<< Chapter < Page Chapter >> Page >

Always keep in mind that field lines serve only as a convenient way to visualize the electric field; they are not physical entities. Although the direction and relative intensity of the electric field can be deduced from a set of field lines, the lines can also be misleading. For example, the field lines drawn to represent the electric field in a region must, by necessity, be discrete. However, the actual electric field in that region exists at every point in space.

Field lines for three groups of discrete charges are shown in [link] . Since the charges in parts (a) and (b) have the same magnitude, the same number of field lines are shown starting from or terminating on each charge. In (c), however, we draw three times as many field lines leaving the + 3 q charge as entering the q . The field lines that do not terminate at q emanate outward from the charge configuration, to infinity.

Three pairs of charges and their field lines are shown. The charge on the left is positive in each case. In part a, the charge on the right is negative. The field lines are represented by curved arrows starting at the positive charge on the left, curving toward and terminating at the negative charge on the right. Between the charges, the field lines are dense. In part b, the charge on the right is positive. The field lines represented by curved arrows start at each of the positive charges and diverge outward. Between the charges, the field lines are less dense, and there is a black region midway between the charges. In part c, the charge on the right is negative. The field lines start at the positive charge. Some of the lines, those that start closest to the negative charge, curve toward the negative charge and terminate there. Lines that start further from the negative charge curve toward it but then diverge outward. There is an area with very low density of lines to the right of the pair of charges.
Three typical electric field diagrams. (a) A dipole. (b) Two identical charges. (c) Two charges with opposite signs and different magnitudes. Can you tell from the diagram which charge has the larger magnitude?

The ability to construct an accurate electric field diagram is an important, useful skill; it makes it much easier to estimate, predict, and therefore calculate the electric field of a source charge. The best way to develop this skill is with software that allows you to place source charges and then will draw the net field upon request. We strongly urge you to search the Internet for a program. Once you’ve found one you like, run several simulations to get the essential ideas of field diagram construction. Then practice drawing field diagrams, and checking your predictions with the computer-drawn diagrams.

One example of a field-line drawing program is from the PhET “Charges and Fields” simulation.

Summary

  • Electric field diagrams assist in visualizing the field of a source charge.
  • The magnitude of the field is proportional to the field line density.
  • Field vectors are everywhere tangent to field lines.

Conceptual questions

If a point charge is released from rest in a uniform electric field, will it follow a field line? Will it do so if the electric field is not uniform?

yes; no

Got questions? Get instant answers now!

Under what conditions, if any, will the trajectory of a charged particle not follow a field line?

Got questions? Get instant answers now!

How would you experimentally distinguish an electric field from a gravitational field?

At the surface of Earth, the gravitational field is always directed in toward Earth’s center. An electric field could move a charged particle in a different direction than toward the center of Earth. This would indicate an electric field is present.

Got questions? Get instant answers now!

A representation of an electric field shows 10 field lines perpendicular to a square plate. How many field lines should pass perpendicularly through the plate to depict a field with twice the magnitude?

Got questions? Get instant answers now!

What is the ratio of the number of electric field lines leaving a charge 10 q and a charge q ?

10

Got questions? Get instant answers now!

Problems

Which of the following electric field lines are incorrect for point charges? Explain why.

Figure a shows field lines pointing away from a positive charge. The lines are uniformly distributed around the charge. Figure b shows field lines pointing away from a negative charge. The lines are uniformly distributed around the charge. Figure c shows field lines pointing away from a positive charge. The lines are denser on the right side of the charge than on the left. Figure d shows field lines pointing toward a positive charge. The lines are uniformly distributed around the charge. Figure e shows field lines pointing toward a negative charge. The lines are uniformly distributed around the charge. Figure f shows two positive charges. Field lines start at each positive charge and point away from each. The lines are uniformly distributed at the charges and bend away from the midline. Some lines intersect each other. Figure g shows a positive 5 micro Coulomb charge and a negative micro Coulomb charge. Several field lines are shown. Long the line connecting the charges is a field line that points away from the positive charge and toward the negative one. Another field line forms an ellipse that starts at the positive charge and ends at the negative charge. Another field line also forms an ellipse that points away from the positive and ends at the negative charge but appears to envelop the charges rather than start and end at the charges.
Got questions? Get instant answers now!

In this exercise, you will practice drawing electric field lines. Make sure you represent both the magnitude and direction of the electric field adequately. Note that the number of lines into or out of charges is proportional to the charges.

(a) Draw the electric field lines map for two charges + 20 μ C and −20 μ C situated 5 cm from each other.

(b) Draw the electric field lines map for two charges + 20 μ C and + 20 μ C situated 5 cm from each other.

(c) Draw the electric field lines map for two charges + 20 μ C and −30 μ C situated 5 cm from each other.


Figure a shows a positive 20 micro Coulomb charge on the left, a negative 20 micro Coulomb charge on the right, and the field lines due to the charges. The field lines come out of the positive charge and converge coming into the negative charge. The outer field lines extend beyond the drawing area and so we see them bending to the right, toward the negative charge, but only see part of the line. The density of lines coming out of the positive is the same as the density going into the negative. Figure b shows a positive 20 micro Coulomb charge on the left, a positive 20 micro Coulomb charge on the right, and the field lines due to the charges. The field lines come out of the positive charges and diverge, bending away from the far charge. The density of lines is the same near each of the charges. Figure c shows a positive 20 micro Coulomb charge on the left, a negative 30 micro Coulomb charge on the right, and the field lines due to the charges. The field lines come out of the positive charge. More lines go into the negative 20 micro Coulomb charge than come out of the positive 20 micro Coulomb charge. All of the lines coming out of the positive charge terminate at the negative, while the outer lines going into the negative start at infinity.

Got questions? Get instant answers now!

Draw the electric field for a system of three particles of charges + 1 μ C , + 2 μ C , and −3 μ C fixed at the corners of an equilateral triangle of side 2 cm.

Got questions? Get instant answers now!

Two charges of equal magnitude but opposite sign make up an electric dipole. A quadrupole consists of two electric dipoles are placed anti-parallel at two edges of a square as shown.

Four charges are shown at the corners of a square. At the top left is positive 10 nano Coulombs. At the top right is negative 10 nano Coulombs. At the bottom left is negative 10 nano Coulombs. At the bottom right is positive 10 nano Coulombs.

Draw the electric field of the charge distribution.


Four charges are shown at the corners of a square. At the top left is positive 10 nano Coulombs. At the top right is negative 10 nano Coulombs. At the bottom left is negative 10 nano Coulombs. At the bottom right is positive 10 nano Coulombs. The field lines are also shown. They come out of the positive charges and curve toward and end at the negative charges. The lowest density is near the center of the square.

Got questions? Get instant answers now!

Suppose the electric field of an isolated point charge decreased with distance as 1 / r 2 + δ rather than as 1 / r 2 . Show that it is then impossible to draw continous field lines so that their number per unit area is proportional to E .

Got questions? Get instant answers now!

Questions & Answers

what is thermodynamics
Aniket Reply
the study of heat an other form of energy.
John
heat is internal kinetic energy of a body but it doesnt mean heat is energy contained in a body because heat means transfer of energy due to difference in temperature...and in thermo-dynamics we study cause, effect, application, laws, hypothesis and so on about above mentioned phenomenon in detail.
ing
It is abranch of physical chemistry which deals with the interconversion of all form of energy
Vishal
what is colamb,s law.?
Muhammad Reply
it is a low studied the force between 2 charges F=q.q`\r.r
Mostafa
what is the formula of del in cylindrical, polar media
Birengeso Reply
prove that the formula for the unknown resistor is Rx=R2 x R3 divided by R3,when Ig=0.
MAXWELL Reply
what is flux
Bundi Reply
Total number of field lines crossing the surface area
Kamru
Basically flux in general is amount of anything...In Electricity and Magnetism it is the total no..of electric field lines or Magnetic field lines passing normally through the suface
prince
what is temperature change
Celine
a bottle of soft drink was removed from refrigerator and after some time, it was observed that its temperature has increased by 15 degree Celsius, what is the temperature change in degree Fahrenheit and degree Celsius
Celine
process whereby the degree of hotness of a body (or medium) changes
Salim
Q=mcΔT
Salim
where The letter "Q" is the heat transferred in an exchange in calories, "m" is the mass of the substance being heated in grams, "c" is its specific heat capacity and the static value, and "ΔT" is its change in temperature in degrees Celsius to reflect the change in temperature.
Salim
what was the temperature of the soft drink when it was removed ?
Salim
15 degree Celsius
Celine
15 degree
Celine
ok I think is just conversion
Salim
15 degree Celsius to Fahrenheit
Salim
0 degree Celsius = 32 Fahrenheit
Salim
15 degree Celsius = (15×1.8)+32 =59 Fahrenheit
Salim
I dont understand
Celine
the question said you should convert 15 degree Celsius to Fahrenheit
Salim
To convert temperatures in degrees Celsius to Fahrenheit, multiply by 1.8 (or 9/5) and add 32.
Salim
what is d final ans for Fahrenheit and Celsius
Celine
it said what is temperature change in Fahrenheit and Celsius
Celine
the 15 is already in Celsius
Salim
So the final answer for Fahrenheit is 59
Salim
what is d final ans for Fahrenheit and Celsius
Celine
what are the effects of placing a dielectric between the plates of a capacitor
Bundi Reply
increase the capacitance.
Jorge
besides increasing the capacitance, is there any?
Bundi
mechanical stiffness and small size
Jorge
why for an ideal gas internal energy is directly proportional to thermodynamics temperature?
Anne Reply
two charged particles are 8.45cm apart. They are moved and the force on each of them is found to have tripled. How far are they now?
Martin Reply
what is flux
Bundi
Bundi, flux is the number of electric field crossing a surface area
Mubanga
you right
Muhammad
martin,F/F`=(r`×r`)÷(r×r)
Mostafa
determining dimensional correctness
PATRICK Reply
determine dimensional correctness of,T=2π√L/g
PATRICK
somebody help me answer the question above
PATRICK
d=dQ+w
Muhammad
calculate the heat flow per square meter through a mineral roll insulation 5cm thick if the temperature on the two surfaces are 30degree Celsius and 20 degree Celsius respectively. thermal conduction of mineral roll is 0.04
akuribire Reply
what are the elementary compositions of a cell?
jackson Reply
poles, chemical
prabir
when a current pass through a material does the velocity varies
lovet Reply
no.
prabir
what is spin entropy ?and disorder in ferromagnetic material
Nepal Reply
diagram of an hall effect sensor
Aweda Reply
if a magnetised wire having dipole moment M is bent in the form of arc subtending angle of 45°at centre,new magnetic moment is
Pravin Reply
tell the answer
prabir
Practice Key Terms 2

Get the best University physics vol... course in your pocket!





Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'University physics volume 2' conversation and receive update notifications?

Ask