Physics Review Question

Unit: electricity and magnetism
Year: 1993 Question#: 23
Question: The diagram below represents the electric field lines in the vicinity of two isolated electric charges, A and B

Which statement identifies the charges of A and B?
(1 ) A is negative and B is positive
(2 ) A is positive and B is negative
(3 ) A and B are both positive
(4 ) A and B are both negative

What is this question really asking?




Explanation by: Christine A

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Answer 1
A is negative and B is positive

CORRECT

This electric field map represents the actions of a positive test charge when placed in the vicinity of these isolated charges. To find the correct answer, you must place an imaginary positive charge in various positions around A and B, and see which answer will fit when applied to the diagram. Answer 1 is correct because, when you place a positive charge anywhere between A and B, it would be repelled from B were it positive and attracted to A were it negative. This is correctly shown by the diagram. When placed to the left of A, a positively charged particle would move toward A as shown in the diagram, and if a positively charged particle were placed to the right of B, it would be repelled and move away, as shown by the diagram. Because all of these "work," answer 1 must be the correct answer. return to top























Answer 2
A is positive and B is negative

INCORRECT
This electric field map represents the actions of a positive test charge when placed in the vicinity of these isolated charges. To find the correct answer, you must place an imaginary positive charge in various positions around A and B, and see which answer will fit when applied to the diagram. Answer 2 is incorrect because the diagram does not match the actions of a positively charged particle placed around A and B. For example, if A were positive and a positively charged particle were to be placed to the left of A, the particle would be repelled rather than attracted as the diagram shows. In addition, if placed between A and B, a positively charged particle would move towards B, in the opposite direction from what is shown by the diagram. return to top



























Answer 3
A and B are both positive

INCORRECT
This electric field map represents the actions of a positive test charge when placed in the vicinity of these isolated charges. To find the correct answer, you must place an imaginary positive charge in various positions around A and B, and see which answer will fit when applied to the diagram. Answer 3 is incorrect because it does not accurately represent what would happed to a positively charged particle were it placed in positions around A and B. For example, if a positively charged particle were placed either to the left of A or to the right of B, it would be repelled, and move away from the charge. The diagram shows this on B, but not on A. Thus, A and B cannot have the same charge, and answer 3 is incorrect. return to top


























Answer 4
A and B are both negative

INCORRECT
This electric field map represents the actions of a positive test charge when placed in the vicinity of these isolated charges. To find the correct answer, you must place an imaginary positive charge in various positions around A and B, and see which answer will fit when applied to the diagram. Answer 4 is incorrect because the diagram does not represent what the answer implies. For expample, if A and B were both negative, and a positively charged particle were placed either to the left of A or the right of B, the two charges would attract. The diagram shows this correctly on A but not on B. Thus, the two charges cannot both be the same and cannot both be negative, so answer 4 is incorrect. return to top


























What's this question really asking?

Do you know about charged objects and their respective fields? How are elctric fields defined? return to top

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