Question
A beam of various particles is launched into the space between two oppositely charged parallel plates as shown in the figure. It is known that the particles are all traveling at the same initial velocity as they enter the region, and that particle P is a proton. What else can be inferred?
(A) Particle A could be a -particle.
(B) Particle B could be an a-particle.
(C) Particle Chas too much mass to be affected by the external force field.
(D) Particle D could be an electron.
▶️Answer/Explanation
Ans:
A—Particle A bends in the opposite direction with a tighter radius, indicating that it is negative with a smaller mass. A §-particle would satisfy these requirements. Particle B must be negative and have a larger mass. All charges would be affected in some way by the electric field; thus particle C must have no charge. Particle D must be positive and have a smaller mass than a proton. Perhaps it is a positron?
Question
Which of the following correctly represents the relationship between the electric field strengths at locations A, B, and C?
(A) \(E_A> E_B> E_e\)
(B) \(E_A =E_B> E_e\)
(C) \(E_B> E_A> E_e\)
(D) \(E_B>E_A=E_e\)
▶️Answer/Explanation
Ans:
C—The electric fields from the two charges add up at location B, making it the strongest. The electric field from the two charges are in opposite directions at A and C. However, at location A, the electric field must be larger than that at location C, due to its proximity to the largest charge.
Question
In which of the following cases would it be appropriate to ignore the gravitational force? (Select two answers.)
(A) A teacher charges a balloon with her hair and demonstrates how the balloon can be stuck to the ceiling by the electrostatic force.
(B) A scientist suspends a charged droplet of oil between two charged horizontal capacitor plates.
(C) Electrons move through a wire in an electrical circuit.
(D) A physics student is asked to calculate the force between the proton and electron in a hydrogen atom.
▶️Answer/Explanation
Ans:
C and D—The gravitational force can only be neglected when it has little influence on the behavior of the object, or when it is very small in comparison to other forces. Even though the electric force is larger than the gravitational force acting on the balloon, gravity is still a sizable component of the overall forces on the balloon. The gravity force and electric force must be equal for the droplet of oil to levitate. However, the gravitational force on subatomic particles is negligible compared to the electric force experienced in electrical circuits and between charged particles in an atom.
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
In an experiment, a scientist sends an electron beam through a cloud chamber and observes that the electron accelerates in a downward direction at a constant rate as shown in the figure. Which of the following could the scientist conclude?
(A) Earth’s gravitational field is causing the beam to change direction.
(B) A uniform electric field is causing the beam to change direction.
(C) A uniform magnetic field is causing the beam to change direction.
(D) The electron beam collided with another particle.
▶️Answer/Explanation
Ans:B
Since the acceleration is constant in direction, this cannot be a collision or a force from a magnetic field. The electron mass is very small, so any gravitational acceleration on the election will be too small to see in a cloud chamber. However, a uniform electric field could easily produce this parabolic trajectory effect.