Electric Field Lines
Electric Field lines
- To visualize electric field lines between charges.
- To determine the equipotential surfaces for a given field line configuration.
An electric field is a region in space in which electric forces act on electric charges, if present. The electric field strength for any point in space is defined as the net electric (Coulomb) force per unit of positive charge acting on a charge placed at that point, i.e.,
The SI unit for electric field is newton / coulomb N/C, or (more practically) volt/meter, V/m. The direction of an electric field at any point is defined as the direction of the net electric force on a positive charge placed at that point.
Faraday introduced the concept of lines of force to aid in visualizing the magnitude and direction of the total electric field about a charge or collection of charges. These concepts are listed below.
1.A line of force is always tangent to the direction of electric field.
2.The lines of force originate on positive charges and terminate on negative charges.
3The density of the lines of force (i.e. the lines/cm or lines/cm2) in a region of space is used to represent the electric field strength in that region of space.
4Lines of force will not cross over or touch one another.
Electric fields can be represented by a scaled drawing, by first choosing a scale factor (proportionality factor) so that n number of lines/cm2 represent a certain value of field strength (volts/m).
Examine the figure above. The two figures represent a uniform electric field. If we let figure 1(a) represent an electric field with field strength of E, figure 1(b) would represent an electric field with field strength of 2E. (Twice the numbers of field lines passes through the same region and are equally spaced).
In this experiment, we may estimate the electric field at certain points from the potential gradient, at these points
Where V1 and V2 are the potential of two adjacent equipotential lines and (x2 – x1) is the distance between the lines in meters.
It is possible to find any number of points in an electric field, all of which are at the same potential (voltage). If a line or surface is constructed such that it includes all such points, the line or surface is known as an equipotentialline or surface.
Part 1- Virtual Lab- Electric Field Lines
Computer and software program
1. Open the program “Start Exploration of Physical Science” by double clicking on the icon shown below on your desktop.
2. Click on the “electricity” tab on the top menu bar.
3. Click on “Electric field lines and vectors” in the list.
4. You will study the electric field line patterns produced by charges in different arrangements.
Study the following charge configurations:
iii) +1 and -1 (placed about two centimeters apart)
iv) +1 and +1
v) -1 and -1
viii) +3 and -3
ix) Two plates
5. To study the field lines, click on the charge, click “Add”. Click on the charge and drag it to the middle of the window. Click on “Electric Field Lines” at the bottom. Click on “Electric Field Vectors” at the bottom and then click on the green lines representing the electric field lines. The direction of the arrows indicates the direction of the electric field. Copy (“Alt + Print Screen”) and paste (“Ctrl + v”) your pattern in the designated sections below.
The above pattern shows the electric field lines and electric field vectors for an isolated +2 charge.
To reset the pattern, click on the “Reset” button:
Resetting will refresh the screen and allow you create a new pattern.
Note: Pressing “Alt + Tab” will help you navigate between the program window and your Microsoft Word Document.
Please copy (“Alt + Print Screen”) and paste (“Ctrl + v”) your patterns in the table below. You may use the “Paint” program to crop your pictures.Charge Configuration / Observed Electric Field Lines Pattern
+1 and -1
-1 and -1
+3 and -3
Answers to questions: Type your answer in complete sentences and in a different color than black.
i) Is electric field a scalar or vector quantity?
ii) As the strength of the charge increases what happens to the number of electric field lines?
iii) If you placed a small positive test charge close to a +3 charge what would happen to the test charge? (Remember that a test charge is always unit positive, i.e., 1 C of positive charge.)
iv) What happens to the field lines at the edges of the “two plates” pattern? Can you explain this effect?
Part 2- Practical Lab- Fields and Equipotentials
Introduction and Objectives:
Describe the concept of a force field. Distinguish between lines of force and equipotential surfaces, and describe their relationships to work.
The electric force per unit charge is called the electric field intensity, or simply the electric field. By determining the electric force on a test charge at various points in the vicinity points of a charge configuration, the electric field may be “mapped” or represented graphically by lines of force. If a charge is moved along a path at right angles or perpendicular to the field lines, no work is done (W= 0), since there is no force component along the path. Such paths are called equipotentials.
Equipotentials can be determined using a simple galvanometer as a detector. When no current flows between two points, as indicated by a zero deflection on the galvanometer, the points are said to be equipotential. Subsequently, lines perpendicular to the equipotentials will represent electric field lines.
Fig.3: Lines of force and equipotential lines near two charges of equal magnitude but opposite sign.
Equipment Required: Electric field mapping setup, galvanometer, voltage source, connecting wires, metal push pins, graph paper.
Part 1: Electric Field:
An electric field mapping setup with a galvanometer is shown in the Figure 4. The apparatus consists of a flat board on which is placed a sheet of carbonized conducting paper imprinted with a grid. The sheet has an electrode configuration of conducting silver paint, which provides an electric field when connected to a voltage source.
The common electrode configurations ordinarily provided are two dots representing point charges of an electric dipole configuration and two parallel linear electrodes representing a two-dimensional cross section of a parallel-plate capacitor. You might have a different one.
1. Place the conducting sheet with the pattern on the board, and fix the metal push pin contact terminals firmly to the painted electrode connections. Connect these terminals to the power supply using appropriate wires. Do NOT turn on the power supply yet. Note: a voltage of 1V to 3V is enough.
2. Connect the movable and stationary probes to the galvanometer using connecting wires. These probes are used to locate equipotential points in the field.
3. Replicate the pattern and grid on a graph paper, ensuring that you label the positive and negative terminals.
4. You can now begin mapping the equipotential lines. First place the stationary probe towards one end of the pattern. Turn on the power supply (use 1 to 3 V) and locate points on the grid that give you NO deflection on the galvanometer. Draw the exact locations of these points on your graph paper. Map 5- 6 points to the left, and 5-6 points to the right of the stationary probe. Connect these points to get equipotential surfaces.
5. Turn off the power supply. Move the stationary probe about 1-2 cm to another location (closer to the other side or terminal). Turn on the power supply and repeat step 4 to find the new equipotential points and line.
6. Repeat step 5 so that you get about 6 to 7 equipotential lines.
7. Draw perpendicular lines to the equipotential lines (preferably in another color). These lines represent the electric field for the given pattern. Don’t forget to indicate the direction of the field lines (positive to negative) with a little arrow head.
Caution! The galvanometer is a delicate instrument and may get damaged due to excessive currents.
For the parallel plate pattern, also investigate field lines at the end of the plates to see how they differ.
8- Scan your drawing and save it with this document.
Fig. 4: Setup for Equipotential Lab
Please type your answers below in complete sentences and in a color different than black.
1. Directions of the fields are indicated on field lines. Why are there no directions indicated on equipotential lines?
2. Comment on the electric field of the parallel plates: a) Between the plates, and b) near the edges of the plates.
3. Is it possible for two different equipotential lines or two lines of force to cross? Explain.
4. How does the electric field strength vary with distance from an isolated charged particle?