<< Chapter < Page Chapter >> Page >

Test prep for ap courses

An electric dipole (with +2 q and –2 q as the two charges) is shown in the figure above. A third charge, − q is placed equidistant from the dipole charges. What will be the direction of the net force on the third charge?

Got questions? Get instant answers now!

A dashed square with four dots is shown. The dot w is in the upper left corner, x is center top, y is upper right corner and z is center bottom. A double-headed vertical line points to short horizontal lines even with the top and bottom of the grid and is on the right side.

Four objects, each with charge + q , are held fixed on a square with sides of length d , as shown in the figure. Objects X and Z are at the midpoints of the sides of the square. The electrostatic force exerted by object W on object X is F . Use this information to answer questions 39–40.

What is the magnitude of force exerted by object W on Z ?

  1. F /7
  2. F /5
  3. F /3
  4. F /2


Got questions? Get instant answers now!

What is the magnitude of the net force exerted on object X by objects W , Y , and Z ?

  1. F /4
  2. F /2
  3. 9 F /4
  4. 3 F
Got questions? Get instant answers now!

There is a horizontal x line and vertical y line splitting each other at their centers and marked 0 at the intersection. A large white circle seems to bend dashes centered on the 0 and is marked S. To the left center of the horizontal line is a dot surrounded by a white circle marked R above the line and -d below the line. On the right center of the line is a dot surrounded by a white circle marked T above the line at the dot and d below the line around the dot. There are evenly closely spaced dashes in the entire grid except where the two small circles R and T and larger circle S are. The dashed seemed to be pushed away from the center of each circle and lines appear to form in elliptical patterns with R and T on the right and left edges of the ellipses.
Electric field with three charged objects.

The figure above represents the electric field in the vicinity of three small charged objects, R , S , and T . The objects have charges − q , +2 q , and − q , respectively, and are located on the x -axis at − d , 0, and d . Field vectors of very large magnitude are omitted for clarity.

(a) i) Briefly describe the characteristics of the field diagram that indicate that the sign of the charges of objects R and T is negative and that the sign of the charge of object S is positive.

ii) Briefly describe the characteristics of the field diagram that indicate that the magnitudes of the charges of objects R and T are equal and that the magnitude of the charge of object S is about twice that of objects R and T .

For the following parts, an electric field directed to the right is defined to be positive.

(b) On the axes below, sketch a graph of the electric field E along the x -axis as a function of position x .

There is an arrow pointing up in the center of the diagram labeled E above the arrow. A horizontal ticked line shows bisects the E line and 0 marks the intersection. X is the label on the to the right of the horizontal line. –d appears below the x line about 2/3 of the length from the 0 on the left and d appears about 2/3 of the length from the 0 to the right.
An Electric field (E) axis and Position (x) axis.

(c) Write an expression for the electric field E along the x -axis as a function of position x in the region between objects S and T in terms of q , d , and fundamental constants, as appropriate.

(d) Your classmate tells you there is a point between S and T where the electric field is zero. Determine whether this statement is true, and explain your reasoning using two of the representations from parts (a), (b), or (c).

(a) i) Field vectors near objects point toward negatively charged objects and away from positively charged objects.

(a) ii) The vectors closest to R and T are about the same length and start at about the same distance. We have that q R / d 2 = q T / d 2 , so the charge on R is about the same as the charge on T . The closest vectors around S are about the same length as those around R and T . The vectors near S start at about 6 units away, while vectors near R and T start at about 4 units. We have that q R / d 2 = q S / D 2 , so q S / q R = D 2 / d 2 = 36 / 16 = 2.25 , and so the charge on S is about twice that on R and T .


A horizontal line divides the top and bottom half of the picture. Three equally spaced vertical lines bisect the horizontal line. The lines divide the figure into eight sections, where the top first and third sections have curves. The bottom second and fourth sections also have curves. The curve in the top left starts horizontally and near the end quickly curves to vertical. The last curve in the bottom right starts from the bottom of the picture and raises quickly to the horizontal line and then approaches and goes below along the horizontal line. The second section on the bottom is an upside down U starting on the bottom of the figure and raising quickly to the horizontal line, staying at the horizontal for a long time and then quickly descends to go along the vertical line in that section. The third curve is a U mirror image and is in the third section on the top. The line starts at the top of the diagram, quickly descends to the horizontal line then goes along the horizontal line and near the end of the section, quickly rises to the top of the section.
A vector diagram.


E = k [ q ( d + x ) 2 + 2 q ( x ) 2 + q ( d x ) 2 ]

(d) The statement is not true. The vector diagram shows field vectors in this region with nonzero length, and the vectors not shown have even greater lengths. The equation in part (c) shows that, when 0 < x < d , the denominator of the negative term is always greater than the denominator of the third term, but the numerator is the same. So the negative term always has a smaller magnitude than the third term and since the second term is positive the sum of the terms is always positive.

Got questions? Get instant answers now!

Section summary

  • Drawings of electric field lines are useful visual tools. The properties of electric field lines for any charge distribution are that:
  • Field lines must begin on positive charges and terminate on negative charges, or at infinity in the hypothetical case of isolated charges.
  • The number of field lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.
  • The strength of the field is proportional to the closeness of the field lines—more precisely, it is proportional to the number of lines per unit area perpendicular to the lines.
  • The direction of the electric field is tangent to the field line at any point in space.
  • Field lines can never cross.

Conceptual questions

Compare and contrast the Coulomb force field and the electric field. To do this, make a list of five properties for the Coulomb force field analogous to the five properties listed for electric field lines. Compare each item in your list of Coulomb force field properties with those of the electric field—are they the same or different? (For example, electric field lines cannot cross. Is the same true for Coulomb field lines?)

Got questions? Get instant answers now!

[link] shows an electric field extending over three regions, labeled I, II, and III. Answer the following questions. (a) Are there any isolated charges? If so, in what region and what are their signs? (b) Where is the field strongest? (c) Where is it weakest? (d) Where is the field the most uniform?

Five field lines represented by long arrows horizontally from left to right are shown. Two arrows diverge from other three, one arrow runs straight toward right and two arrows end abruptly.
Got questions? Get instant answers now!

Problem exercises

(a) Sketch the electric field lines near a point charge + q . (b) Do the same for a point charge –3.00 q .

Got questions? Get instant answers now!

Sketch the electric field lines a long distance from the charge distributions shown in [link] (a) and (b)

Got questions? Get instant answers now!

[link] shows the electric field lines near two charges q 1 size 12{q rSub { size 8{1} } } {} and q 2 size 12{q rSub { size 8{2} } } {} . What is the ratio of their magnitudes? (b) Sketch the electric field lines a long distance from the charges shown in the figure.

Field lines between a positive and a negative charge represented by curved lines is shown
The electric field near two charges.
Got questions? Get instant answers now!

Sketch the electric field lines in the vicinity of two opposite charges, where the negative charge is three times greater in magnitude than the positive. (See [link] for a similar situation).

Got questions? Get instant answers now!

Questions & Answers

Determine the total force and the absolute pressure on the bottom of a swimming pool 28.0m by 8.5m whose uniform depth is 1 .8m.
Henny Reply
for the answer to complete, the units need specified why
muqaddas Reply
That's just how the AP grades. Otherwise, you could be talking about m/s when the answer requires m/s^2. They need to know what you are referring to.
Suppose a speck of dust in an electrostatic precipitator has 1.0000×1012 protons in it and has a net charge of –5.00 nC (a very large charge for a small speck). How many electrons does it have?
Alexia Reply
how would I work this problem
how can you have not an integer number of protons? If, on the other hand it supposed to be 1e12, then 1.6e-19C/proton • 1e12 protons=1.6e-7 C is the charge of the protons in the speck, so the difference between this and 5e-9C is made up by electrons
what is angular velocity
Obaapa Reply
angular velocity can be defined as the rate of change in radian over seconds.
Why does earth exert only a tiny downward pull?
Mya Reply
Why is light bright?
Abraham Reply
what is radioactive element
Attah Reply
an 8.0 capacitor is connected by to the terminals of 60Hz whoes rms voltage is 150v. a.find the capacity reactance and rms to the circuit
Aisha Reply
thanks so much. i undersooth well
Valdes Reply
what is physics
Nwafor Reply
is the study of matter in relation to energy
physics can be defined as the natural science that deals with the study of motion through space,time along with its related concepts which are energy and force
a submersible pump is dropped a borehole and hits the level of water at the bottom of the borehole 5 seconds later.determine the level of water in the borehole
Obrian Reply
what is power?
aron Reply
power P = Work done per second W/ t. It means the more power, the stronger machine
e.g. heart Uses 2 W per beat.
A spherica, concave shaving mirror has a radius of curvature of 32 cm .what is the magnification of a persons face. when it is 12cm to the left of the vertex of the mirror
Alona Reply
did you solve?
my name is Abu m.konnek I am a student of a electrical engineer and I want you to help me
the magnification k = f/(f-d) with focus f = R/2 =16 cm; d =12 cm k = 16/4 =4
what do we call velocity
A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in a horizontal plane. A typical weather vane has a large cross-sectional area perpendicular to the direction the arrow is pointing, like a “One Way” street sign. The purpose of the weather vane is to indicate the direction of the wind. As wind blows pa
Kavita Reply
what about the wind vane
If a prism is fully imersed in water then the ray of light will normally dispersed or their is any difference?
Anurag Reply
the same behavior thru the prism out or in water bud abbot
If this will experimented with a hollow(vaccum) prism in water then what will be result ?
Practice Key Terms 4

Get the best College physics for ap... course in your pocket!

Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics for ap® courses' conversation and receive update notifications?