Question
The figure shows the electric field in a region surrounding two charges. The vectors in the diagram are not scaled to represent the strength of the electric field but show only the direction for the field at that point. Which two points have electric fields of the same magnitude?
(A) A andB
(B) B andC
(C) C andD
(D) D and A
▶️Answer/Explanation
Ans:D
The electric field is symmetrical along a horizontal axis through the two charges. Points A and D are the same distance away from both charges where the electric fields will have the same strength.
Question
A grounded metal tank containing water has a spout at the bottom through which water flows out in a steady stream. A negatively charged ring is placed near the bottom of the tank such that the water flowing out of the tank passes through without touching the ring. See figure above. Which of the following statements about the charge of the water flowing out of the tank is correct?
(A) Water exiting the tank is neutral because the tank is grounded.
(B) Water exiting the tank is negative because charges jump from the ring to the water.
(C) Water exiting the tank will be positive because the ring repels negative charges in the water and tank toward the top of the tank and ground.
(D) Water exiting the tank will be positive at first but negative as the water level in the tank goes down. The ring attracts positive charges to the bottom of the tank and pushes negative charges to
the top of the tank. The positive water flows out first and negative water flows out last.
▶️Answer/Explanation
Ans:C
The negative ring polarizes the tank and drives negative charges toward the ground, creating a tank of positively charged water.
Question
Two parallel metal plates are connected to a battery as shown in the figure. A small negative charge is placed at location O and the force on the charge is measured. Compared to location 0, how does the force on the charge change as it is moved to locations A, B, C, and D?
(A) The force at location A is the same magnitude as at location O but in the opposite direction.
(B) The force at location Bis smaller than at location O but in the same direction.
(C) The force at location C is smaller than at location O but in the same direction.
(D) The force at location D is the same magnitude as at location O but in the opposite direction.
▶️Answer/Explanation
Ans:B
The electric field produced by a charged parallel plate capacitor is uniform in strength and direction everywhere between the plates away from the edges of the capacitor. Therefore, the electric force on the charges must be identical for locations A, C, and 0. Near the edges of the capacitor, the field bows out a bit and is not quite as strong. This is consistent with answer choice B. Outside the capacitor the electric field is much weaker.
Question
The circuit shown in the figure consists of three identical resistors, two ammeters, a battery, a capacitor, and a switch. The capacitor is initially uncharged and the switch is open. Which of the following correctly compares the original open switch readings of the ammeters to their readings after the switch has been closed for a very long time?
A1 A2
(A) decreases decreases
(B) decreases stays the same
(C) decreases increases
(D) stays the same stays the same
▶️Answer/Explanation
Ans:D
After the switch is closed and the capacitor has been connected to the circuit for a very long time, it has had time to fully charge and it will behave like an open circuit line. No current will bypass the middle resistor and, for all intents and purposes, the circuit looks just like it did before the switch was ever closed, with the exception that the capacitor now stores both charge and energy.