Suppose a rod is given a negative charge by rubbing it with wool. Which one of the following statements is correct in this case ?
- A.The positive charges are transferred from rod to wool
- B.The positive charges are transferred from wool to rod
- C.The negative charges are transferred from rod to wool
- D.The negative charges are transferred from wool to rod
Correct answer
Explanation
The correct answer is D, The negative charges are transferred from wool to rod. The rod becomes negative because it gains electrons from the wool.
When two materials are rubbed together, only electrons move, because they are the loosely held outer particles of atoms. Protons, the positive charges, sit inside the nuclei and do not move from one body to the other. The material that holds electrons more strongly pulls some away from the other. Here the rod gains extra electrons and becomes negatively charged, while the wool loses the same number and is left with an equal positive charge. Charge is neither created nor destroyed in rubbing, only transferred.
A is wrong: positive charges do not move in charging by friction, and losing positive charge is not how the rod turns negative. B is wrong: if positive charges entered the rod it would become positive, not negative. C is wrong: if electrons left the rod for the wool, the rod would become positive.
Exam tip: In charging by friction only electrons move; the body that gains electrons turns negative, the one that loses them turns positive.
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