Question
Points P and Q lie between the plates of a fully charged parallel plate capacitor as shown above. The lower plate is negatively charged and the upper plate is positively charged. How do the magnitudes of the electric fields at points P and Q compare?
(A) The field is o N/C at both points.
(B) The field is the same at both points, but not o N/C.
(C)Ep > EQ
(D)EQ > Ep
▶️Answer/Explanation
Ans:B
Between the plates of a parallel plate capacitor, the field will be constant and non-zero.
Question
Which statement correctly characterizes the work done by the gas during the ABCA cycle shown in the above P-V diagram?
(A) There is no work done by the gas because the system both starts and concludes in state A.
(B) There is no work done because the work done during the transition from A→B cancels out the work done in transition from C→D.
(C) The work done by the gas is positive because the work done during the transition from A→B is greater than the work done in transition from C→D.
(D) The work done by the gas is positive because the work done during the transition from B→C is greater than the work done in transition from D→A.
▶️Answer/Explanation
Ans:C
Work is done when there is a change in volume. At constant pressure, the equation W = -PΔV tells us that when P is higher (as it is at path A→B) a greater amount of negative work is done on the gas than at lower P. Thus, the work done by the gas during the entire cycle is positive, and the constant volume paths have no influence on the amount of work.
Question
What is the equation for the electric field given by the above plot?
The electric field crosses the axis at times o ps, 1.43 ps, 2.85 ps, and 4.28 ps.
(A) \(E = \left ( \frac{0 N}{C} \right )sin ((2.85 THz)t)\)
(B) \(E = \left ( \frac{10 N}{C} \right )sin ((2.85 THz)t)\)
(C) \(E = \left ( \frac{10 N}{C} \right )sin ((2.20 THz)t)\)
(D) \(E = \left ( \frac{10 N}{C} \right )cos ((0.45 THz)t)\)
▶️Answer/Explanation
Ans:C
The field is a sine curve with an amplitude of 10 N/C. From the graph, the period of the wave is 2.85ps. The argument of sine needs to be 2πt/T.
Question
Charges are distributed as shown above. At point A is a charge of +3Q. At Bis a charge of +1Q. At C is a charge of -1Q. What is the direction of the force on a proton located at point P?
(A) Up and to the left.
(B) Down and to the left.
(C) Up and to the right.
(D) Down and to the right.
▶️Answer/Explanation
Ans:C
From the charge at point A, the field will be up and to the right. From B, the field will be up and to the left. The right component of A will be stronger than the leftward component of B. From the charge at C, the field will point down and to the right. The upward field lines from either A or B will be greater than the downward field lines from C.
Question
All of the following observations of electric field diagrams are correct EXCEPT:
(A) Longer arrows correspond to a greater field magnitude.
(B) Field lines will be larger at positions close to sources or sinks.
(C) Arrow lengths will decrease linearly with distance from sources or sinks.
(D) Arrows point in the direction in which a positive charge would experience a force.
▶️Answer/Explanation
Ans:C
Field lines represent the force a test charge would experience at a given position relative to a charge distribution. For point charges, field strength will decrease as an inverse-square as positions get farther from the sources or sinks.