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HSPTA MALAPPURAM, , PHYSOL-The Solution for Learning Physics, , Question Bank, Chapter 2 : Electrostatic Potential and Capacitance, Each question scores One ( Focus Area ), 1 Q: Pick out a vector quantity from the following, (i) electric potential (ii) electric potential gradient (iii) electric potential energy (iv) electric flux, Ans: (ii) Electric potential gradient, 2 Q: 5J of work is done in moving a positive charge of 0.5C between two points., What is the potential difference between the points (i) 2.5V (ii) 10V (iii) 0.1V (iv) 5V, Ans: (ii) 10V, 3 Q: “Electric field lines are always............. to equipotential surfaces”., Choose the correct one (parallel, perpendicular), Ans: perpendicular, 4 Q: Draw the symbol of a variable capacitor., Ans:, , 5 Q: You are given two capacitors of capacitance 20 μF each. Draw a diagram to show how you will, connect these capacitors to get 40 μF capacitance., Ans:, , 6 Q: Two capacitors C1 and C2 are connected in series. This combination is connected in parallel with, a third capacitor C3. Draw a diagram of the above combination?, Ans:, , 7 Q: In a charged capacitor energy is stored in the............ (i) positively charged plate (ii) negatively, charged plate (iii) electric field between the plates (iv) none of these, Ans: (iii) electric field between the plates, 8 Q: Define electric potential at a point?, Ans: Electric potential at a point is defined as the work done to bring a unit positive charge from, infinity to that point., , Prepared by Higher Secondary Physics Teachers Association Malappuram

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9 Q: SI unit of electric potential is............, Ans: J/C or volt (V), 10 Q: Is electric potential a vector or a scalar?, Ans: Scalar, 11 Q: A device used to store electrical charge is called..........., a) Transformer b) Capacitor c) Resistor d) Inductor, Ans: b) Capacitor, 12 Q: Define potential energy of a system of charges?, Ans: Potential energy of a system of charges is the work done to bring the charges from infinity, to their present positions., 13 Q: Define one farad?, Ans: Capacitance of a capacitor is said to be one farad if one coulomb of charge raises its, potential by one volt., 14 Q: One coulomb of charge initially at rest is accelerated through a potential difference of 1 volt., During this process the kinetic energy acquired by the charge is ………, Ans: KE = W = qV = 1 x1 = 1J, 15 Q: The work done to move a charge from one point to another on an equipotential surface is........., Ans: zero, 16 Q: What is meant by equipotential surface?, Ans: It is a surface with a constant value of the potential at all points on the surface., 17 Q: Electric field is the direction in which potential........, Ans: Reduces at the steepest., 18 Q: For a single charge equipotential surface will be......, Ans: Spherical., 19 Q: In the case of uniform electric field equipotential surface will be...., Ans: A plane., 20 Q: Who invented capacitor?, Ans: Michael Faraday, 21 Q: If we introduce a dielectric in a capacitor, its capacitance will......., Ans: Increase, 22 Q: The area under Q-V graph of a capacitor will give......, Ans: Energy of that capacitor, 23 Q: Equation for capacitance of a parallel plate capacitor with air as dielectric is......., Ans: ε0A/d, 24 Q: Equation for capacitance of a parallel plate capacitor with a dielectric in between the plates, is......., Ans: ε0εrA/d, 25 Q: Equation for capacitance of a parallel plate capacitor with a dielectric slab of thickness t in, between the plates is......., Ans: ε0A/(d-t+t/εr), 26 Q: Equation for capacitance of a sphere is...., Ans: 4πε0r, , Prepared by Higher Secondary Physics Teachers Association Malappuram

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27 Q: Capacitance of earth is......, Ans: 0.7 mF, 28 Q: 1eV = ........... J, Ans: 1.6X10-19, 29 Q: 1MeV = ........... J, Ans: 1.6X10-13, 30 Q: An insulated capacitor with air between its plates has a potential difference of Vo and a, charge Qo. When the space between the plates is filled with oil, the potential difference becomes V, and charge become Q. Which of the following relation is correct? a) Qo = Q, V > Vo, (b) Qo = Q, V < Vo (c) Qo > Q, V= Vo (d) Qo < Q, V = Vo., Ans: Charge will not change, Qo = Q, C = KCo, So by K = C/V, V = Vo/K, V > Vo Hence Option (a) is correct, 31 Q: Name any two factors on which the capacitance of a parallel plate air capacitor depends?, Ans: Area of plates, separation between two plates and the medium which is placed in between, the plates., 32 Q: What is the SI unit capacitance?, Ans: SI unit of capacitance is C/V or farad (F), 33 Q: What is meant by the energy density of a parallel plate capacitor?, Ans: Energy stored per unit volume of a capacitor is called energy density., 2, Energy density, u = (1/2)ε0E, 34 Q: Two capacitors of capacitances C1 & C2 with potentials V1 & V2 are connected in parallel with, each other. Their common potential is........., Ans: (C1V1 + C2V2) / (C1+C2), Each question scores One ( Non Focus Area ), 35 Q: What are dielectrics?, Ans: Dielectrics are non-conducting substances or insulators. But they allow electric field to pass, through them., 36 Q: What is the maximum value of potential energy/work done on a dipole?, Ans: 2PE, 37 Q: What is the minimum value of potential energy/work done on a dipole?, Ans: Zero, 38 Q: If we place a metal bock of thickness d in between the plates of a parallel plate capacitor with, plate separation d. Its capacitance is......, Ans. Infinity., 39 Q: - dV/dr =........., Ans: E, 40 Q: If charge is given to a soap bubble, its radius will......, Ans: Increase., 41 Q: What will happen if an electric dipole is placed in a uniform electric field?, Ans: It will experience torque., Prepared by Higher Secondary Physics Teachers Association Malappuram

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Each question scores Two ( Focus Area ), 1 Q: Draw the equipotentials for a single positive point charge., Ans:, , 2 Q: What are the different uses of a capacitor?, Ans: (i) to store charge (ii) to generate electromagnetic radiation (iii) to tune radio circuits, (iv) to reduce voltage fluctuation in power supply, 3 Q: Draw the graph showing the variation of V with distance r., Ans:, , 4 Q: Define electric potential difference., Ans: Potential difference between two points is defined as the work done to bring a unit +ve charge, from one point to another., , 5 Q: Draw the equipotential surfaces for a uniform electric field., Ans:, , 6 Q: Write any 4 properties of equipotential surfaces., Ans:, 1. The work done to move a charge from one point to another on an equipotential surface is zero., 2. Two equipotential surfaces will never intersect., 3. Electric lines of force pass normal to an equipotential surface., 4. For a point charge, the equipotential surfaces are concentric spherical shells., 7 Q: What is the amount of work done in moving a 2C charge between two points at 3cm apart of an, equipotential surface?, Ans: Zero, Prepared by Higher Secondary Physics Teachers Association Malappuram

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8 Q: Three capacitors of capacitances 2 pF, 3 pF and 4 pF are connected in series. Determine the, charge on each capacitor if the combination is connected to a 100 V supply., Ans: In series,, , 9 Q: You are given two capacitors of 2F and 3F. What are the maximum and minimum values of, capacitance that can be obtained by combining them?, Ans: We get maximum capacitance when it is connected parallel., Cmax = C1 + C2 = 2 + 3 = 5 F, We get minimum capacitance when it is connected in series., 1/Cmin = 1/C1 + 1/C2 = 1/2 + 1/3 = 5/6 F, Cmin = 6/5 F, 10 Q: What is the area of plates of a 0.1𝜇F parallel plate capacitor (with air medium), given that the, separation between the plates is 0.1mm, Ans:, , 11 Q: Explain the principle of a parallel plate capacitor?, Ans: Consider a positively charged plate, , A. If another plate B with no charge, is, brought near to A (and placed without, touching), then on the inside of the plate, negative charges are induced and on the, outside positive charges are induced. If, the second plate is earthed all the, positive charges, get neutralised. Now, due to the presence of negative charges on the plate B, the potential (V) of A decreases., Therefore, by equation C = Q/V, When potential decreases capacitance increases. This is the principle of capacitor., 12 Q: What happens if a dielectric material is introduced between the plates of the capacitor?, Ans: The capacitance increases 𝜀𝑟 times., 13 Q: Calculate the electrical capacitance of earth. The radius of earth is 6400km., 3, 9, Ans: C = 4𝜋ε0r = 6400 x 10 x 1/ (9 x 10 ) = 0.7 mF, 14 Q: What is the use capacitor? Define capacitance., Ans: It is a device used to store electrical charges. Capacitance is the ratio of electric charge stored, on a conductor to the difference in electric potential., 15 Q: What are the advantages of introducing dielectric slab between the plates of a capacitor., Ans: 1) The capacitance increases 𝜀𝑟 times, 2) Dielectric medium avoids sparking between the plates., 16 Q: If the air medium between the two charges is replaced by water what change you expect in the, electrostatic force and why?, Ans: The dielectric constant or relative permittivity of water is 81. Hence the force between two, Prepared by Higher Secondary Physics Teachers Association Malappuram

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charges is reduced by 1/81., 17 Q: Derive an expression for the capacitance of a parallel plate capacitor., Ans:, , 18 Q: A parallel plate capacitor connected to a cell gets fully changed. After disconnecting the cell, a, thin sheet of mica is placed between the plates of the capacitor., What happens to its a) charge b) capacitance, Ans: a) charge remains constant (b) capacitance increase, 19 Q: Calculate the effective capacitance between a and b from the figure given below, , Ans: Resultant of C1 and C2 = 300μF and resultant of C3 and C4 = 300μF, 1/C = 1/300 + 1/300, C = 150μF, 20 Q: Derive the relation between electric field and potential or Show that electric field is negative, gradient of potential?, Ans:, , 21 Q: The figure shows the Q (charge) versus V (potential) graph for series and parallel combination of, two capacitors. Identify the graph representing the parallel combination., Ans: Graph A (Slope will be more in parallel combination), , 22 Q: Obtain the expression for energy stored in a capacitor using Q-V graph?, Ans: Area under the graph, = 1/2 QV, = Energy stored in the capacitor, Prepared by Higher Secondary Physics Teachers Association Malappuram

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23 Q: What is the area of the plates of a 2F parallel plate capacitor, given that the separation between, the plates is 0.5 cm?, Ans:, , 24 Q: A 12pF capacitor is connected to a 50V battery. How much electrostatic energy is stored in the, capacitor?, 2, -12, 2, -8, Ans: UE = (1/2)CV = (1/2) x12x10 x(50) = 1.5x10 J, 25 Q: Figure shows charge (q) versus voltage (V) graph for two given capacitors. Calculate the values, of both the capacitors?, , Ans: CA = Slope of A = 500/10 = 50 μF, CB = Slope of B = 80/10 = 8 μF, , 26 Q: Two charges 3×10−8 C and −2×10−8 C are located 15 cm apart. At what point(s) on the line, joining the two charges is the electric potential zero?, Ans:, , 27 Q: Give the relation connecting electric field and electric potential., Ans: E = q/(4𝜋ε0r2), V = q/(4𝜋ε0r), E = V/r, 28 Q: Find out the dimensional formula of electric potential?, Ans:, , 29 Q: Find out the dimensional formula of capacitance?, , Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans:, , Each question scores Two ( Non Focus Area ), 30 Q: Derive expressions for potential energy of a single charge., Ans:, , 31 Q: Derive expressions for potential energy of a two charge system in an external electric field., Ans:, , 32 Q: Derive expressions for potential energy of a two charges system in the absence of external, electric field., Ans:, , 33 Q: Derive expressions for potential energy of a three charge system, in the absence of external, electric field., Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans:, , 34 Q: The electric field lines of a positive charge is shown in the figure., a) Give the sign of potential difference VP -VQ?, b) Give the sign of work done by the field in moving a small positive charge, from Q to P?, c) What is the shape of equipotential surface near the of charge? (1+1+1), Ans: a) Positive (b) Negative (c) Spherical, 35 Q: A permanent electric dipole of dipole moment p is placed in a uniform external electric field E,, as shown in Figure. (a) Redraw the figure and show the magnitude and direction of force acting on, the charges. (b) Write an expression of the torque acting on this dipole in vector form., Ans:, , 36 Q: Explain the polarization in non-polar molecules., Ans: In the absence of external electric field,, non-polar molecules have no permanent dipole, moment. In an external electric field, the positive, and negative centres of the non-polar molecule, are displaced in the opposite directions. Thus the, molecule develops an induced dipole moment., Then the dielectric is said to be polarized. The, induced dipole moments of different molecules, add up giving a net dipole moment of the, dielectric in the presence of external electric, field., , 37 Q: Explain the polarization in polar molecules., Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans: In a polar dielectric, each molecule has permanent dipole moment but in the absence of, external electric field, the dipoles are arranged randomly due to thermal agitation; so the total dipole, moment is zero. When an external electric field is applied, the individual dipoles tend to align with, the field. Then a net dipole moment is developed., , 38 Q: Define Polarization and electric susceptibility., Whether polar or non-polar, a dielectric develops a net dipole moment in the presence of an external, electric field., The dipole moment per unit volume of the dielectric is called polarization (P)., For linear isotropic dielectrics,, , is called electric susceptibility of the dielectric medium., , Each question scores Three ( Focus Area ), 1 Q: Derive expressions for effective capacitance when capacitors are connected in parallel., Ans:, , 2 Q: Derive an expression for capacitance of a parallel plate capacitor with a dielectric slab is, introduced between the plates?, , Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans:, , 3 Q: Derive an expression for capacitance of a parallel plate capacitor?, Ans:, , 4 Q: You are given two capacitors of 2𝜇F and 3𝜇F. What are the maximum and minimum values of, capacitance that can be obtained by combining them?, Ans:, , 5 Q: Derive an expression for electric potential at a point due to an electric dipole., Ans:, Prepared by Higher Secondary Physics Teachers Association Malappuram

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6 Q: You are given two capacitors of 2𝜇F and 3𝜇F. What are the maximum and minimum values of, capacitance that can be obtained by combining them?, Ans:, , 7 Q: Derive an expression for energy density of a parallel plate capacitor., Ans:, , 8 Q: Derive an expression for the potential due to a point charge., , Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans:, , 9 Q: Derive an expression for energy of a parallel plate capacitor?, Ans:, , 10, , Q: Can we produce a sphere of capacitance 1F?, Ans: No., C = 4πε0r, r = C/4πε0, r = 1/(1/9X109) = 9X109m, Prepared by Higher Secondary Physics Teachers Association Malappuram

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r = 9X106 km. This is very large and practically impossible., 11 Q: Derive an expression for the work done in rotating a dipole in an external electric field., Ans:, , −7, , 12 Q: Calculate (a) the potential at a point P due to a charge of 4 × 10 C, located 9 cm away. Hence, −9, (b) obtain the work done in bringing a charge of 2 × 10 C from infinity to the point P. Does the, answer depend on the path along which the charge is brought?, Ans:, , 13 Q: Derive expressions for effective capacitance when capacitors are connected in parallel., Ans:, , Prepared by Higher Secondary Physics Teachers Association Malappuram

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14 Q: A parallel plate capacitor with air between the plates has a capacitance of 8 pF. What will be the, capacitance if the distance between the plates is reduced by half, and space between them is filled, with a substance of dielectric constant 6., Ans:, , 15 Q: Can we produce a parallel plate capacitor of capacitance 1F with plate separation 1 cm?, Ans: No., C = ε0A/d, A = Cd/ε0, A = 1X10-2 /8.854X10-12, A = 11200 km2, Which is practically impossible, 16 Q: Four metal plates are arranged as shown. Capacitance between X and Y is (A→ Area of each, plate, d→distance between the plates), , Ans:, , 17 Q: How many 8 μF 2V capacitors are required to produce a capacitor of 12 μF 8V?, Prepared by Higher Secondary Physics Teachers Association Malappuram

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Ans: 24, , Each question scores Three (Non Focus Area ), 18 Q: Distinguish between polar and non-polar, molecules., Ans : In certain molecules, the centre of gravity, of positive charges and centre of gravity of, negative charges do not coincide. These, molecules are called polar molecules., Eg: HCl, H2O, NH3, etc., But in some other molecules the centre of gravity, of positive charges and centre of gravity of, negative charges coincide. These molecules are, called non-polar molecules., Eg: O2, N2, H2, CO2, etc., , Prepared by Higher Secondary Physics Teachers Association Malappuram

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Each question scores Four ( Focus Area ), 1 a) 3 capacitors are connected in series to a power source. The quantity remains constant is........?, b) n capacitors each of capacitance C1 are connected first in series and then in parallel. Find the, ratio of their capacitances., c) If 2 capacitors are connected in series. The effective capacitance is less than........?, Ans: a) charge, b) Cs = C/n, Cp = nC1, Cs/Cp = C1/(n/nC1), 2, , 2, , Cs/Cp = 1/n = 1 : n, c) minimum of individual, 2 A capacitor is charged with a source. After charging source is removed. Then the plates are moved, away using insulating handle. What will happen to the a) electric field in between the plates, b) potential c) charge d) capacitance., Ans: a) remains the same, b) increases, c) remains the same, d) decreases, , Prepared by Higher Secondary Physics Teachers Association Malappuram