Download E Field Map

Allan Hancock College
Physics 163 lab
Mapping the Electric Field
Purpose: To explore the relationship between the electric potential, the electric field, and various
charge configurations. The electric field, which is so important in electric phenomena,
can not be measured directly. What we can measure however is the electric potential,
and then, using the relationship
Er = - V/r
==>
Ex = - V/x
and
Ey = -V/y,
we can calculate the electric field. The negative sign indicates that the electric field
points in the direction of decreasing potential.
In this lab we will measure the potential at different points to find regions where the
potential is the same. By connecting these points we have lines that are called
“equipotential contours”. These are much like the contour lines on a topographical map
that show areas where the height of the land is the same, or isobars on a weather map
which show regions of constant pressure.
Having marked the equipotential contours, we can now map the electric field lines,
which will cross the equipotentials at right angles to them, and point from the high
potential to the low potential.
Apparatus: Masonite boards covered with weakly conducting paper with an electrode pattern
(silver paint), board holders which create a conducting path to the electrodes, voltage
probe, power supply, digital volt-meter, and various connective wires.
Experiments: We will use the Pasco power supplies. Turn on the Science Workshop interface
and power supply, then turn on the computer. Open Science Workshop, connect the
power supply to the interface box, then make the same connection on the interface
screen on your monitor. Now tell the computer that it is a power supply. Set it to 10
volts DC.
The electrode boards attach on the bottom of the board holders, with the silver paint
pattern facing downward, so that the screws holding the board in place make contact
with the silver paint. Place a piece of paper on the top of the board holder, fix it so it
remains stationary, and mark the position of the electrodes on it. With the power
supply connected to the electrodes, set the voltage of the power supply to 10 volts.
Connect the positive lead from the voltmeter to the probe, and the ground lead to the
ground connection of the board. Now with the bottom half of the probe in contact with
the conducting paper board, the top half is on the top of the board holder and can be
used to plot equipotential points.
1)
Using the board with 2 parallel strips, plot 4 or 5 equipotential contours. Is the voltage
near the positively charged electrodes close to 10 volts? Is the voltage near the
grounded electrode close to 0 volts? Now map the Electric field lines perpendicular to
the equipotential contours. Which way do the E-field lines point? The electric field
between 2 parallel plates is constant. To calculate the magnitude of the electric field,
measure the change in potential at several points and the distance between the points
and then find the average value of the electric field between the plates.
E = V/x
2)
Repeat the procedure for the 2 dot pattern. This pattern represents positively and
negatively charged point charges, an electric dipole. The E field will not be constant
between the point charges, so its magnitude is not so easily measured. However it will
be constant in a circular pattern around the electrodes. Try calculating it at different
points for the same radial distance r for one of the electrodes. Note: the pattern will not
make a perfect circle near the outside edge of the board due to edge effects, so limit
your measurements to the inside of the board.
3)
Repeat the above procedures for 2 more boards of different patterns. You will not be
able to calculate the magnitude of the magnetic field for these asymmetric patterns, but
try to deduce the pattern and direction of the electric field lines.