Questions- Electric field and charges
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Here i am providing you important questions on chapter 1 class12th physics.. you will find all the important questions and pyqs from electric charges and fields class12th physics… Practice more questions so that your concepts becomes more clear…
Questions on charges & properties of charges
Q.1 What is a charge? Write its SI unit and Dimensional formula.
Q.2 How is charge created? Explain briefly.
Q.3 Why does one can ignore quantization at macroscopic level?
Q.4 What do you mean by quantization of charge?
Q.5 What do you mean by q1q2>0 and q1q2<0?
Q.6 A polythene piece rubbed with wool is found to have a negative charge of 3 x 10^-7 C. Estimate the number of electrons transferred from wool to polythene. Is there any transfer of mass ? Ans. n= 1.875 × 10¹² electrons
Q.7 Write the properties of electric charges.
Q.8 Calculate the total positive or negative charge on 3.11g copper penny. Given Avogadro Number 6.02 x 10^23 , atomic number of copper 29 and atomic mass 63.5. Ans. Positive or negative charge on it will be 1.37 ×10⁵ C
Q.9 Can a body have a charge of 0.8 x 10^-19C? Justify your answer by comment? Ans. No, a body cannot have a charge of 0.8 × 10-¹⁹C
Q.10 What is meant by Conservation of charge? Give one example.
Q.11 What is the basic cause of quantisation of charge?
Questions on Electrostatic force and force due to multiple charges
Q.12 Find the force on a charge q1 (20uC) due to the charge q2 ( -10uC), if the positions of the charges are given as P(1,-1,2) and Q( -1,1,1). Ans. Force will be -0.06 ( 2i -2j+k)
Q.13 Two equal balls having equal positive charge q coulombs are suspended by two insulating strings of equal length. What would be the effect on the force when a plastic sheet is inserted between the two?
Q.14 Plot a graph showing the variation of Coulomb force (F) versus 1/r², where r is the distance between the two charges of each pair of charges (1uC,2uC) and (1uC,-3 uC). Interpret the graphs obtained.
Q.15 The sum of two point charges is 7uC. They repel each other with a force of 1N when kept 30cm apart in free space. Calculate the value of each charge. Ans. x = 2μC or 5μC
Q.16 A charged rod P attracts rod R where as P repels another charged rod Q what type of force is developed between Q and R.
Q.17 Explain meaning of the statement 'electric charge of a body is quantised’.
Q.18 Is the mass of a body affected on charging?
Q.19 Calculate the Coulomb force between two alpha particles separated by 3.2×10^-15 m. Ans. 90N
Q.20 Similar charges repel each other can they attract each other also?
Q.21 Charges Q1 and Q2 are separated by distance 1 m find the minimum possible force between them when kept in air or free space. Ans. F min = 2.3 × 10-²⁸N.
Q.22 The electrostatic force of repulsion between two equal positively charged ion is 3.7×10^-9 N, when they are separated by a distance of 5 Angstrom how many electrons are missing from each ion? Ans. q = 3.2 ×10-¹⁹C and n = 2.
Q.23 A positive charge is given to an isolated sphere will its mass increase or decrease or remains unchanged? Ans. Its mass will decrease .
Q.24 A free pith ball of 8 g carries a positive charge of 5×10^-8 C what must be the nature and magnitude of charge that should be given to a second pith ball fixed 5 cm vertically below the former pith ball so that the upper pit ball is stationary. Ans. q = 4.4 × 10-⁷ C
Q.25 Four point charges QA is equal to 2 µC, QB is equal to -5 µC, QC is equal to 2 µC and QD is equal to -5 µC are located at the corners of a square ABCD of side 10 cm. What is the force on charge of 1 µC pleased at the centre of the square? Ans. Net force at centre O charge of 1μC due to four charges kept at corners is zero.
Q.26 State coulombs law in vector form and show F12= - F21
Q.27 Three charges 10 µC, 5 µC and -5 µC are placed in air at the three corners A, B and C of an equilateral triangle of side 0.1 m. Find the resultant force experienced by charge placed at corner A. Ans. F = 45N , 60°
Q.28 Two identical point charges of charge Q are kept at a distance r from each other. A third point charge is placed on the line joining the above two charges, such that all the three charges are in equilibrium. Calculate the magnitude and location of the third charge. Ans. x= r/2 and q= Q/4
Q.29 Two point charges of charge values Q and q are placed at a distance of x and x/2 respectively from a third charge of charge value 4q, all charges being in the same straight line. Calculate the magnitude and nature of charge Q, such that the net force experienced by the charge q is zero. Ans. Q = -4q
Q.30 Two equally charged identical metal sphere A and B repel each other with a force 2.0x10^-5 N. Another identical uncharged sphere C is touched to A and then placed at midpoint between A and B. What is the net electric force on C. Ans. Net force on C is 2 × 10-⁵ N , away of sphere B.
Q.31 ABC equilateral triangle of side 10m and D is the mid point of BC. Charges of +100,-100& +75 C are placed at B,C&D respectively. What is the force experienced by 1 Coulomb positive charge placed at A? Ans. F= 9 × 10^9 N ,
Q.32 (a) Two insulated charged copper spheres A and B have their centres separated by a distance of 50cm. What is the mutual force of electrostatic repulsion, if charge on each is 6.5 × 10-⁷ C ?The radii of A and B are negligible as compared to the distance of separation. b) What is the force of repulsion if I) each sphere is charged to double the above amount and distance between them is halved? II) two spheres are placed in water? (K=80) Ans. a- 1.512 ×10^-5 , b- I)— 0.243 II)—1.352× 10^-10
Q.33 Q Suppose in the problem above (a) the spheres A and B have identical sizes . A third sphere of the same size but uncharged is brought in contact with the first, then brought in contact with the second and finally removed from both.What is the new force of repulsion between A and B ? Ans. 5.70375×10^-3 N
Q.34 A small conducting sphere having a positive charge of 1uC is made to touch another sphere of the same radius having a negative charge of -3uC. Find the force between them when they are separated by a distance of 3 cm. What will be the force between them when they are immersed in an oil of dielectric constant 2? Ans. 10N , F in oil medium- 5 N
Q.35 Four equal point charges each q are placed on the four corners of a square of side a. Calculate the force on any one if the charges.
Q.36 Find the force on charge q kept at centre of a square having charge at its vertices A= -2q, B= q,C = -q and D= 2q.
Q. 37 How far apart should the two electrons be, if the force each exerts on the other is equal to the weight of the electron? Ans. 5.03 m
Q. 38 The force between two equal charges placed in a medium at a distance of 9cm from each other is 16dyne. On increasing one of the charges by 56statC, it is found that the distance between the charges must be changed by 3cm in order to keep force between them same. Calculate the magnitude of the charges and the dielectric constant of the medium. Ans. Charge is 72 statC and dielectric constant is 4 .
Q. 39 Two point charges of charge value q and -9q are placed at d distance apart. Find the location of third charge on the line joining the charges, where it will remain in equilibrium.
Questions on electric field due to multiple charges
Q. 40 Two point charges of 5×10-¹⁹C and 20×10-¹⁹C are separated by a distance 2m. Find the point on the line joining them at which electric field intensity is zero. Ans. x= 2/3
Q. 41 Four charges, each having a charge q are placed on the vertices of a regular pentagon. The distance of each corner from the centre is 'a'. Determine the electric field at the centre of pentagon.
Q. 42 On x-axis at x= 1,2,4,8,.... Infinite charges each value q are placed. Calculate the intensity of field at x= 0. Ans. 1.2 x 10^10 q N/C .
Q. 43 Draw a plot showing the variation of E with r due to a point charge q.
Q. 44 Two point charges q1 and q2 are placed at a distance d apart as shown. The electric field intensity is zero at point p on the line joining them. Write two conclusions that you can draw from this.
q1_________________q2....p
Q. 45 Four charges +q,+q,-q,-q are placed at the four corners of a square of side a. Calculate net electric field at the centre of square.
Q. 46 Find out that electric field intensity which can hold a water drop of radius 10^-5cm in air , charge on drop is of one electron.
Q. 47 A Uniform electric field of strength 2×10 to the power 3 N per coulomb is established between two parallel plates of length 0.1 m held horizontally at a distance of 0.02 m apart. An electron is projected at a speed of 6×10 to the power 6 m/s making an angle of 45°. The field is directed vertically upwards. Will the electron strike the either plate?? if it strikes the plate, where does it do so?? Ans. The electron will strike the upper plate. x= 2.71 from left end of upper plate. ( Use eq of motion. & projectile concept)
Questions on electric field due to dipole
Q. 48 A dipole of length 0.1 m consist of two charges of ± 500µC. What is its electric dipole moment? Calculate the electric field due to the dipole at a point on the axis at distance 0.2 m from one of the charges in air. Ans. 6.25 × 10⁷ N/C.
Q. 49 Two Charges of +0.2 µ µC and -0.2 µ µ C are placed 10 to the power -6 cm apart. Calculate the electric field at an axial point at a distance of 10 cm from their mid-point. Ans. 3.6 × 10-⁸ N/C.
Q. 50 Calculate the field due to an electric dipole of length 10 cm and consisting of charges of ±100 µC at a point 20 cm from each charge. Ans. 1.125 × 10⁷ N/C.
Questions on torque experienced by dipole
Q. 51 An electric dipole, when held at 30° with respect to a uniform electric field of 10⁴N/C experiences a torque of 9× 10²⁶Nm. Calculate dipole moment of the dipole. Ans. 1.8 × 10^23 Cm.
Q. 52 Two charges of plus minus 1000 microcoulomb are separated by 2 mm. The dipole so formed is held at an angle of 30° with uniform electric field of 15×10 to the power 4 N per coulomb calculate the torque acting on the dipole. Ans. 0.15 Nm.
Q. 53 An electric dipole consist of two charges of 0.1 µC separated by a distance of 2.0 cm. The dipole is placed in an external field of 10 to the power 5 N per coulomb. What maximum torque does the field exert on the dipole? Ans. 2 x 10^-4 Nm.
Questions on continuous charge distribution
Q. 54 Find the net electric field at p due to linear distribution of q.
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Q. 55 Find the electric field on the axis of a uniformly charged a ring.
Questions on flux and Gauss’s law:-
Q. 56 A charge Q is placed at a distance a/2 above the centre of a horizontal, square surface of edge a ,as shown. Find the flux of the electric field through the square surface. Ans. Q/6 ε0
Q. 57 A large plain sheet of charge having surface charge density 5×10 to the power -6 C per metre square lies in XY plane find the electric flux through a circular area of radius 0.1 m, if the normal to the circular area makes an angle of 60° with the Z axis. Ans. 4.44 x 10^3 N m²/C.
Q. 58 If the electric field is given by ( 6i + 3j + 4k) . Calculate the electric flux through a surface of area 20 units lying in Y-Z plane. Ans. 120 units.
Q. 59 The electric field in a certain region of space is (5i + 4j -4k) x 10^5 N/C. Calculate the electric flux due to this field over an area of (2i -j) x 10-² m². Ans. 6 x 10^3 Nm²/C.
Q. 60 A charge of 2 x 10^-9C is placed on a corner of a cube of side 1 m. Find the electric flux passing through this cube. Ans. 28.25Nm² /C.
Q. 61 Given a uniform electric field E= 2×10^3 i N/C , find the flux of this field through a square of side 20 cm, whose plane is parallel to the YZ plane. What would be the flux through the same Square, if the plane makes an angle of 30° with the X axis? Ans. 40 Nm²/C.
Questions on Applications of Gauss’s law
Q. 62 An infinite line charge produces a field of 9×10^4 N/C at a distance of 0.02 m. Calculate the linear charge density. Ans. 10^-7 C/m
Q. 63 A plastic rod of lent 2.2 m and radius 3.6 mm carries a negative charge of 3.8×10^ -7 C spread uniformly over its surface. What is the electric field near the midpoint of the rod, at a point on its surface? Ans. λ = -1.73 x 10^-7 C/m , E = -8.6 x 10^ 5 N/C.
Q. 64 Considered two hollow concentric spheres S1 and S2 including charges 2Q and 4Q respectively, as shown in the figure. 1) Find out the ratio of the electric flux through them. 2) How will the electric flux through the spheres S1 change if a medium of dielectric constant K is introduced in the space inside S1 in place of air? Deduce the necessary expression.
Q. 65 A sphere S1 of radius r1 encloses a net charge Q. If there is another concentric sphere S2 of radius r2 including charge 2Q, find the ratio of the electric flux through S1 and S2. How will the electric flux through spheres S1 change if a medium of dielectric constant K is introduced in the space inside S2 in place of air?
Q. 66 Two concentric metallic spherical shells of radii R and 2R are given charge Q1 and Q2 respectively. The surface charge densities on the outer surfaces of the shells are equal. Determine the ratio Q1: Q2 .
Q. 67 Two infinitely large plane thin parallel sheets having surface charge density sigma one and sigma2 as shown in figure. Write the magnitudes and directions of the net fields in the region marked II and III.
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