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AP® Physics C: E&M Conservation of Electric Energy MCQ | Exam Style Question

AP® Physics C: E&M - Conservation of Electric Energy - MCQs

Question

A particle with charge \(Q\) and mass \(M\) has instantaneous speed \(u_1\) when it is at a position where the electric potential is \(V_1\). At a later time, the particle has moved a distance \(R\) away to a position where the electric potential is \(V_2\). Which of the following equations can be used to find the speed \(u_2\) of the particle at the new position?

(A) \(\frac{1}{2} M\left(u_2^2-u_1^2\right)=Q\left(V_1-V_2\right)\)

(B) \(\frac{1}{2} M\left(u_2-u_1\right)^2=Q\left(V_1-V_2\right)\)

(C) \(\frac{1}{2} M u_2^2=Q V_1\)

(D) \(\frac{1}{2} M u_2^2=\frac{1}{4 \pi \varepsilon_0} \frac{Q^2}{R}\)</

▶️Answer/Explanation

Ans:A

Question

A helium nucleus (charge +2q and mass 4m) and a lithium nucleus (charge +3q and mass 7m) are accelerated through the same electric potential difference, V0. What is the ratio of their resultant kinetic energies, \(\frac{K_{lithium}}{K_{Helium}}\)  ?
(A) 2/3             (B) 6/7              (C) 7/6                (D) 3/2

Answer/Explanation

Ans:D

Solution: K = qΕV so K1/K2 = q1/q2

Question

Two protons and an electron are assembled along a line, as shown above. The distance between the electron and each proton is a. What is the work done by an external force in assembling this configuration of charges?
(A) –2ke2/a              (B) –3ke2/2a             (C) ke2/2a             (D) 3ke2/a

Answer/Explanation

Ans:B

Solution: The work to assemble the charges is the potential energy of the system, which is the sum of the potential energies of each pair of charges UE = –ke2/a – ke2/a + ke2/2a

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