Question 7

Three small identical metal spheres, P, S and T, on insulated stands, are initially
neutral. They are then charged to carry charges of -15 x 10^-9 C, Q and +2 x 10^-9 C
respectively, as shown below.

The charged spheres are brought together so that all three spheres touch each other
at the same time, and are then separated. The charge on each sphere, after
separation, is -3 x 10^-9 C.

7.1 Determine the value of charge Q. (2)

7.2 Draw the electric field pattern associated with the charged spheres, S and T,
after they are separated and returned to their original positions. (3)

The spheres, each with the new charge of -3 x 10^-9 C, are now placed at points on the
x-axis and the y-axis, as shown in the diagram below, with sphere P at the origin.

7.3 State Coulomb’s law in words. (2)

Calculate the magnitude of the:

7.4 Net electrostatic force acting on sphere P (5)

7.5 Net electric field at the origin due to charges S and T (3)

7.6 ONE of the charged spheres, P and T, experienced a very small increase in
mass after it was charged initially.

7.6.1 Which sphere, P or T, experienced this very small increase in
mass? (1)

7.6.2 Calculate the increase in mass by the sphere in QUESTION 7.6.1. (3)

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