Física Resolvida - Vídeo aula: Questão/Exercício Vestibular UFSC Campo Elétrico

Summary

This video explains a physics problem involving a negatively charged ball suspended between two charged plates. The ball experiences both gravitational and electric forces, with the dynamometer registering 3 N while the weight is only 2 N. The electric field and the charges on the plates are examined, leading to a breakdown of the propositions. The video uses formulas and principles to calculate the ball's charge and determine the correct propositions about the direction of the electric field and the magnitude of the ball’s charge. The solution is explained step-by-step, with a final answer that combines the correct propositions.

Takeaways

• 😀 The problem involves a negatively charged ball suspended between two parallel plates with an electric field of 4 × 10⁶ N/C.
• 😀 The weight of the ball is 2 N, but the dynamometer reads 3 N, indicating an additional force due to the electric field.
• 😀 The force pulling the ball downward is 1 N, which is caused by the electric field between the plates.
• 😀 The negative charge on the ball is indicated by the downward force caused by the electric field.
• 😀 The correct configuration of the plates is Plate A with a negative charge and Plate B with a positive charge.
• 😀 The charge on the ball is calculated to be 0.25 × 10⁻⁶ C using the electric field and force values.
• 😀 The proposition stating the magnitude of the ball's charge as 4 × 10⁻⁶ C is incorrect.
• 😀 If the ball's charge were positive, it would experience an upward force, not a downward force, and would not remain in equilibrium.
• 😀 The question asks to identify which propositions are correct about the charge distribution and behavior of the ball.
• 😀 The correct answer to the problem is the sum of propositions 2 and 4, resulting in a total of 10.

Q & A

What is the main focus of the problem in the script?

The main focus of the problem is the behavior of a negatively charged ball hung between two parallel plates with the same magnitude of charge, and understanding the forces acting on the ball, including its weight and the electric force.

Why does the dynamometer register 3 Newtons instead of the expected 2 Newtons?

The dynamometer registers 3 Newtons because in addition to the 2 Newtons of weight pulling the ball down, there is an additional 1 Newton of electric force acting downward, resulting in a total force of 3 Newtons.

What is the significance of the charge of the ball being negative in this context?

The negative charge of the ball means that the ball experiences a downward electric force because the upper plate is positively charged and the lower plate is negatively charged, which causes the ball to move downward.

How can the charge on the ball be determined from the electric field data?

The charge on the ball can be determined using the formula for the electric field: E = F/q. Given the electric field strength (4 x 10^6 N/C) and the force (1 Newton), the charge is calculated as q = F/E, which results in a charge of 0.25 x 10^-6 C.

Why is item 1 in the list of propositions incorrect?

Item 1 is incorrect because plate A does not have a positive charge. Based on the forces and direction of the electric force on the ball, plate A must be negatively charged, and plate B positively charged.

What does item 8 in the proposition list state about the magnitude of the ball's charge?

Item 8 states that the magnitude of the ball's charge is 0.25 x 10^-6 C, which is confirmed by the calculation derived from the electric field data.

Why is item 32 incorrect in the list of propositions?

Item 32 is incorrect because if the charge of the ball were positive, the forces acting on the ball would reverse. Instead of being pulled downwards, the ball would experience an upward force and would not remain in equilibrium in the same position.

What does the problem indicate about the role of the hole in the wire's support?

The problem indicates that the hole through which the wire supporting the ball passes does not change the electric field between the plates. This suggests that the hole does not influence the electric field or the forces acting on the ball.

How do the charges of the plates affect the motion of the ball?

The charges on the plates create an electric field that exerts a force on the charged ball. The negative charge on the ball causes it to be attracted to the positively charged plate and repelled by the negatively charged plate, resulting in a downward force.

What conclusion can be drawn about the equilibrium condition of the ball in this problem?

The ball reaches equilibrium when the upward force from the dynamometer balances the downward forces of the weight and the electric field. If the charge on the ball were reversed, the ball would not remain in equilibrium, indicating that the polarity of the charge is crucial for maintaining the balance of forces.