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Chapter 21
Electric Fields
21.1 Creating and Measuring
Electric Fields
The Electric Field
– The electric field is a vector quantity that relates the
force exerted on a test charge to the size of the test
charge.
– Electric field
E = F on q’
q’
– The direction of the field is the direction of the force
on the positive test charge.
– A picture of an electric field can be made by using
arrows to represent the field at various locations.
Picturing the Electric Field
– Notice the field lines always leave a positive charge
and enter a negative charge. Lines do not really
exist. They are simply a means of providing a model
of an electric field. Electric fields do exist!
21.2 Applications of Electric Fields
Energy and Electric Potential
– The electric potential difference, ΔV, is defined
as the work done moving a test charge in an
electric field divided by the magnitude of the
test charge (q’)
– Electric Potential Difference
ΔV = W on q’
q’
The Electric Potential in a Uniform Field
– The electric potential difference, ΔV, between
two points a distance d apart in a uniform field
E, is represented by:
ΔV = Ed
– The produce of the units E and d is (N/C)∙(m).
This is equivalent to one J/C, the definition of
one volt.
Millikan’s Oil-Drop Experiment
– Robert A. Millikan in 1909 used the uniform
electric field to determine the charge on a
single electron.
– He found experimentally that the changes in
the charges of the oil drops were always
multiples of 1.60 X 10-19 C.
Sharing of Charge
– All systems come to equilibrium when the
energy of the system is at a minimum.
Charges do the same thing when moving
between charged conductors. They distribute
themselves so that the net force on each
charge is zero.
– Earth is a very large conductor and can
absorb all excess charge on a body. Touching
a body to Earth to eliminate excess charge is
called grounding.
Electric Fields near Conductors
– Charges on a conductor spread as far as
possible to make the energy of the system as
low as possible. The result is that all charges
are on the surface of a conductor. The shape
of the conductor is also important and
charges will be closer together at sharp
points. A lightning rod is an example of this.
Storing Charges: The Capacitor
– As charge is added to an object, the electric
potential difference between the object and
Earth increases. The ratio of the charge
stored to the potential difference is called the
capacitance, C. Capacitance is measured in
farads, F. One farad is one coulomb per volt.
Capacitance
C = q/ ΔV
The capacitance depends only on the
construction of the capacitor, not on the
charge, q.
PSS
1. Sketch the problem.
2. Draw the vector diagram.
3. List the known and unknowns.
4. Use the equations to relate the variables.
5. Check that the units are correct.
6. Solve the problem.
7. Check your answer.