- Four point charges +5 mC, +2 mC, +10mC and +2 mC are kept at the corners of a square of side 10 cm. A charge q=+1mC is placed at its centre. Find the net force on q.
- Calculate the distance between two prorons such that the electrostatic force between them is equal to the weight of either.
- Two point charges are 0.1 m apart and their combined charge is 9 mC. If they repel each other with a force 18N, then calculate the magnitude of each charge.
- Calculate the coulomb forcebetween two alpha particles separated by a distance of 3.2 x 10
^{-15}m - A proton moves through a uniform electric field of 5.01 x 10 3 N/C. Calculate (a) the accelerationwith which the proton is moving and (b) the time taken by the proton to cover a distance of 4.8 cm.
- How many electrons would have to be removed from or added to a penny to leave it charged with 1.0 x 10
^{-6}C?*[Ans: 6.25 x 10*^{12}] ^{-1}*[Ans: 1.32 x 10*^{5}*m**C*- What is the Coulomb’s force between two small charged spheres having charges of 2.0 x 10
^{-7 }C and 3.0 x 10^{-7}C placed 30 cm in air?*[Ans: 6.0 x 10*^{-3}N] - Find the electrostatic force between two protons placed in free space separated by a distance 20 cm
*[Ans: 5.76 x 10*^{-27}N] - The electrostatic force between 200mC and 500 mC placed in free space is 5 gf. Find the distance between the two charges.
*[Ans: 135.5 m]* - A pith ball A of mass 9 x 10
^{-5}kg carries a charge of 5mC. What must be the magnitude and sign of the charge on another pith ball B held 2 cm directly above the pith ball A so that the pith ball remains stationary?*[Ans: – 7.84 x 10*^{-12}C] - Two similar charges repel each other with a force of 44.1 N when placed two cm apart in air. Calculate the strength of the charges.
*[Ans: ± 1.4**m**C]* - The force between two charged objects is to be kept unchanged even though the charge on one of the objects is halved keeping the other the same. If d is the original separation between charges, then what is the new separation?
*[Ans: d/**Ö**2]* - The electrostatic force of repulsion between two positively charged ions carrying equal charges = 3.7 x 10
^{-9}N when they are separated by a distance of 5 Å. How many electrons are missing from each ion? [Ans: 2] - Two fixed point charges +4q and +2q units are separated by a distance
*x*. Where should the third point charge be placed for it to be in equilibrium? [Ans: at a distance of 0.59*x*from +4q] - How far apart should two electrons be if the force of repulsion between them is equal to the weight of electron?
*[Ans: 5.1 m]* - Two identical charges repel each other with force of 0·1 N when placed O· 5 m apart in ail (a) Find the value of charges (b) Determine the magnitude of the charges if they were placed in a liquid whose permittivity is tell times that of the vacuum. [
*Ans. (a) 1·67**m**C (b) 0·53**m**C]* - Two small spheres each of mass 10 mg are suspended from a point by threads 0·5 m long. They are equally charged and repel each other to a distance of 0·28 m. What ia the charge on each? g = 10 ms
^{-2}.*[Ans. 1·59 x 10*^{-8}C] - The normal hydrogen atom consists of proton and an orbital electron. Assume the orbit to be circular with radius 5·3 X 10
^{-11}m. Find the force between the proton and electron by using Coulomb’s law.*[Ans. 8·2 x 10*^{-8}N] - Two point charges q1 and q2 are 3 m apart and their combined charge is 20 mC. If one repel the other with a force of 0·075 N, what are the two charges?
*[Ans. 15**m**C and 5**m**C ]* - Two equally charged identical spheres P and Q repel each other with a force of 2 x IO
^{-5}N. Another identical uncharged sphere R is touched to P, then placed at the midpoint between P and Q. Find net force on R.*[Ans. 2 x 10*^{-5}N ] - Two sphere of charges + 10 C and + 40 C are placed 12 cm apart. Find the position of the point between them where force on + 1 C is zero.
*[Ans. 0·04 m from + 10 C]* - There are two equal charges each of 1.0 C. What should be the separation between them so that the force between them is equal to the weight of a 20 kg child?
*[Ans. 6·78 km]* - Two equal charges are separated by 1 m. Calculate the magnitude of the charges so that the force between them is equal to the weight of a 20 kg child.
*[Ans. 1·476 x 10*^{-4}C] - Two equal charges each of 1·0 C are separated by a distance of 1 cm. Calculate the mass of a body so that the force between these charges equals the weight of the body.
*[Ans. 9·18 X 10*^{12}kg] - Two insulating spheres are rubbed against each other and are placed 1 mm apart. If they attract each other with a force of 2.0 N, how many electrons were transferred from one sphere to the other during rubbing?
*[Ans. 9 x 10*^{10}] - Two small pith balls are 1 cm apart in air and carry charges of 2 x 10
^{-9}C and – 6 x 10^{-9}C respectively. Calculate the force of attraction. If the balls are touched and then separated by a distance of 1 cm, what will be the force between them?*[Ans. 108 X 10*^{-5}N ; 36 X 10^{-5}N repulsive] - Calculate the value of two equal charges if they repel one another with a force of I kgf when separated by a distance of 1 cm in air. Now, if the charges are situated in an insulating liquid having relative permittivity 8 times that of air, then what would be the magnitude of the charges if they exert the same force on each other?
*[Ans. 0·33**m**C ; 0·87**m**C]* - Five electrons have been removed from each atom to form ions. Find the electrostatic force between two such ions when separated by a distance of 4Å in air.
*[Ans, 3·6 x 10*^{-8}N] - Four charges + 4 mC, + 5 mC, + 4 mC and + 5 mC are placed at the corners A, B, C and D of a square of side 10 cm. Calculate the force on a charge of + I mC placed at the centre of the square.
*[Ans. 0 N]* - Two identical metallic spheres P and Q. have charge on each as 6 x 10
^{-7}C. They are placed half meter away from each other. Another identical sphere R having no initial charge is first touched with sphere P and then with Q. Sphere R is then with drawn. Find the new force between P and Q.*[Ans.4·86 x 10*^{-3}N] - Calculate the magnitude and direction of force on unit positive charge at the centre of a square of each side 2 m, when three charges of +8 mC each are placed at the three corners of the square.
*[Ans. 3·6 x 10*^{4}N towards the corner without charge] - Four equal charges each of + 2mC are placed at the corners of a square of side 50 cm. Find the force on anyone of the charges.
*[Ans. 275·6 x 10*^{-3}N repulsive along the diagonal of square] - Two identical balls each carrying a charge Q are suspended from a common point by two strings of equal length l. Calculate the mass of each ball if the angle between “the strings is 2q in equilibrium.
- Two equally charged identical metal spheres A and B repel each other with a force of 2·0 x 10-5 N. Another identical uncharged sphere C is touched to A and then placed at the mid point between A and B. What is the net force on C?
*[Ans. 2.0 x 10*^{-5}N along BC]

## 4 thoughts on “Practice Numericals from Electrostatics”

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i need the answers plz my id siyad_16@yahoo.com. i liked the questions

Nice questions. Can you send me the solutions ??

I need them. Please send them to ashish.sword@gmail.com

Keep up the good work !!

ans to first ques is 450 N;

ans to sec ques is 4*5^1/2