Lab Exercise 1: Electric Fields
Refer to the specific outline in your manual for the lab Electric Fields.
Follow the instructions and directions below for this lab. Disregard the outline in the manual for your LabPaq Kit. Use the packaging at the tray instead of the petri dish. See Figure 3 below for reference.
Read this document entirely before starting your work.
Do not forget to record your measurements and partial results.
Submit to your instructor the answers to the questions below as well as a Laboratory Report. Remember that the Laboratory Report should include the answers to the questions below.
(1) To investigate experimentally the concept of the electric field.
(2) To determine the shape of equal potential lines surrounding charged objects.
An electric field is defined as electric force per unit charge, , with being the electric field vector, being the force vector and q being the charge immersed in the electric field. However, since this lab is focused on studying the relationship between electric field and equipotential lines, we can use the following equation:
We can determine E, the electric field strength at a given point by measuring the potential difference (ΔV) between two points along a line in the direction of the electric field and dividing the potential difference (ΔV) by the distance between these two points (Δx). The units of electric field strength are Newtons/Coulomb or Volts/meter which are their equivalent.
An electric field or force line represents the direction in which a charge would accelerate if moving only under the influence of the electric force. Potential difference denotes the amount of work which would have to be done when a unit charge is moved from one point to another, and equipotential lines are lines of equal electric potential or equal voltage. Electric field lines or electric force lines originate at positive charges and terminate at negative charges; while equipotential energy lines are perpendicular to electric field lines (see figure 1).
Since the direct measurement of electric field lines is quite difficult, and requires specialized equipment, we will use the electric potential energy or the electric potential to map the electric field. We will measure and record the electric potential of numerous points in an electric field using a voltmeter, and then connect points with equal electrical potential thereby producing a “contour plot” of “equipotential” lines. Once such a contour plot of equipotential lines has been produced, the electric field lines can be visualized by superimposing electric field lines so that they intersect the equipotential lines perpendicularly as shown in Figure 2.
Figure 1: (left) Electric fields around positive and negative charges. (right) Equipotential lines (dashed)
IMPORTANT !! Before you start the experimental part of this lab, you MUST read and understand the instructions of the Digital Multimeter (DMM) included in your kit.
Place the sheet of graph paper on a table and center the clear tray over the grid. Use the rectangular and not the circular tray for this exercise.
Attach the end of each jumper cable to a metal nut by clamping the free alligator clip onto it as shown in Figure 2.
Figure 2: Probe connection
Place the two metal nut conductors in opposite ends of the clear tray. They should be approximately centered and about 2.5 cm away from the ends of the tray. If needed, you can use a different tray.
Position the battery holder with a 1.5V battery outside of and slightly away from the tray so it cannot get wet. Attach the jumper cables from the two conductors to the battery holder, one to the positive terminal and the other to the negative terminal.
Fill the tray with sufficient water to just barely cover the conductors.
Set your DMM to measure voltage by moving the dial to DCV, and its range to a voltage equal to or higher than that of the 1.5V battery. Again, make sure that you understand how the DMM works.
It is very important that the DMM is set to measure Volts (DCV) rather than current (DCA).
If it is set to measure current, the internal fuse will likely blow.
Attach the negative black lead from the DMM to the negative terminal of the battery holder.
Attach a jumper cable to the positive red lead that comes from the DMM. To the other end of the jumper cable attach the washer.
Figure 3 shows the experimental setup for this lab.
Figure 3: Experimental setup
With the DMM’s positive red lead, touch each of the conductors in the tray and record your findings. When doing this, keep the probes perpendicular to the tray. Keep in mind that:
Touching the negative conductor in the tray should result in a zero volt reading.
Touching the positive conductor should result in a reading that is the same as the battery output.
Touching a distance halfway between the conductors should record a voltage equal to approximately one-half the voltage of the battery. If it does not, stir a few grains of salt into the water in the tray.
What is the DMM reading for the voltage of your battery?
You can find grid paper at: http://www.classroomjr.com/printable-graph-paper-and-grid-paper/
Using the second sheet of graph paper, draw the conductors’ locations and label them with the voltage readings of your voltmeter.
Place the positive red lead of the DMM in the water again and note the voltage reading. Move the lead around in such a way that the voltage reading is kept at approximately the same value. How far does this path go?
Sketch this pattern on your graph paper and label the line with the voltage you chose.
Move the positive lead along additional voltage value paths and similarly sketch their patterns on the graph paper until you have well mapped out the area between and around the conductors.
With a color pen or pencil draw a point any place on your map to represent a moveable positive charge. Predict the path it would take by drawing a line with your colored pen or pencil.
Figure 4 below shows an example of the results of a similar experiment. However, in this experiment, the tray was square and the voltage of the probe was 5 V. Your results should be different. We include it here to give you an indication of the type of results you will obtain.
Figure 4: Example of results of a similar experiment
What generalizations can you make from this exploration?
Imagine that we drop a positive test charge into the tray? Where would it have the least potential energy?
Create a laboratory report using Word or another word processing software that contains at least these elements:
Introduction: what is the purpose of this laboratory experiment?
Descriiption of how you performed the different parts of this exercise. At the very least, this part should contain the answers to questions 1-3 above. You should also include procedures, etc. Adding pictures to your lab report showing your work as needed always increases the value of the report.
Conclusion: What area(s) you had difficulties with in the lab; what you learned in this experiment; how it applies to your coursework and any other comments.