Question

5. Metal sphere A and B in Fig 1.24A are initially neutral
Coulomb’s law describes the
and are touching. A positive charge rod is brought near A,
electrostatics forces between
but not touching.
small point electric chares q1 and
i) Is A now positive, negative, or neutral? Use both charge
q2 at rest or nearly at rest and
diagrams and words to explain.
separated by distance r.
ii) If you bring your finger near a lightweight, negatively
charged hanging ball, the ball swings over toward your
finger as show in Fig 1.24B. Use charge diagrams and
푠2
words to explain this observation

Answer 1

i) Two metal spheres A and B are touching and a positive rod is brought near A (no contact).

Diagram (side view):
rod(+) A B
+ [ − − ] [ + + ]
← electrons move from B to A

Words:
- The positive rod attracts electrons in the connected pair of spheres. Electrons flow through the contact toward the side of A nearest the rod, so that side of the combined conductor becomes negative.
- Because electrons left the far region (mainly on B), that region becomes positive. Thus while they remain touching A ends up with excess negative charge (near the rod) and B with positive charge.
- If you now separate the spheres while the rod is still nearby, A will remain negatively charged and B positively charged. If you remove the rod while they remain touching, the induced separation disappears and the pair returns to neutral.

ii) Negatively charged hanging ball and your neutral finger.

Diagram (front view):
finger ball
[ + − ] ( − − − )
nearest side: induced + ball: excess −

Words:
- The negative ball repels electrons in the material of your finger, pushing them away from the finger’s surface that faces the ball. That leaves an induced positive charge on the near surface and a slight excess negative further back.
- The induced positive charges on the near side of your finger are closer to the ball than the repelled negatives are, so the attractive force between the ball’s negative charge and the induced positive charge is stronger than the repulsive force from the farther-away negatives.
- The net result is an attractive force, so the negatively charged ball swings toward your finger. (If your body is well grounded, electrons can flow away more easily and the induced attraction can be even stronger.)