College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300



College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300
Textbook Problem

Two hard rubber spheres, each of mass m = 15.0 g, are rubbed with fur on a dry day and are then suspended with two insulating strings of length L = 5.00 cm whose support points axe a distance d = 5.00 cm from each other as shown in Figure P15.65. During the rubbing process, one sphere receives exactly twice the charge of the other. They are observed to hang at equilibrium, each at an angle of θ = 10.0° with the vertical. Find the amount of charge on each sphere.


Figure P15.65

To determine
The charge on each sphere.


Given info: The mass of the sphere is 15.0 g. The length of the string is 5.00 cm. The magnitude of one charge is twice the other.

The free body diagram is,

  • FE is the net electric force.
  • T is the tension.
  • m is the mass of the ball.
  • g is the acceleration due to gravity.

From the diagram,



Dividing the 2nd equation by the 1st,


Formula to calculate the net electric force is,


  • ke is the Coulomb constant.
  • r is the distance between the spheres.
  • q is the magnitude of smaller charge.

The distance between the spheres is,




On Re-arranging,


Substitute 9.8ms2 for g, 15

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