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EXPERIMENT 1：
NAME
ELECTRIC POTENTIAL & FIELD MAPS
STUDENT ID
Scores
ROOM：Fundamental teaching Building, 1103
DATE:
TABLE
NUMBER:
PURPOSE: To map and study the equipotential lines and electric field lines around two
electrically charged conductors.
APPARATUS: DC power supply, voltmeter, field map board, coordinate paper
INTRODUCTION:
The electric field E at a point is the force per unit charge on a test
charge located at that point.
It is numerically equal to the force on a unit (one Coulomb)
positive test charge. Because similar charge s repel each other, a positive test charge placed
near another positive charge experiences a repulsive force, and the field due to a positive
charge points away from the charge. The field direction near a negative charged object is
towards the object.
Electric-field lines: Electric field lines are a way of visualizing the electric field near
charged objects. Field lines define the direction of the force that a positive "test" charge
experiences near other charges. The field lines are also used to define the magnitude of
the electric field. The lines are drawn such that their density (the number of lines that cross
a unit area) is proportional to the magnitude of E.
An electric field, E, exists wherever an electric force, F, acts on a charge, q . E and F are
vector quantities; they have both magnitude and direction.
These fields can be
represented by lines of force which show the magnitude and direction of the force that
would act on a small positive charge placed in the field.
Electric field lines follow two
rules:
1.
Lines of force originate from positive charges or infinity and terminate at negative
charges or infinity.
2.
Lines of force are close together where the field is strong and far apart where the field
is weak.
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ELECTRIC POTENTIAL & FIELD MAPS
The electric lines of force between point charges are shown in Figure 1.
Force lines for a
pair of oppositely charged plates are shown in Figure 2.
If you place a positive charge in the field it will move along a field line towards the
negative electrode. The field will do work on the charge.
If you move a charge from the
negative to the positive electrode you will have to do work against the field.
Rather than trying to measure electric fields directly, it is easier to measure the electrical
potential near the charges. This is an easier measurement because electric potential does
not have a direction (it’s a scalar) and can be measured with a voltmeter.
The electrical
potential is the potential energy of the test charge divided by the test charge.
Equipotential lines: Consider a test charge that is made to move in the presence of one or
more other charges in such a direction that it does not experience any change in electrical
potential energy. The path of this test charge is perpendicular to the electric field at every
point of its motion, and is called an equipotential line. No work is done to move the test
charge; its potential energy does not change. We define electrical potential as the electrical
potential energy divided by the test charge. The unit of electrical potential is the Volt. Work
against the electric field happens only when the charge is moved with a component parallel
to the E field; thus, equipotential lines are perpendicular to the E field. Devices that
measure electric potential are voltmeters which are inexpensive and readily available. The
equipotential lines, V, measured with the voltmeter can then be used to m a p the electric
field lines, E.
From the definitions of E and V, it follows that:
• Electric field lines at any location are perpendicular to the equipotential lines.
• Electric field lines go from higher to lower potential.
• Electric fields are vectors: they define force directions.
Some properties of electrically charged conductors in electrostatic equilibrium:
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ELECTRIC POTENTIAL & FIELD MAPS
• All points on the conductor's surface are at the same potential.
• The electric field at a conductor's surface is perpendicular at each point to the surface
(since the entire conductor surface is at the same potential).
• The electric field at the surface of a charged conductor is large at a sharp point (a
convex region with small radius of curvature).
MAPPING PROCEDURE:
You will measure electric potential and draw
equipotential lines for three different configurations of oppositely charged conductors:
1. two parallel plates (a capacitor);
2. two points;
3. a hollow circular conductor between oppositely charged parallel plates.
5.00V
10
Figure 3 shows the experimental arrangement for simulating the electric field between
two point charges. A dc power supply set just under 10 volts is connected across the
two so. For the parallel strip electrodes measure potential beyond and to the sides of the
electrode region. For the circular electrode between parallel strips make measurements
inside the ring to test shielding. For the two points electrode configuration measure
behind the points as well as between, to map the dipole (two-pole) shape. In all cases,
be careful to take sufficient additional data to define the shape of the equipotential lines
and corresponding E field lines close to the conducting electrodes.
Connect points at the same potential with smooth lines to produce equipotential lines.
Label the potentials. Draw the associated electric-field in a different color. The E
field lines should be everywhere perpendicular to the equipotential lines, from the
defining relation between E and V. The direction of E is toward decreasing potential V.
Indicate the direction of the E field lines on your plot.
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