AP Physics C Electricity and Magnetism – 1.2 Electrostatics: Electric Field and Electric Potential -MCQ Exam style question

Question

Two conducting plates hold equal and opposite charges that create an electric field of magnitude E = 95 N C that is directed to the right, as shown in the figure above. Points A and B are 0.75 cm apart with A closer to the positive plate. A proton is released from rest at point A. What is the kinetic energy of the proton when it reaches point B ?

(A) 0

(B)\(+1.14\times 10^{-19}J\)

(C)\(+1.52\times 10^{-17}J\)

(D)\(+1.92\times 10^{-7}J\)

(E)\(+71J<0\)

Answer/Explanation

Ans:B Setting the kinetic energy of the proton at point B equal to the change in the potential energy of the system as the proton moves from point A to point B yields the following. \(\Delta U_{E}=qE\Delta d=K\) \(K = (1.6 × 10^-19C) (95N/C) \left ( 0.75cm\times \frac{1m}{100cm} \right )\)

 

Question

Four isolated arrangements of charged spheres are created for an experiment, as shown above. Which of the following correctly compares the electric potential energy U of the arrangements?
Assume U = 0 when the charges are an infinite distance apart.
(A) \(U _AU_ BU_ C U_ D \)
(B) \(U _AU_ CU_ B U_ D \)
(C) \(U _BU_ DU_ A U_ C \)
(D) \(U _DU_ CU_ B U_ A \)
(E) \(U _AU_ BU_ D U_ C\)

Answer/Explanation

Ans:E

Question

An electric dipole consisting of a positive charge and a negative charge held a fixed distance apart is at rest in an external, nonuniform electric field E, as shown in the figure above. Which of the following best describes the net torque and net force exerted on the dipole? 

Net Torque                                               Net Force
(A) Clockwise                                         To the left
(B) Clockwise                                         To the right
(C) Counterclockwise                           To the left
(D) Counterclockwise                           To the right
(E) Zero                                                      Zero

Answer/Explanation

Ans:B

The force on the top part of the dipole is to the right and on the bottom part is to the left; thus, the net torque will be clockwise. Also, because the positive side of the dipole is in a position where the electric field is stronger, the net force is to the right.

Question

Points A and B shown above are in the plane of the page and 5 meters apart. The points are located in a uniform electric field of magnitude 1000 V m directed toward the bottom of the page. When a proton (of charge +e) moves from point A to point B, how much work is done on the proton by the electric field?
(A) -5000 eV
(B) -3000 eV
(C) +3000 eV
(D) +4000 eV
(E) +5000 eV

Answer/Explanation

Ans:B

 

Question

Two positive point charges, both of magnitude \(4.0\times 10^{-6}C\) , are situated along the x-axis at \(x_{1}=-2.0m\) and \(x_{2}=+2.0 \). What is the electric potential at the origin of the xy-coordinate system?

(A)\(-3.6\times 10^{4}V\)
(B)\(-1.8\times 10^{4}V\)
(C) \(0v\)
(D)\(1.8\times 10^{4}V\)
(E)\(3.6\times 10^{4}V\)

Answer/Explanation

Ans:E . Electric potential due to multiple point charges can be determined using the principle of superposition. Substituting into the equation for the electric potential yields

\(V=\frac{1}{4\pi\varepsilon _{0}}\sum i \frac{qi}{r_i}\)

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