Electrostatic Potential and Capacitance
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2.21 Two charges –q and +q are located at points (0, 0, –a) and (0, 0, a), respectively.
(a) What is the electrostatic potential at the points (0, 0, z) and (x, y, 0) ?
(b) Obtain the dependence of potential on the distance r of a point from the origin when r/a >> 1.
(c) How much work is done in moving a small test charge from the point (5,0,0) to (–7,0,0) along the x-axis? Does the answer change if the path of the test charge between the same points is not along the x-axis?
2.21 Two charges –q and +q are located at points (0, 0, –a) and (0, 0, a), respectively.
(a) What is the electrostatic potential at the points (0, 0, z) and (x, y, 0) ?
(b) Obtain the dependence of potential on the distance r of a point from the origin when r/a >> 1.
(c) How much work is done in moving a small test charge from the point (5,0,0) to (–7,0,0) along the x-axis? Does the answer change if the path of the test charge between the same points is not along the x-axis?
2.21 Charge –q is located at (0,0,-a) and charge +q is located at (0,0,a). Hence, they form a dipole. Point (0,0,z) is on the axis of this dipole and point (x,y,0) is normal to the axis of the dipole. Hence electrostatic potential at point (x,y,0) is zero. Electrostatic potential at point (0,0,z) is
2.20 Two charged conducting spheres of radii a and b are connected to each other by a wire. What is the ratio of electric fields at the surfaces of the two spheres? Use the result obtained to explain why charge density on the sharp and pointed ends of a conductor is higher than on its flatter portions.
2.20 Two charged conducting spheres of radii a and b are connected to each other by a wire. What is the ratio of electric fields at the surfaces of the two spheres? Use the result obtained to explain why charge density on the sharp and pointed ends of a conductor is higher than on its flatter portions.
2.20 For Sphere A: We assume, radius = a, Charge on the sphere = , Capacitance = , Electric field =
For Sphere B: We assume, radius = b, Charge on the sphere = , Capacitance = , Electric field =
and
= ……….(
2.19 If one of the two electrons of a molecule is removed, we get a hydrogen molecular ion . In the ground state of an , the two protons are separated by roughly 1.5 Å, and the electron is roughly 1 Å from each proton. Determine the potential energy of the system. Specify your choice of the zero of potential energy.
2.19 If one of the two electrons of a molecule is removed, we get a hydrogen molecular ion . In the ground state of an , the two protons are separated by roughly 1.5 Å, and the electron is roughly 1 Å from each proton. Determine the potential energy of the system. Specify your choice of the zero of potential energy.
2.19

Let the charge in Proton 1 be = 1.6 C, in Proton 2, = 1.6 C and the charge in the electron, = 1.6 C
Distance between Proton 1 & Proton 2, = 1.5 Å = 1.5 m
Distance between Proton 1 and Electron,
2.18 In a hydrogen atom, the electron and proton are bound at a distance of about 0.53 Å:
(a) Estimate the potential energy of the system in eV, taking the zero of the potential energy at infinite separation of the electron from proton.
(b) What is the minimum work required to free the electron, given that its kinetic energy in the orbit is half the magnitude of potential energy obtained in (a)?
(c) What are the answers to (a) and (b) above if the zero of potential energy is taken at 1.06 Å separations?
2.18 In a hydrogen atom, the electron and proton are bound at a distance of about 0.53 Å:
(a) Estimate the potential energy of the system in eV, taking the zero of the potential energy at infinite separation of the electron from proton.
(b) What is the minimum work required to free the electron, given that its kinetic energy in the orbit is half the magnitude of potential energy obtained in (a)?
(c) What are the answers to (a) and (b) above if the zero of potential energy is taken at 1.06 Å separations?
2.18 The distance between electron – proton of a hydrogen atom, d = 0.53 Å = 0.53 m
Charge of an electron, = 1.6 C
Charge of a proton, = 1.6 C
Potential at infinity = 0
Potential energy of the system = Potential energy at infinity – Poten
2.17 A long charged cylinder of linear charged density is surrounded by a hollow co-axial conducting cylinder. What is the electric field in the space between the two cylinders?
2.17 A long charged cylinder of linear charged density is surrounded by a hollow co-axial conducting cylinder. What is the electric field in the space between the two cylinders?
2.17

Let charge density of the long charged cylinder, with length L and radius r be
Another cylinder of same length surrounds the previous cylinder with radius R.
Let E be the electric field produced in the space between the two cylinders.
Electric flux through the Gaussian surface is given by Gau
2.15 A spherical conducting shell of inner radius r1 and outer radius r2 has a charge Q.
(a) A charge q is placed at the centre of the shell. What is the surface charge density on the inner and outer surfaces of the shell?
(b) Is the electric field inside a cavity (with no charge) zero, even if the shell is not spherical, but has any irregular shape? Explain.
2.15 A spherical conducting shell of inner radius r1 and outer radius r2 has a charge Q.
(a) A charge q is placed at the centre of the shell. What is the surface charge density on the inner and outer surfaces of the shell?
(b) Is the electric field inside a cavity (with no charge) zero, even if the shell is not spherical, but has any irregular shape? Explain.
2.15 Charge placed at the centre of the shell is +q. Hence a charge of magnitude of –q will be induced to the inner surface of the shell. Therefore, the total charge on the inner surface of the shell is –q.
Surface charge density at the inner surface of the shell is given by the relation,
…………(1
2.14 Two tiny spheres carrying charges 1.5 μC and 2.5 μC are located 30 cm apart. Find the potential and electric field:
(a) At the mid-point of the line joining the two charges, and
(b) At a point 10 cm from this midpoint in a plane normal to the line and passing through the mid-point.
2.14 Two tiny spheres carrying charges 1.5 μC and 2.5 μC are located 30 cm apart. Find the potential and electric field:
(a) At the mid-point of the line joining the two charges, and
(b) At a point 10 cm from this midpoint in a plane normal to the line and passing through the mid-point.
2.14

Two charges are at point A and B. O is the midpoint of line joining A & B
Magnitude of charge at point A, = 1.5 = 1.5 and
at point B, = 2.5
Distance between A & B, d = 30 cm = 0.3 m
Let and be the electric potential and electric
2.13 A cube of side b has a charge q at each of its vertices. Determine the potential and electric field due to this charge array at the centre of the cube.
2.13 A cube of side b has a charge q at each of its vertices. Determine the potential and electric field due to this charge array at the centre of the cube.
2.13

Length of the side of a cube = b
Charge at each vertices= q
d = diagonal of each face, d = = b
l = length of the solid diagonal, l = = b
If r is the distance from the centre of the cube to its corner, then r = l/2 =
The electric potential (V) at the centre of t
2.12 A charge of 8 mC is located at the origin. Calculate the work done in taking a small charge of –2 × 10–9 C from a point P (0, 0, 3 cm) to a point Q (0, 4 cm, 0), via a point R (0, 6 cm, 9 cm).
2.12 A charge of 8 mC is located at the origin. Calculate the work done in taking a small charge of –2 × 10–9 C from a point P (0, 0, 3 cm) to a point Q (0, 4 cm, 0), via a point R (0, 6 cm, 9 cm).
2.12

Charge located at the origin O, q=8 mC= 8 C
Magnitude of a small charge, which is taken from point P to Q, = -2 C
Pont P is at a distance, = 3 cm = 0.03 m from origin, along Z axis
Point Q is at a distance, = 4 cm = 0.04 m from origin, along y
2.11 A 600pF capacitor is charged by a 200V supply. It is then disconnected from the supply and is connected to another uncharged 600 pF capacitor. How much electrostatic energy is lost in the process?
2.11 A 600pF capacitor is charged by a 200V supply. It is then disconnected from the supply and is connected to another uncharged 600 pF capacitor. How much electrostatic energy is lost in the process?
2.11 Capacitance of the first capacitor, C = 600 pF = 600 F
Potential difference, V = 200V
Electrostatic energy of the first capacitor, = C = 600 J
= 1.2 J
After the supply is disconnected and the first capacitor is connected t