MCAT Physics Question 73: Answer and Explanation
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Question: 73
13. A proton and an alpha particle (a helium nucleus) repel each other with a force of F while they are 20 nm apart. If each particle combines with three electrons, what is the magnitude of the new force between them?
- A. 9F
- B. 3F
- C. F
- D.
Correct Answer: C
Explanation:
The magnitude of the electrostatic force is given by the equation 
Because the distance does not change during the interaction in the question, the value of r is irrelevant to the answer. Currently, q1 and q2 are equal to +1e and +2e, respectively;the addition of three electrons (each of which carries a charge of ?e) will change the charges to ?2e and ?1e. Therefore, the product q1q2 before the interaction is equal to the product q1q2 after the interaction (+2e2). Because k and r also remain constant in this system, the magnitude of the force does not change.