outside (circular) cross-section of the pill box.

Since over the small area δS, E may be considered constant and E and δS are parallel or antiparallel, the flux equals ± E δS (positive for σ > 0, negative for σ < 0),

The charge enclosed by the pill box is σ δS.

By Gauss’s law

E δS=σ δSεo 

 E=σεo  

 E=σεon̂ 

  1. Electrostatic shielding

    Consider a conductor with a cavity, with no charges inside the cavity, the electric field inside the cavity is zero, whatever be the size and shape of the cavity and whatever be the charge on the conductor and the external fields in which it might be placed.

    • The effect can be made use of in protecting sensitive instruments from outside electrical influence.

Vidyarthi Academy

Dielectrics and Polarisation

Dielectrics are non-conducting substances. They have no (or negligible number of) charge carriers. When a conductor is placed in an external electric field, the free charge carriers move and charge distribution in the conductor adjusts itself in such a way that the electric field due to induced charges opposes the external field within the conductor.

Vidyarthi Academy

In a dielectric, this free movement of charges is not possible. The external field induces dipole moment by stretching or re-orienting molecules of the dielectric. The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field. Unlike in a conductor, however, the opposing field so induced does not exactly cancel the external field. It only reduces it. The extent of the effect depends on the nature of the dielectric.

Polarisation in non-polar dielectric

Vidyarthi Academy

In an external electric field, the positive and negative charges of a nonpolar molecule are displaced in opposite directions. The displacement stops when the external force on the onstituent charges of the molecule is alanced by the restoring force (due to internal fields in the molecule).

Polarisation in polar dielectric

A dielectric with polar molecules also develops a net dipole moment in an external field, but for a different reason.

In the absence of any external field, the different permanent dipoles are oriented randomly due to thermal agitation; so the total dipole moment is zero.

When an external field is applied, the individual dipole moments tend to align with the field. When summed over all the molecules, there is then a net dipole moment in the direction of the external field, i.e., the dielectric is polarised.

Vidyarthi Academy

The factors affecting polarisation

The extent of polarisation depends on the relative strength of two mutually opposite factors:

The dielectric is thus said to be polarised in either case by the external field.

Definition of Polarisation

The dipole moment per unit volume is called polarisation and is denoted by P. For linear isotropic dielectrics, P = 𝜒eE where 𝜒e is a constant characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.

How does the polarised dielectric modify the original external field inside it?

Let us consider, for simplicity, a rectangular dielectric slab placed in a uniform external field E0 parallel to two of its faces. Every volume element Δv of the slab has a dipole moment PΔv in the direction of the field.

Anywhere inside the dielectric, the volume element Δv has no net charge (though it has net dipole moment). This is, because, the positive charge of one dipole sits close to the negative charge of the adjacent dipole.

Vidyarthi Academy

At the surfaces of the dielectric normal to the electric field, there is a net charge density, the +ve ends of the dipoles remain unneutralised at the right surface and the –ve ends at the left surface. The unbalanced charges are the induced charges due to the external field. Thus the polarised dielectric is equivalent to two charged surfaces with induced surface charge densities, say σp and –σp.

The field produced by these surface charges opposes the external field. The total field in the dielectric is, thereby, reduced from the case when no dielectric is present. We should note that the surface charge density ± σp arises from bound (not free) charges in the dielectric.

Capacitors and Capacitance

Vidyarthi Academy

A capacitor is a system of two conductors separated by an insulator.

Parallel Plate Capacitor Without Dielectric

A parallel plate capacitor consists of two large plane parallel conducting plates separated by a small distance.

Vidyarthi Academy

Let A be the area of each plate and d the separation between them. The two plates have charges Q and –Q. Since d is much smaller than the linear dimension of the plates (d2 << A), we can use the result on electric field by an infinite plane sheet of uniform surface charge density.

Plate 1 has surface charge density σ = QA and plate 2 has a surface charge density –σ. The electric field in different regions is:

Outer region I (region above the plate 1),

E =σ2εo -σ2εo = 0

Outer region II (region belo the plate 2),

E =σ2εo -σ2εo = 0

In the inner region between the plates 1 and 2, the electric fields due to the two charged plates add up, giving

E =σ2εo+σ2εo =σεo

=σAεoA

E=qεoA 

The direction of electric field is from the positive to the negative plate.

Thus, the electric field is localized between the two plates and is uniform throughout. For plates with finite area, this will not be true near the outer boundaries of the plates.

The field lines bend outward at the edges – an effect called ‘fringing of the field’.

Since electric field is uniform, potential difference between the plates is,

V = Ed =qdεoA

The capacitance C of the parallel plate capacitor is,

C=qV =εoAd

which depends only on the geometry of the capacitor and not on V or q.

Parallel Plate Capacitor With Dielectric Inserted

We have two large plates, each of area A, separated by a distance d. The charge on the plates is ± q, corresponding to the charge density ± σ (with σ = qA).

Case I

When there is vacuum between the plates

Eo =σoεo

And the potential difference Vo = Eo d

Therefore the capacitance,

Co= εoAd

Case II

Vidyarthi Academy

Consider next a dielectric inserted between the plates fully occupying the intervening region.

The dielectric is polarised by the field, the effect is equivalent to two charged sheets (at the surfaces of the dielectric normal to the field) with surface charge densities σp and –σp and net surface charge density on the plates is ± (σ – σp ).

That is,

E =σ  σpεo

And

V = E d =σ  σpεod

For linear dielectrics, σp is proportional to Eo, i.e., to σ. Thus, (σ – σp) is proportional to σ,

σ  σp=σK

where K is a constant characteristic of the dielectric.

Clearly, K >1.

That is,

V = σ εoKd= q AεoKd  

And

C=εoK Ad

ε = εo K is called the permittivity of the medium.

Relative Permittivity/Dielectric Constant

The dimensionless ratio K = εεo is called the dielectric constant or relative permittivity of the substance.

Also K = CCo K

The dielectric constant of a substance is the factor (>1) by which the capacitance increases from its vacuum value, when the dielectric is inserted fully between the plates of a capacitor.

Capacitors in series

When +ve plate of one capacitor is connected to the negative plate of another, the capacitors are said to be in series. In the series combination, charges on the two plates (± q) are the same on each capacitor. The total potential drop V across the combination is the sum of the potential drops V1 and V2 across C1 and C2, respectively.

Vidyarthi Academy

V = V1 + V2 =qC1+qC2   

We can regard the combination as an effective capacitor with charge q and potential difference V.

The effective capacitance of the combination is,

That is

VQ=1C=1C1+1C2

Capacitors in parallel

Vidyarthi Academy

In this case, the same potential difference is applied across both the capacitors. But the plate charges (± q1) on capacitor 1 and the plate charges (±q2) on the capacitor 2 are not necessarily the same:

The equivalent capacitor is one with charge q = q1 + q2 and potential difference V. ie.

Or the effective capacitance is,

If the +ve plates of both the capacitors are connected to one point and the –ve plates to another, then

V =C1V1+C2V2C1+C2

If the +ve plates of one is connected to –ve plate of another, then

V =C1V1-C2V2C1-C2

Energy Stored in a Capacitor

Consider initially two uncharged conductors 1 and 2. Imagine next a process of transferring +ve charge from conductor 2 to conductor 1 bit by bit, so that at the end, conductor 1 gets charge q. By charge conservation, conductor 2 has charge –q at the end.

Vidyarthi Academy

In transferring +ve charge from conductor 2 to conductor 1, work will be done externally, since at any stage conductor 1 is at a higher potential than conductor 2.

To calculate the total work done, we first calculate the work done in a small step involving transfer of an infinitesimal amount of charge. Consider the intermediate situation when the conductors 1 and 2 have charges q' and –q' respectively. At this stage, the potential difference V' between conductors 1 to 2 are q'/C, where C is the capacitance of the system.

Next, a small charge δq' is transferred from conductor 2 to 1. Work done in this step (δW'), resulting in charge q' on conductor 1 increasing to q'+ δq', is given by

W=V'q'=q'q'C

We can write,

 q'q' 12q'+q'2-q'2

Therefore,

W=12Cq'+q'2-q'2

Both the equations are identical because the term of second order in δq′, i.e., δq2/2C, is negligible, since δq′ is arbitrarily small.

The total work done (W) is the sum of the small work (δW) over the very large number of steps involved in building the charge q' from zero to q.

W =Sum over all stepsδW

=Sum over all steps12Cq'+q'2-q'2

= 12C(q'2-0)+2q'2-(q')2+3q'2-(2q')2 .. + q2-(q-q')2

= q22C

The same result can be obtained directly by integration

W= 0qq'C dq'=  q'22C 0q= q22C

Or Finally,

W= 12CV2=12q V=12q2C=A22×dεoA

This is the energy stored in a capacitor.

Since

E =σεo

We can write

U = 12εo E2 × A d= 12εo E2 v

Here, A d = v is the volume of the region between the plates (where alone the electric field exists).

If we define energy density u, as energy stored per unit volume of space,

u= 12εo E2

Spherical Capacitor

The capacitance for spherical or cylindrical conductors can be obtained by evaluating the voltage difference between the conductors for a given charge on each.

For a charged conducting sphere, by applying Gauss' law, the electric field outside it is,

E=q4πεor2

http://2.bp.blogspot.com/-Yl6bFbwtJOU/UOJA_hQPR-I/AAAAAAAAG9g/ZwI1Zf3hzU4/s1600/csph.gif

The voltage between the spheres can be found by integrating the electric field along a radial line:

ΔV=q4πεoab1r2 dr 

= q4πεo1a-1b

From the definition of capacitance,

C =qΔV=4πεo1a-1b

Cylindrical Capacitor

http://1.bp.blogspot.com/-m-DZKSLQQjM/UOJBbCw0sNI/AAAAAAAAG9o/hkVl1JxJ85Y/s1600/ccyl.gif

For a cylindrical geometry like a coaxial cable, the capacitance is usually stated as a capacitance per unit length. The charge resides on the outer surface of the inner conductor and the inner wall of the outer conductor.

The capacitance expression is

CL=qΔV=2π Kεolnba

Van de Graaf Generator

Defintion of Van de Graaf Generator

It is a machine capable of building up potential difference of a few million volts, and fields close to the breakdown field of air which is about 3 × 106 V/m.

Structure of Van de Graaf Generator

A Van de Graaff generator consists of a large spherical conducting shell (a few metre in diameter). By means of a moving belt and suitable brushes, charge is continuously transferred to the shell and potential difference of the order of several million volts is built up, which can be used for accelerating charged particles.

Principle of Van de Graaf Generator

Suppose we have a large spherical conducting shell of radius R, on which we place a charge Q. This charge spreads itself uniformly all over the sphere.

The field outside the sphere is just that of a point charge Q at the centre; while the field inside the sphere vanishes.

So the potential outside is that of a point charge; and inside it is constant, namely the value at the radius R. We thus have:

Potential inside conducting spherical shell of radius R carrying charge Q = constant

=Q4πεoR

Now, let us suppose that in some way we introduce a small sphere of radius r, carrying some charge q, into the large one, and place it at the centre.

Vidyarthi Academy

Potential due to small sphere of radius r carrying charge q, at the surface of small sphere = q4πεor 

Potential due to small sphere of radius R carrying charge Q, at the surface of bigger sphere = Q4πεoR

Taking both charges q and Q into account we have the total potential, at the surface of smaller sphere,

Vr= 14πεoQR+qr

And, at the surface of the bigger sphere,

VR= 14πεoQR+qR 

Vr-VR = q4πεo1r-1R

Assume now that q is positive. We see that, independent of the amount of charge Q that may have accumulated on the larger sphere and even if it is positive, the inner sphere is always at a higher potential: the difference V(r) –V(R) is positive. The potential due to Q is constant upto radius R (present in both terms) and so cancels out in the difference!

This means that if we now connect the smaller and larger sphere by a wire, the charge q on the former will immediately flow onto the matter, even though the charge Q may be quite large.

We can thus keep piling up larger and larger amount of charge on the latter. The potential at the outer sphere would also keep rising, at least until we reach the breakdown field of air.

This is the principle of the Van de Graaff generator.

Vidyarthi Academy