Download 7. Two fixed charges +4q and +q are kept at

electrostatics
1. Why should electrostatic field be zero inside a conductor?
2. Why must electrostatic field be normal to the surface at every point of a charged
conductor?
3. Why is the potential inside a hollow spherical charged conductor constant and has the
same value as on its surface?
4. Why is there no work done in moving a charge from one point to another on an
equipotential surface?
5. A point charge Q is placed at point O as shown in the figure. Is the potential difference
VA – VB positive, negative or zero, if Q is (i) positive (ii) negative?
o---------------------o-----------o
O
A
B
6. Two point charges having equal charges separated by 1m distance experience a force
of 8N. what will be the force experienced by them, if they are held in water, at the same
distance? (Given : Kwater = 80 )
7. If the radius of the Gaussian surface enclosing a charge is halved, how does the electric
flux through the Gaussian surface change?
8. Net capacitance of three identical capacitors in series is 1 μF. What will be their net
capacitance if connected in parallel? Find the ratio of energy stored in the two
configurations if they are both connected to the same source.
9. Suppose the electric field lines in a region of space are straight lines. If a charged particle
is released from rest in that region, will the trajectory of the particle be along a field
line?
10. An electric dipole is placed in a region of uniform electric field E , with the electric
dipole moment P , pointing in the direction opposite to E. Is the dipole (i) in stable
equilibrium , (ii) in unstable equilibrium , or (iii) neither?
11. A proton is placed in a uniform electric field and then released. Then an electron is
placed at this same point and released. Do these two particles experience the same
force? The same acceleration ? do they move in the same direction when released?
12. At a distance R from a point charge its electric field is Eo . At what distance (in terms of
R) from the point charge would the electric field be ⅓ Eo
13. A charge Q μc is placed at the centre of a cube. What is the electric flux coming out from
any one surface
14. Why is it necessary that the field lines from a point charge placed in the vicinity of a
conductor must be normal to the surface of the conductor at every point ?
15. Electrostatic potential is constant throughout the volume of the conductor and has the
same value on its surface. Why ?
16. If the electric potential at a certain point is zero, does the electric field at that point have
to be zero?
17. If the electric field at a certain point is zero, does the electric potential at that point have
to be zero?
18. Draw equipotential surface for (i) A single positive charge (ii) An electric dipole (iii)
Two equal positive charge .
19. You want to connect a 4μF capacitor and an 8μF capacitor. With which type of
connection will the 4μF capacitor have greater (i) Potential difference (ii) charge (iii)
stored energy than the 8μF capacitor?
20. The space between the plates of an isolated parallel plate capacitor is filled by a slab of
dielectric with dielectric constant K. the two plates of the capacitor have charges Q and
–Q . You pull out the dielectric slab. If the charges do not change, how does the energy
in the capacitor change when you remove the slab?
21. A parallel plate capacitor is charged by being connected to a battery and is kept
connected to the battery. The separation between the plates is then doubled. How does
the electric field change? The charge on the plates? The total energy? Explain your
reasoning.
22. A parallel plate is capacitor is charged by being connected to a battery and is then
disconnected from the battery. The separation between the plates is then doubled. How
does the electric field change? The potential difference? The total energy? Explain your
reasoning.
23. A parallel plate capacitor is connected to a power supply that maintains is fixed
potential difference between the plates . (a) If a sheet of dielectric is then slid between
the plates, what happens to (i) the electric field between the plates (ii) the magnitude
of charge on each plate, and (iii) the energy stored in the capacitor ? (b) Now suppose
that before the dielectric is inserted, the charged capacitor is disconnected from the
power supply. In this case, what happens to (i) the electric filed between the plates ,
(ii) the magnitude of charge on each plate (iii) the energy stored in the capacitor?
NUMERICALS
1. Two point charges, q1 = 10x 10-8C and q2 = - 2 x 10-8C are separated by a distance of
60 cm in air.
(i)
(ii)
2.
3.
4.
Find at what distance from the 1st charge, q1, would the electric potential be
zero.
Also calculate the electrostatic potential energy of the system.
Two point charges 4Q, Q are separated by 1m in air. At what point on the line
joining the charges in the electric field intensity zero ? also calculate the electrostatic
potential energy of the system of charges, taking the value of charge,
Q = 2 x 10-7 C.
A parallel plate capacitor with air between the plates has a capacitance of 8pF. What
will be the capacitance if the distance between the plates be reduced by half and the
space between them is filled with a substance of dielectric constant K=6?
Three point charges of +2 μC are kept at the vertices A, B and C respectively of an
equilateral triangle of side 20cm as shown in the fig. what should be the sign and
magnitude of the charge to be placed at the mid-point (M) of side BC so that the
charge at A remains in equilibrium ?
2μC
A
B
mm
B C
20cm
-3μC
-3μC
5. Two parallel plate capacitor, X and Y, have the same area of plates and same
separation between them. X has air between the plates while Y contains a dielectric
medium of ϵr = 4
X
Y
+
-
12V
(i)
Calculate capacitance of each capacitor if equivalent capacitance of the combination is 4μF.
(ii)
(iii)
Calculate the potential difference between the plates of X and Y.
What is the ratio of electrostatic energy stored in X and Y?
6.
Electric field intensity at point 'Bʻ due to a point charge 'Qʻ kept at point A is 24 NC-1 and
the electric potential at point B due to same charge is 12 JC-1. Calculate the distance AB and
also the magnitude of charge Q.
7. Two fixed charges +4q and +q are kept at adistance r from each other.At what point
between the charges must a third charge q be placed to keep them in equilibrium ?