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Consider the following diagram which reveals a positive test charge that is located between two charged spheres, X and Y. Sphere X has a charge of +3Q and is located 0.6 meters from the test charge. Sphere Y has a charge of -3Q and is located 0.3 meters from the test charge.
If the magnitude of the force on the test charge due to sphere X is F, what is the magnitude of the force on the test charge due to sphere Y?

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Correct Answer: A. 4F
Keeping Coulomb’s law in mind, if you reduce the distance by 1/2, then the force increases by the inverse squared of 1/2 which is 4 times. Note the question is only asking about the magnitude of the forces as opposed to their directions. If the direction were asked, then the sign of the charge (i.e. positive or negative) would matter.
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