Download electric field lines. the electric dipole.

Laboratory #2
Phys 2426
Dr. Cristian Bahrim
The purpose of this experiment is plotting the electric field produced by an
electric dipole. If a test charge, q, is located at a point P in a uniform electric field, E ,
it experiences a constant electric force, F = qE . If the charge q moves through a
displacement, ds , the electric force does a work:
dW = F ⋅ ds = q E ⋅ ds = q E (cosθ ) ds
[1]
where θ is the angle between the vectors F and ds . The electric force is a
conservative force and has associated an electrical potential energy U. Based on the
principle of conservation of energy one can prove that the work done by the electric
force F (or implicitly by the electric field E ) equals the negative change in the electric
potential energy
dW = − dU
[2]
It is conventional to define a concept called “the change in electric potential” by
dV =
dU
q
[3]
Equations [1], [2] and [3] lead us to the formula:
dV = − E ( cosθ ) ds
[4]
If the test charge, q, moves on a path which is perpendicular to the electric field
lines, so that the angle θ is 90 degrees and implicitly cosθ is 0, there is no work done
by the electric field, and also no change in the electric potential. A line of constant
potential is called “equipotential line”.
The electric field lines are always perpendicular to equipotential lines.
In this lab, first you are going to measure the equipotential lines. Next, you have to
find the electric field pattern created by two distributions of charges drawing a number of
field lines perpendicular on each equipotential line they cross. The electric field line
should start on the positive charge and end on the negative charge.
Place a sheet of transparent graph paper under a clear plastic sheet in the
bottom of an enamel pan. Put just enough water into the pan to cover completely the
plastic sheet. Force out all air bubbles.
Center two point electrodes in the pan at 10 big squares apart. One electrode is
connected through a black wire to the terminal (-) of a DC generator, which is built in the
Pasco interface. This electrode simulates a negative point charge and is going to be
the reference electrode of zero potential. The other electrode is connected through a
colored (blue or red) wire to the terminal (+) of the DC generator. This electrode
simulates a positive point charge. The two electrodes simulate an electric dipole.
Also the terminal (-) of the DC generator is connected through a voltage sensor
to a movable probe, which can measure the electric potential at any point between the
electrodes. The voltage sensor allows displaying on a computer screen any changes in
the electric potential with respect to the reference (black) electrode.
You have to measure seven equipotential lines between the two electrodes.
Place the movable probe in water at two big squares in front of the electrode at zero
potential. Record the value of the electric potential and go with the movable probe
around the negative electrode keeping constant the electric potential you read on the
voltmeter. On a graph paper you need to record the exact location of the two electrodes
in the pan and also, the position of the movable probe for the same potential. After you
measure enough points of same electric potential, you need to joint them with a smooth
curved line. This is an equipotential line. You need to measure six more equipotential
lines. One equipotential line should be for the value of the electric potential you
measure at the midpoint between the two electrodes. Three equipotential lines
should be on the right hand side of this central equipotential line and other three
lines should be on its left hand side. Try to draw equipotetial lines equally spaced!
The first and last equipotential line should be at 2 big squares in front of the
electrodes.
When seven equipotential lines have been drawn, carefully construct
curved lines perpendicular to them. These are electric field lines. The electric field
lines originate on the positive electrode and terminate on the negative electrode.
Draw all equipotential lines with one color and all electric field lines with a
different color. In this way you can distinguish two patterns on the same graph paper:
the electric field pattern and the equipotential pattern.
Each student in the group should draw one graph.
1. Sketch the electric field pattern around
a. Two positive charges of magnitude +1 µC, placed close together.
b. Two negative charges of magnitude -2 µC, placed close together.
c. A positive charge of +1 µC and a negative charge of –2 µC placed close
together.