Download a16.3Physics Section 16.3

Physics Section 16.3
Apply the Properties of Electric Fields
Recall: An electrically charged object will exerts a force on another
charged object without any contact.
http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html
An electric field is a region in space surrounding a
charged object in which another charged object
experiences an electrical force.
+q experiences an electrical
force due to the charge +Q.
The closer the charges are
to each, the larger the
force. A measure of this
force is called the electric
field strength.
The strength of the electric field is the ratio of the
magnitude of the electric force acting on a test charge
and the magnitude of the test charge.
E = Fe
qo
E = electric field strength N/C
Fe = electric force (N)
qo = test charge (C)
Note: The electric field is
created by +Q, not
by +q.
Example:
A test charge of 3.0 x 10-18 C experiences an electric
force of 6.0 x 10-15 N. Find the electric field strength.
example
A charged object creates an electric field with a
strength of 4.0 x 105 N/C at a given point. Find the
force exerted on an electron at this point.
Note: the electric field strength varies inversely as the distance from
the object creating it.
E = KcQ_
r2
E = electric field strength N/C
Kc = 8.99 x 109 N m2/C2
r = distance from the object creating
the electric field (m)
Q = charge on object creating electric
field (C)
example
An object with a charge of 8.2μC creates an electric
field. Find the electric field strength at a distance of
65 cm.
Note: A convenient way to represent an electric field is to use electric lines of force.
Electric field lines are lines that indicate the path a
test point would follow in an electric field.
Properties of electric field lines
1. Electric field lines originate from a positively charged
object and terminate at a negatively charge object.
2. Electric field lines originate and terminate perpendicular
to the surface of a charged object.
3. The density of the electric field lines indicate the
strength of the electric field.
4. No two field lines from the same source can cross.
Electric fields from compound systems
http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html
A charge can be given to a conductor.
Properties of an isolated charged conductor.
1. The electric field is zero everywhere inside the conductor.
2. All excess electric charge lies on the outer surface.
3. The electric field is perpendicular to the conductor’s surface.
4. On an irregular shaped conductor, charge accumulates at the
point(s) of greatest curvature.
A Faraday Cage is an enclosure made of a conducting
material. No electric field can exist in a Faraday Cage.
http://www.youtube.com/watch?v=9tzga6qAaBA
The repairman wears a protective suit made of
shielding material. The helicopter must ground off
before he steps onto the line since the chopper and the
line have different electrical charges. Bonding, or
equalizing the charges is a very risky operation. Too close
and the chopper gets fried, too far away and the
bonding process fails. The power line repairman's suit
acts as a "faraday cage" or electrical shield. 150,000
volts flows around his body and down the powerline,
leaving him unharmed.
assignment
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