2015 Sec 4 Physics Assignment Answers

AS 12

1 (a) (i) The positively charged sphere C induces negative charges at the left side of sphere S and positive charges at the right side of sphere S.

Since the unlike charges of spheres C and S are closer than their like charges, the force of attraction is stronger than the force of repulsion. Hence, sphere S moves towards C.

(ii) When S touches C, since unlike charges attract, some negative charges from S will move towards C to neutralise some of the positive charges in C. This will cause sphere S to become positively charged and C will be less positive. Since like charges repel, S will be repelled away from C.

(b)

(c) Thermal energy ionises the air particle around S to become negative and positive ions. The negative ions will be attracted to S and slowly discharge the excess positive charges in S. Eventually, S will falls back towards C due to gravity and a weaker repulsion force. Once all the excess positive charges are discharged, S will be attracted to C again.

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electrostatic force of repulsion

tension

gravitational force

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3 (a) Since unlike charges attract, some negative charges in P and Q will move to the near end of P where the perspex is located. As a result there are more negative charges in P than Q. Likewise there will be more positive charges in Q than P.

(b) P becomes negatively charged and Q becomes positively charged.

(c) No, as there is no contact throughout the whole process.