Transcript Bell Quiz - Bryn Mawr School Faculty Web Pages

January 30, 2010
Electric Force Review and then an
Introduction to Electric Fields
Static Electricity Experiment
• Cut 2 20-cm strips of transparent tape (mass of
each 65 mg). Fold about 1 cm tape over at one end
of each strip to create a handle. Press both pieces
of tape side-by-side on your lab table and rub your
finger back and forth across the strips. Quickly
pull the strips off the lab table. Hold the handles
together and the strips will repel each other,
forming an inverted “V”. Estimate the charge on
each strip using a protractor. Assume the charges
act as though they are at the center of mass of the
strip.
• Hint: Begin by drawing a Free Body Diagram!
Two identical balls of mass m and
charge q are hanging from strings
of length L. Derive an expression
for q in terms of m, , L, and
fundamental constants.

L
m,q
L
m,q
The Electric Field
• A charge distribution changes the empty
space around it such that this empty space
will affect other charges brought nearby.
• We say that a charge distribution creates an
electric field in the empty space
surrounding it.
Electric Field
qo
kqo
E
E 2 r
2
4 o r
r
This equation can be used to calculate the electric field a
distance r away from a the center of a spherically
symmetric charge distribution of qo Coulombs.
Another charge q entering the electric field created by qo
will experience a force F, which can be calculated by
the equation F
= qE.
Electric Field Direction
• The direction of the electric field at a point
in space is the direction that a small positive
charge at rest (a “test charge”) wishes to
move if it is placed at that location.
• Thus, the electric field points away from
positive charges and toward negative
charges.
Electric Field Direction
• The electric field around point charges is
spherically symmetric.
• When multiple point charges are present, the fields
due to the individual charges “superimpose” to
create a more complex picture.
• http://www.edumedia-sciences.com/m195_l2electric-field.html
• http://www.falstad.com/emstatic/
• http://www.cco.caltech.edu/~phys1/java/phys1/EF
ield/EField.html
Caution…
• Electric field lines are NOT VECTORS, but
may be used to derive the direction of
electric field vectors at given points.
• Electric field vectors are always tangent to
the electric field line at any given point in
space.
Superposition
problems involve
vector addition.
E2 P
E
E1
-q1
q2