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Chapter 14: Q14Q (page 579)

A small glass ball is rubbed all over with a small silk cloth and acquires a charge of +5nC. The silk cloth and the glass ball are placed 30 cm apart.

(a) On a diagram like that shown in Figure 14.71, draw the electric field vectors qualitatively at the locations marked 脳. Pay careful attention to directions and to relative magnitudes. Use dashed lines to explain your reasoning graphically, and draw the final electric field vectors with solid lines.

(b) Next, a neutral block of copper is placed between the silk and the glass.

On a diagram like that shown in Figure 14.72, carefully show the approximate charge distribution for the copper block and the electric field vectors inside the copper at the locations marked 脳.

(c) The copper block is replaced by a neutral block of plastic. Carefully show the approximate molecular polarization of the plastic block at the locations marked 脳 in Figure 14.73.

(d) Even if you have to state your result as an inequality, make as quantitative a statement as you can about the electric field at the location of the glass ball and the net force on the ball when the plastic block is in place compared to when there is no block. Explain briefly.

Short Answer

Expert verified

(a)

(b)

(c)

(d)The electric field at the location of the glass ball would be zero and the net electric force on the ball in the presence of the plastic block is lesser in compared to when there is no block.

Step by step solution

01

Understanding about the electric field

In this problem, the concept of the electric field at a specific point due to a specific electric charge will be used to evaluate the electric field's direction. The electric field depends on the electric charge鈥檚 nature.

02

(a) Representation of electric field vectors

When the small glass ball is rubbed with a small silk cloth, then the glass ball will acquire an electric charge as, and the small silk cloth will acquire charge.

The representation of electric field vectors qualitatively at the marked location is given below.

The solid lines represent the final electric field vectors, and a dashed line represents the magnetic field vectors at every marked location.

Thus, the magnetic field vectors at the marked location are represented by the dashed lines, and solid lines represent the final electric field vector.

03

(b) Representation of the approximate charge distribution for the copper block and the electric field vectors inside the copper block

A neutral copper block is placed between the silk cloth and the glass ball then, the copper block's surface closer to the glass ball would be charged as positive, and the surface of the copper block close to the silk cloth would be charged as negative.

The approximate charge distribution for the copper block and the electric field vectors inside the block is represented below.

All the electric charges lie on the copper block's outer surface, so the electric field inside the conductor will be zero, and no electric field vector exists inside the copper block at the marked locations.

Thus, the electric field vectors inside the copper block at the marked locations don鈥檛 exist.

04

(c) Representation of the approximate molecular polarization of the plastic block

Now the copper block is replaced by a neutral block of plastic then the surface of the plastic block closure to the glass ball would be charged as positive whereas the surface of the plastic block closure to the silk cloth would be charged as negative.

The approximate molecular polarization of the plastic block at the marked location inside the plastic block is given below.

The approximate molecular polarization at the marked location inside the plastic block is represented by the solid lines with an arrow.

Thus, the approximate molecular polarization inside the plastic block is directed towards the negative charge side (towards silk cloth).

05

(d) Comparison of the net electric force on the ball when the plastic ball is in place compared to when there is no block   

When there is a plastic block in place compared to when there is no block, then the external electric field polarizes the plastic block, and it results in the field being intensely reduced. Theelectric field at the location of the glass ballwould be zero.

The net electric force on the ball would be less in comparison to when there is no block (only air exists) because the plastic block would offer extra resistance that is absent when there is no block.

Thus, the net electric force on the ball in the presence of the plastic block is lesser in compared to when there is no block.

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Most popular questions from this chapter

Blocks A and B are identical metal blocks. Initially block A is neutral, and block B has a net charge of5nC.Using insulating handles, the blocks are moved so they touch each other. After touching for a few seconds, the blocks are separated (again using insulating handles). (a) What is the final charge of block A? (b) What happened while the blocks were in contact with each other? (1) Protons moved from block B to block A. (2) Positrons moved from block B to block A. (3) Electrons moved from block A to block B. (4) Both protons and electrons moved. (5) No charged particles moved.

Two small, negatively charged plastic spheres are placed near a neutral iron block, as shown in Figure 14.89. Which arrow (a鈥搄) in Figure 14.89 best indicates the direction of the net electric field at location A?

You take two invisible tapes of some unknown brand, stick them together, and discharge the pair before pulling them apart and hanging them from the edge of your desk. When you bring an uncharged plastic pen withinof10CM either the U tape or the L tape you see a slight attraction. Next you rub the pen through your hair, which is known to charge the pen negatively. Now you find that if you bring the charged pen withinrole="math" localid="1655718752350" 8CMof the L tape you see a slight repulsion, and if you bring the pen withinrole="math" localid="1655718766744" 12CMof the U tape you see a slight attraction. Briefly explain all of your observations.

: A thin, hollow spherical plastic shell of radius \({\bf{R}}\)carries a uniformly distributed negative charge \({\bf{ - Q}}\). A slice through the plastic shell is shown in Figure 14.95. To the left of the spherical shell are four charges packed closely together as shown (the distance \({\bf{s}}\) is shown greatly enlarged for clarity). The distance from the center of the four charges to the center of the plastic shell is \({\bf{L}}\) , which is much larger than \({\bf{s}}\left( {{\bf{L}} \gg {\bf{s}}} \right)\). Remember that a uniformly charged sphere makes an electric field as though all the charge were concentrated at the center of the sphere.

(a)Calculate the \({\bf{x}}\) and \({\bf{y}}\) components of the electric field at location B, a distance \({\bf{b}}\) to the right of the outer surface of the plastic shell. Explain briefly, including showing the electric field on a diagram. Your results should not contain any symbols other than the given quantities \({\bf{R,Q,q,s,L}}\), and \({\bf{b}}\)(and fundamental constants). You need not simplify the final algebraic results except for taking into account the fact that \({\bf{L}} \gg {\bf{s}}\).

(b)What simplifying assumption did you have to make in part (a)?

(c)The plastic shell is removed and replaced by an uncharged metal ball, as in Figure 14.96. At location Ainside the metal ball, a distance \({\bf{b}}\)to the left of the outer surface of the ball, accurately draw and label the electric field\({{\bf{\vec E}}_{{\bf{ball}}}}\) due to the ball charges and the electric field \({{\bf{\vec E}}_{\bf{4}}}\) of the four charges. Explain briefly.

(d)Show the distribution of ball charges.

(e)Calculate the \({\bf{x}}\) and \({\bf{y}}\) components of the net electric field at location A.

You rub a clear plastic pen with wool, and observe that a strip of invisible tape is attracted to the pen. Assuming that the pen has a net negative charge, which of the following could be true? Select all that apply. (1) The tape might be negatively charged. (2) The tape might be positively charged. (3) The tape might be uncharged. (4) There is not enough information to conclude anything.
See all solutions

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