Electrostatics. 1. Show does the force between two point charges change if the dielectric constant of the medium in which they are kept increase?

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1 Electostatics 1. Show does the foce between two point chages change if the dielectic constant of the medium in which they ae kept incease? 2. A chaged od P attacts od R whee as P epels anothe chaged od Q. What type of foce is developed between Q and R? 3. A fee poton and a fee electon ae placed in a unifom field. Which of the two expeience geate foce and geate acceleation? 4. No two electic lines of foce can intesect each othe? Why?

2 5. A paticle of mass m and chage is eleased fom est in a unifom electic field of intensity E. calculate the kinetic enegy it attains afte moving a distance s between the plates? 6. Two point chages + and +9 ae sepaated by a distance of 10 a. Find the point on the line joining the two changes whee electic field is zeo?

3 7. Define the tem dipole moment P of an electic dipole indicating its diection. Wite its S.I unit. An electic dipole is placed in a unifom electic field E. Deduce the expession fo the Toue acting on it.

4 8. (1) The electic field E due to a point change at any point nea to it is defined F E = Lim as: whee is the test chage and F is the foce acting on it. c What is the significance of lim in this expession? o (2) Two chages each 2 x 10-7 C but opposite in sign foms a system. These chages ae located at points A (0,0, -10) cm and B(0,0, +10) cm espectively. What is the total chage and electic dipole moment of the system?

5 9. (a) Sketch electic lines of foce due to (i) isolated positive change (ie >0) and (ii) isolated negative change ( ie <0) (b) Two point changes and ae placed at a distance 2a apat. Calculate the electic field at a point P situated at a distance along the pependicula bisecto of the line joining the chages. What is the field when >> a?

6 ANSWERS FV foce between the chages in vaccuum Ans 01. Since K= FM = foce between two chages in medium FV Fm = k if k inceases, Fm deceases. Ans 02. Suppose od P be negatively chaged since it attacts od R R is positively chaged since it epels od Q Q is negatively chaged. So foce between Q and R is attactive in natue. Ans 03. Since F = E and a = F/m Since chage on poton and electon ae same but mass fo electon is smalle, hence foce and acceleation expeienced by an electon is geate. Ans 04. Two electic lines of foce neve intesect each othe because if they intesect then at the point of intesection thee will be two tangents which is not possible as the two tangents epesents two diections fo electic field lines. E 1 E 2 Ans 05. Since F = E F E a = m = m Using thid euation of motion v 2 u 2 = 2as Initially chaged paticle is at est u = o v 2 = 2as KE = mv = m (2 as) = mas Substituting 1 in e. 2 E KE = m x S m KE = ES

Ans 06. Let P be the pt whee test chage (+o) is pesent then electic field at pt.

P due to + 9------------1 E K( + ) K ( + 9 ) E A = EB = x (10 a x) Substituting in e.

5 a fom change (+) O 10 a - x = 10a - 2.5a = 7.

Electic dipole moment is defined as the poduct of the magnitude of eithe chage and the length of

7 Ans 06. Let P be the pt whee test chage (+o) is pesent then electic field at pt. P will be zeo if Field at pt. P due to + = field at p+. P due to E K( + ) K ( + 9 ) E A = EB = x (10 a x) Substituting in e. 1 K ( + ) K ( + 9 ) = x (10 a x) (10 a x) = 9x 10 a x = 3x 10 10a = 4x x = 9 4 x = 2.5 a fom change (+) O 10 a - x = 10a - 2.5a = 7.5a fom change (+9) P + EB EA +9 A x +v 10a 10a-x B Ans 07. Electic dipole moment is defined as the poduct of the magnitude of eithe chage and the length of dipole. P = (2 l) Its S.I. unit is coulomb mete (cm) Conside a dipole placed in unifom electic field and makes an angle (θ) with the Since two foces acts on the chages constituting an electic electic field ( E) dipole which ae eual and opposite in diection, thus a toue acts on the dipole which makes the dipole otate. And Toue τ = Ethie foce X distance Hee foce (F) = E BN And = sin θ BN = AB sin θ = 2 l sin θ AB (τ ) = E x 2lsin θ (τ ) = PE Sinθ ( P = (2 l)) In vecto fom τ = P E

8 Ans 08. (1) The Significance of witing lim means the test chage o should be vanishingly small so that it should not distub the pesence of souce chage. (-2x10-7 2l=10+10=20cm Z P (0,0,10)cm (2) (i) Total chage of the system = 2 x (-2 x 10-7 ) = zeo. P (ii) P = 2 l P= 2 x 10-7 x 20 x 10-2 P = 4 x 10-8 cm Diection of Dipole moment Along negative x-axis. Y Z (2x10-7 ) (0,0,-10)cm X

2E+ cos 2θ 2 Enet = 2 E + (1 + cos 2 θ ) Enet = 2E + 2cos θ = 4E + cos θ

9 Ans 09. (a) (b) K E + = ( + a ) K E = ( + a ) Since E + = E Enet can be calculated by using a paallelogam law of vecto addition. A (-) + a E+ 2θ P(+o) E=? E net E- + a θ θ B(+) a o a Enet = E + + E + 2 E+ E - cos2θ Enet = 2E + + 2E+ cos 2θ 2 Enet = 2 E + (1 + cos 2 θ ) Enet = 2E + 2cos θ = 4E + cos θ Enet = 2E + cos θ cos θ = a Enet = 2E + + a k a Enet = 2 + a + a k2a K P Enet = = ( + a ) ( + a ) 3/ 2 3/2 Fo >>> a (a can be neglected) KP Enet = 3 a + a

anubhavclasses.wordpress.com CBSE Solved Test Papers PHYSICS Class XII Chapter : Electrostatics

anubhavclasses.wordpress.com CBSE Solved Test Papers PHYSICS Class XII Chapter : Electrostatics CBS Solved Test Papes PHYSICS Class XII Chapte : lectostatics CBS TST PAPR-01 CLASS - XII PHYSICS (Unit lectostatics) 1. Show does the foce between two point chages change if the dielectic constant of