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```					                                   Sutherland High School
Regents Physics

Laboratory 24
Electric Fields

Purpose: 1). To draw electric fields lines to represent the electric field around several symmetric
charge distributions.
2). To determine the number of electrons present on an oil drop in an electric field between
two parallel plates

I. Drawing Electric Fields

Electric Field: An electric field exists around any charged object. An electric field has a magnitude and
direction that is not dependent on another charged object in the electric field. The magnitude and
direction of an electric field at a particular place is measured with a small positive test charge.

   F
E        The electric field produces a force on another charged object. The units of electric field are
q
newtons / coulomb (N/C).

Electric Field Lines:
Provide a picture of the electric field in a region around a charge distribution. There are several rules
for drawing electric field lines. Electric field lines:
 Always leave a positive charge and enter a negative charge.
 Show the direction of the electric force (tangent to the electric field line).
 Are perpendicular to the surface of a charged conductor.
 The spacing between the lines indicates the strength of the electric field.
 Never intersect.

Uniform Electric Fields:
A uniform electric field will result from placing two large, flat conducting plates with opposite
charge near each other.
V
E  , where V is the electric potential difference and d is the distance between the plates.
d
The units for a uniform electric field is expressed in Volts/meter (V/m) which is equivalent to N/C.

Procedure:
1. Open MS INTERNET EXPLORER.
2. Go to the site ELECTROSTATICS SIMULATION. http://www.falstad.com/emstatic/
3. The simulation should be open to a single source.
4. Deselect “Draw equipotentials”
5. Select “E lines/rho/i”
6. Under Setup select the charge distribution for the simulation to show the resultant Electric Field.
7. Explore a number of different charge distributions.
8. On the Lab Report draw at least five (5) electric field lines to represent the Electric Field of the
following charge distributions:
 Single positive charge
 Single negative charge (not shown in simulation)
 Double positive charge
 Positive and negative charge (dipole charge)
 Parallel plates (conducting planes)

II. Millikan’s Oil Drop Experiment:
In 1909 Robert Millikan determined the charge on an electron by finding the force acting on charged oil
drops in an electric field. To find the electrical force, he first determined the mass of the oil drop and
then the gravitational force acting on the oil drop. Next he varied the intensity of the electric field until
the drop either stood still or moved at a constant speed (hard to determine). When this occurred, the sum
of the forces acting on the drop would be zero and he could calculate q. Unfortunately, there could be
more than one electron on the drop so that q could be the charge of several electrons rather than one. His
technique was to do thousands of trials and he concluded that the smallest value found must be that of the
charge of one electron. (A full explanation of Millikan’s Oil Drop experiment and an illustration of his
apparatus is on the web site of the simulation). Since the charge on an electron is known, you will
determine the number of electrons on an oil drop for three different trials of a simulation of Millikan’s
original experiment.
Theory and Information: Some equations and constants that you will find helpful:
Mass of an electron = 9.11 x 10-31 kg
Charge on an electron = 1.6 x 10-19 Coulombs
Acceleration due to gravity (g) = 9.81 m/s2
Mass of the oil drop = 1.0 x 10-15 kg
For charged parallel plates E = F/q = V/d
Fg = mg
at equilibrium Fg = Fe
Procedure:
1.    Go to the site MILLIKAN’S OIL DROP EXPERIMENT.
http://physics.wku.edu/~womble/phys260/millikan.html
2.    Read the description of the apparatus that Millikan used and how he conducted the experiment.
3.    Open the simulation by clicking on here.
4.    Start the simulation by clicking START.
5.    Adjust the electric field E until one or more droplets are suspended in air. It takes a moment for
the effects of the changes in E to show up since they must overcome the momentum of the
droplets. Continue to adjust, as necessary, for the most accurate reading of E in a particular trial
but do not add new drops during a trial.
6.    Record the value of E used when the forces are balanced.
7.    STOP the simulation, then click on NEW DROPS to reset the simulation.
8.    Repeat steps 4-7 until you have 3 different values for E that will produce equilibrium. These
values should be approximately integer multiples or fractions of the first value for E. (This may
take more than 3 trials).
9.    Insert the values in the data table and shut down the simulation.
10.   Determine all values in the calculation table for three different values of E.
11.   Complete the lab report.
Electric Field Simulations

Static Electricity Tutorials       http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/estaticstoc.html
Electroscope                       http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.html

Electric Field Mapping             http://physics.weber.edu/amiri/director/DCRfiles/Electricity/efiel24s.dcr
Electric Field Vectors and Lines   http://qbx6.ltu.edu/s_schneider/physlets/main/efield.shtml

Electric Field Lines (tres cool)   http://www.zahniser.net/~physics/field.html

Electric Field Lines               http://www.surendranath.org/Applets/Electricity/FieldLines/FieldLinesApplet.html
Millikan's Oil Drop Experiment     http://www68.pair.com/willisb/millikan/experiment.html
The E-field Game                   http://www.batesville.k12.in.us/physics/PHYNET/e&m/efields_&_potential/efgApplet/EFieldGa

AFTER you have completed the lab, open up the Electric Field Hockey, and try to get the puck
into the goal. When you feel very confident, try to get the puck into the goal for level 3!
Name ______________________
Laboratory 24                                                Lab Partner __________________
Electric Fields                                              Lab Date/Period ______________
Date Submitted _______________

Pre-Lab:
 Why do electric field lines never intersect?

   What is meant by the concept that “charge is quantized”?

I. Diagrams of Electric Field Lines:

Single positive charge               Single negative charge                   Double positive charge

+

+                                       -

+

Positive and negative charge                                Charged parallel plates

+
+   + +      + + +       + + +         + + +

-
-   -   -   -    -   -   -   -     -   -   -   -
II. Millikan Oil Drop Data:
Trial #         Value of E at equilibrium (kV/m)
1
2
3
4
5

Calculations: Please include all units in SI format.
Trial #    E (N/C)                       Fg                 Fe              q total     # of electrons
1                , 000

2                  , 000

3                  , 000

Observations: What happens to the oil drops in the absence of an electric field? Why?

Analysis: What condition is necessary for equilibrium to be established?

Conclusions: Were the number of electrons on the oil drops always an integer?

Error: By how much did your calculation of the number of electrons vary from an integer value?

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