Question

1- What is the conclusion of the first video activity (Electric Force vs. Charge)?

Explain your answer by referring to the graph “Force vs. Charge” that you created in Part H of the first video activity. Please insert the data table and the graph side by side and then write your answer underneath.

Viedo Link: https://mediaplayer.pearsoncmg.com/assets/_frames.true/secs-coulomb-force-2-force-vs-charge

Part A

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Assume that the Styrofoam slab and the fur are both initially neutral, and that the slab charged negatively after it is rubbed with the fur. Describe what happens.

Mark all the correct statements among those provided below.

Protons move from the fur to the slab.

Protons move from the slab to the fur.

Electrons move from the fur to the slab.

Electrons move from the slab to the fur.

The fur becomes negative after rubbing the slab.

The fur becomes positive after rubbing the slab.

The fur remains neutral after rubbing the slab.

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Since the electrons are the mobile charge carriers, they are transferred in this case from the fur to the slab. As a result, the fur will have fewer electrons than it began with, thus leaving it positively charged.

Part B

Part complete

After the slab is charged, and the electrophorus is placed on the slab, the student briefly touches the electrophorus, effectively grounding it. After that, the electrophorus appears to be charged. By what method is it charged?

Select the best answer from the choices provided.

induction

conduction

rubbing

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Charging by induction means that a charged object (the slab) causes the other one (the electrophorus) to become charged by influencing (inducing) the process of charge transfer; however, little or almost no charge transfer occurs directly between the charging object and the object being charged.

What, exactly, is happening in this particular case?

Part C

Part complete

As the hand touches the electrophorus, in which direction do the electrons move?

Select the best answer from the choices provided.

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The electrons move from the electrophorus to the slab.

The electrons move from the hand to the slab.

The electrons move from the hand to the electrophorus.

The electrons move from the slab to the electrophorus.

The electrons move from the slab to the hand.

The electrons move from the electrophorus to the hand.

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The negatively charged slab is pushing the “free” electrons in the conducting electrophorus away. Before the electrophorus is grounded, there is no escape path for those electrons. Therefore, the electrophorus is polarized; that is, its top surface is negative, and the bottom one, the one nearest the slab, is positive. However, the electrophorus remains neutral until the hand provides a conducting path that allows some electrons escape from the electrophorus, rendering it positive.

Part D

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Observe the process by which the grey and the red spheres are charged using the electrophorus. After each sphere is first charged, what are their charges?

Select the best answer from the choices provided.

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The gray sphere has a negative charge and the red sphere has a positive charge.

The gray sphere has a negative charge and the red sphere has a negative charge.

The gray sphere has a positive charge and the red sphere has a positive charge.

The gray sphere has a positive charge and the red sphere has a negative charge.

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The electrophorus becomes less positive once it pulls some electrons away from the red sphere. However, the electrophorus is replaced on the slab and recharged by grounding it before it proceeds to charge the grey sphere. This ensures that both spheres initially have about the same amount of charge.

Since the red sphere is suspended on an insulating handle, and no other objects toch it, we cna assume that its charge remains constant throughout the experiment. However, this is not the case for the gray sphere.

Part E

Part complete

Before we proceed with the quantitative part of this set, let us make sure that you can see the reading of the electronic balance clearly on your screen. What is the reading of the scale right after the red sphere is first charged, and the electrophorus has been replaced on the slab? Use the “pause” and the “frame-by-frame-advance” buttons to navigate to the appropriate part in the video.

Select the best answer from the choices provided.

0.000005N0.000005N

0.00005N0.00005N

5N5N

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Note that the scale has been adjusted so that it reads approximately zero when the only force acting on it is gravity. A small difference from zero is frequently observed with sensitive scales.

Part F

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What is the reading of the scale after both spheres have been charged, and the student is getting ready to bring the hand-held sphere close to the bottom sphere? Consider the moment when the hand-held sphere is about half-way between the handle of the electrophorus and the bottom sphere. Use the “pause” and the “frame-by-frame-advance” buttons to navigate to the appropriate part in the video.

Express your answer in Newtons. Use three significant figures in your answer.

0.00175

NN

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Now let us see what happens when the charge of the bottom sphere begins to change.

Part G

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The bottom sphere is briefly touched by another conducting sphere of the same size, mounted on an insulating handle. Assuming that the charge of the bottom sphere is +q+q before it is touched by the other sphere, what is its charge after it is touched?

Select the best answer from the choices provided.

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−q/2−q/2

−q−q

−2q−2q

+q/2+q/2

+q+q

+2q+2q

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Note that, after each touch of the spheres, the handheld one is briefly touched by hand—that is, neutralized—through grounding. That means that each subsequent touch would remove half of the charge from the bottom sphere.

Part H

Part complete

We will now try to determine how the magnitude of the Coulomb force between the spheres depends on the charge of the bottom sphere. Now that we understand that the charge of the bottom sphere is halved after each touch, create a graph “force vs. charge of the bottom sphere” using +q+q, +q/2+q/2, +q/4+q/4, etc. as the charge values. What is the most reasonable conclusion that can be made from the graph about the magnitude of the Coulomb force?

Select the best answer from the choices provided.

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It is inversely proportional to the square of the charge of the bottom sphere.

It is inversely proportional to the charge of the bottom sphere.

It is directly proportional to the square of the charge of the bottom sphere.

It is directly proportional to the charge of the bottom sphere.

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It seems reasonable to assume that the magnitude of the force would be also directly proportional to the charge of the top sphere—in other words, directly proportional to the product of the charges of the spheres! (Recall that the magnitude of the force of gravity between two spherical masses is directly proportional to the product of the masses of the spheres.)

Now, let's do one last exercise to make sure you are comfortable with this idea.

Part I

Part complete

Let us suppose the magnitude of the original Coulomb force between the two charged spheres is FF. In this scenario, a third sphere touches the grey sphere and the red sphere multiple times, being grounded each touch. If the grey sphere is touched twice, and the red sphere is touched three times, what is the magnitude of the Coulomb force between the spheres now?

Select the best answer from the choices provided.

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F/5F/5

F/6F/6

F/8F/8

F/16F/16

F/32F/32

In another DMV-based activity, you will explore (if you haven’t already done that) the dependence of the Coulomb force on the distance between the spheres.

Answer 1

Part I) with each touch ,the charge on the sphere (being touched) reduce to half.

Grey sphere touched twice so final charge will be q/4,( q/2 in first ,q/2 by 2 in second)

Red sphere is touched thrice final charge q/8.

So total force is reduced by factor of 8*4 = 32

F= f/32.