Electric field and electric potential

Physics 1051 Laboratory #3 Electric Field and Potential

Electric field and electric potential


Contents

Part I: Objective Part II: Introduction Part III: Apparatus and

Setup

Part IV: Measurements Part V: Analysis Part VI: Summary and

Conclusions


Part I: Objective

In this experiment, you will measure electric potential and use those measurements to plot both equipotential lines and the electric field for a configuration of charge.


Part II: Introduction Electric fields

Electric field is defined as the electric force acting on a test particle divided by the charge of the test particle .

Electric field points away from a positive charge and towards a negative charge.

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Part II: Introduction Electric field lines

Electric field lines are used to visualize electric fields and show the electric field around charges or configurations of charges.

Some examples are on the following slides.


Part II: Introduction Electric field lines around a positive point charge


Part II: Introduction Electric field lines around a negative point charge


Part II: Introduction Electric field lines around a dipole


Part II: Introduction Electric field lines between charged plates


Part II: Introduction Electric potential

Electric potential V is defined as the amount of potential energy per unit charge.

Electric potential is the integral of electric field over distance.

Electric field is the change in electric potential over distance:

ΔV = −!E∫ •d !s

Er = −ΔVΔr


Part II: Introduction Equipotential lines

Equipotential lines are lines showing lines along which the electric potential is constant.

Equipotential lines are always perpendicular to electric field lines.

Some examples are shown on the following slides.


Part II: Introduction Equipotential lines for positive or negative point charges


Part II: Introduction Equipotential lines for a dipole


Part II: Introduction Equipotential lines for parallel plates


Lab Report

Lab Report 1: Write the objective of your experiment. Lab Report 2: Write the relevant theory of this experiment.

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Part III: Apparatus and setup

You have been provided with !  conductive paper (painted with

electrodes) !  1 cm graph paper !  carbon paper !  multimeter !  power supply !  electrical leads


Part III: Apparatus and setup Predictions

Your conductive paper has one of the following patterns (the upper electrode is positive and the lower electrode is negative):

Lab Report 3: For the pattern which matches your conductive paper, draw what you expect for the electric field lines.

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Part III: Apparatus and setup Assembly

Using the leads (banana-alligator): Connect the banana end to the power supply.

•  One lead should be in the negative terminal. •  The other lead should be in the positive terminal.



Connect the alligator clips to the painted portions of the conductive paper:



Connect another lead from the COM port of the multimeter to the negative terminal of the power supply:

Plug one lead on top of the other.



Connect one end of the remaining lead to the VΩmA port of the multimeter. Leave the remaining end free (it will be the probe).



Your completed setup:



Turn on the power supply. Adjust the current and voltage controls until the voltage display on the power supply reads about 15 V. Turn on your multimeter by moving the switch to 20 DCV.



Test the setup: As you touch the free end of the lead to the conductive paper, you should see a voltage reading on the multimeter. If you do not see a reading, consult a member of the lab staff.



To provide each partner with a set of data: To record your data, use two pieces of graph paper with a carbon sheet in between. Make sure the inky side is down.



Copy the electrode pattern on the black conductive paper onto the white graph paper.



Tape the graph paper to the bench next to the black conductive paper.

You will record your data on the white graph paper!


Lab Report

Lab Report 4: List your apparatus and sketch your setup.

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Part IV: Measurements Finding the first equipotential

Touch the probe to the conductive paper. Move it along the paper until you find a place where the potential reads 2.5 V. Mark this location on your white graph paper. Label it ‘2.5 V’. Do not mark on the black conductive paper! Move the probe a few cm and find a nearby place where the potential is also 2.5 V. Mark this on your graph paper and label it ‘2.5 V’. Continue to move the probe and locate at least three more points where the potential is 2.5 V. For best results, make sure your spots are spread approximately

equally across the whole width of the paper. Have an instructor check your work and initial your lab report.


Part IV: Measurements Finding the remaining equipotentials

Use the probe to find and mark places (at least 5 points for each potential) where the potential is:

!  2.5 V !  5.0 V !  7.5 V !  10.5 V !  12.5 V

Be sure to label all points.


Part V: Analysis Drawing the equipotential lines

Draw the equipotential lines by connecting the spots which have the same potential.

Lab Report 5: Are successive equipotential lines equally spaced? Did you expect them to be? Explain.


Part V: Analysis Drawing the electric field lines

Draw the electric field lines by drawing lines from the positive electrode to the negative electrode, being sure that they are always perpendicular to the equipotential lines.

Lab Report 6: Do the electric field lines match your predictions from Question 3?

Lab Report 7: Where is the electric field the strongest? Where is it the weakest? Explain how you know.

Lab Report 8: Find the magnitude of the electric field at the centre of your paper. Include uncertainty. Show our workings.

To answer Question 8, use the equipotential map you drew on the white graph paper.


Part V: Summary

Lab Report 9: Outline briefly the steps of your experiment. Lab Report 10: List your experimental results and comment on

how they agreed with the expected results. Lab Report 11: List at least three sources of experimental

uncertainty and classify them as random or systematic.

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Wrap it up!

Check that you have completed your Lab Report. Include your graph with the equipotential and electric field lines.

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Electric field and electric potential