Physics 180B – Ch. 16-21 – Concepts (HW + Modules) – Flashcards
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A: It is strongly attracted
Correct: This charge is said to be "induced" by the presence of the electric field of the charged ball: It is not transferred by the ball.
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PART A. A small metal ball is given a negative charge, then brought near (i.e., within about 1/10 the length of the rod) to end A of the rod (Figure 1) . What happens to end A of the rod when the ball approaches it closely this first time?
- It is strongly repelled.
- It is strongly attracted.
- It is weakly attracted.
- It is weakly repelled.
- It is neither attracted nor repelled.
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There is negative charge spread evenly on both ends.
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PART B. After a great many contacts with the charged ball, how is the charge on the rod arranged (when the charged ball is far away)?
- There is positive charge on end B and negative charge on end A.
- There is negative charge spread evenly on both ends.
- There is negative charge on end A with end B remaining neutral.
- There is positive charge on end A with end B remaining neutral.
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A: It is strongly repelled.
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PART C. How does end A of the rod react when the charged ball approaches it after a great many previous contacts with end A? Assume that the phrase "a great many" means that the total charge on the rod dominates any charge movement induced by the near presence of the charged ball.
- It is strongly repelled.
- It is strongly attracted.
- It is weakly attracted.
- It is weakly repelled.
- It is neither attracted nor repelled.
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A: It is strongly repelled
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PART D. How does end B of the rod react when the charged ball approaches it after a great many previous contacts with end A?
- It is strongly repelled.
- It is strongly attracted.
- It is weakly attracted.
- It is weakly repelled.
- It is neither attracted nor repelled.
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A:The marble has lost the same number of electrons acquired by the piece of silk.
A:The marble acquires a positive charge and attracts the piece of silk.
Correct: This was a simple example of electrostatic interactions. When you rub a piece of glass against a piece of silk, the glass acquires a positive charge and the silk acquires a negative charge because some electrons were transferred from the glass to the silk in the rubbing process. The silk acquires the same net charge as the glass, but with the opposite sign. This charge distribution causes the silk and the glass marble to be attracted to one another.
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PART A. A glass marble is rubbed against a piece of silk. As a result the piece of fabric acquires extra electrons. What happens to the glass marble?
- The marble has lost the same number of electrons acquired by the piece of silk.
- The marble has acquired the same number of electrons acquired by the piece of silk.
- The marble acquires a positive charge and repels the piece of silk.
- The marble acquires a positive charge and attracts the piece of silk.
- The marble acquires a negative charge and attracts the piece of silk.
- The marble acquires a negative charge and repels the piece of silk.
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A: Marbles 1 and 2 repel each other, but no interaction occurs with marble 3.
Correct: As you have seen here, electrostatic interactions occur between charged objects. Objects with like charges repel each other, whereas objects with opposite charges attract each other.
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PART B. Two glass marbles (1 and 2), each supported by a nylon thread, are rubbed against a piece of silk and then are placed near a third glass marble (3), also supported by a similar thread. Assuming that marble 3 has not been in contact with the piece of fabric, which of the following statements best describes the situation when the three marbles are brought together?
To keep things simple in this Tutorial, we will ignore the effects of polarization and just focus on the overall charge of each object.
- Marbles 1 and 2 attract each other, but no interaction occurs with marble 3.
- Both marbles 1 and 2 attract marble 3.
- The three marbles will repel each other.
- Marbles 1 and 2 repel each other, but no interaction occurs with marble 3.
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Learning Goal:
When a test charge is brought near a charged object, we know from Coulomb's law that it will experience a net force (either attractive or repulsive, depending on the nature of the object's charge). A test charge may also experience an electric force when brought near a neutral object. Any attraction of a neutral insulator or neutral conductor to a test charge must occur through induced polarization. In an insulator, the electrons are bound to their molecules. Though they cannot move freely throughout the insulator, they can shift slightly, creating a rather weak net attraction to a test charge that is brought close to the insulator's surface. In a conductor, free electrons will accumulate on the surface of the conductor nearest the positive test charge. This will create a strong attractive force if the test charge is placed very close to the conductor's surface.
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Problem 3: A Test Charge Determines Charge on Insulating and Conducting Balls
Consider three plastic balls (A, B, and C), each carrying a uniformly distributed charge equal to either +Q, -Q or zero, and an uncharged copper ball (D). A positive test charge (T) experiences the forces shown in the figure when brought very near to the individual balls. The test charge T is strongly attracted to A, strongly repelled from B, weakly attracted to C, and strongly attracted to D.
Assume throughout this problem that the balls are brought very close together.
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A: Strongly attractive
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PART A. What is the nature of the force between balls A and B?
- Strongly attractive
- Strongly repulsive
- Weakly attractive
- Neither attractive nor repulsive
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A: Weakly attractive
Correct: Recall that ball C is composed of insulating material, which can be polarized in the presence of an external charged object such as ball A. Once polarized, there will be a weak attraction between balls A and C, because the positive and negative charges in ball C are at slightly different average distances from ball A. If ball C had a very small negative charge the test charge would have the same response (weakly attractive) but it would have a weak repulsive interaction with ball A. However, a smaller negative charge is not one of the options.
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PART B. What is the nature of the force between balls A and C?
- Strongly attractive
- Strongly repulsive
- Weakly attractive
- Neither attractive nor repulsive
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A: Attractive
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PART C. What is the nature of the force between balls A and D?
- Attractive
- Repulsive
- Neither attractive nor repulsive
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A: Neither attractive nor repulsive
Correct: Because the test charge T is neither strongly attracted to nor repelled from ball C, ball C must have zero net charge. Since ball D also has zero net charge, there will not be any force between the two balls.
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PART D. What is the nature of the force between balls D and C?
- Attractive
- Repulsive
- Neither attractive nor repulsive
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A: F = 14 N
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Problem 4. What is the repulsive electrical force between two protons 4.0×10−15m apart from each other in an atomic nucleus?
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A: F = 7.93×10^6 N
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PART A. Determine the magnitude of the force on each charge.
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A: Along the line between the charge and the center of the square outward of the center
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PART B: Determine the direction of the force on a charge.
- Along the line between the charge and the center of the square toward the center
- Along the line between the charge and the center of the square outward of the center
- Along the side of the square toward the other charge that lies on the side
- Along the side of the square outward of the other charge that lies on the side
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Learning Goal: Coulomb's law gives the electrostatic force F⃗ acting between two charges. The magnitude F of the force between two charges q1 and q2 depends on the product of the charges and the square of the distance r between the charges:
F=k*(|q1*q2|)/(r^2)
where k=1/(4*π*ϵ0) = 8.99×109N⋅m2/C2.
The direction of the force is along the line connecting the two charges. If the charges have the same sign, the force will be repulsive. If the charges have opposite signs, the force will be attractive. In other words, opposite charges attract and like charges repel.
Because the charges are not in contact with each other, there must be an intermediate mechanism to cause the force. This mechanism is the electric field. The electric field at any location is equal to the force per unit charge experienced by a charge placed at that location. In other words, if a charge q experiences a force F⃗ , the electric field E⃗ at that point is: E⃗ = F⃗ q.
The electric field vector has the same direction as the force vector on a positive charge and the opposite direction to that of the force vector on a negative charge.
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Problem 6. An electric field can be created by a single charge or a distribution of charges. The electric field a distance r from a point charge q′ has magnitude: E=k *(|q′|)/(r^2)
The electric field points away from positive charges and toward negative charges. A distribution of charges creates an electric field that can be found by taking the vector sum of the fields created by individual point charges. Note that if a charge q is placed in an electric field created by q′, q will not significantly affect the electric field if it is small compared to q′.
Imagine an isolated positive point charge with a charge Q (many times larger than the charge on a single electron).
