Transcript Electricity - The Lesson Locker

Electricity
Fields
Fields
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electric charges can exert forces on each
other
A force is that it is a push or pull.
What exactly is causing the push or pull
between electric charges ?
Remember there are two types of forces:
contact forces and non-contact forces
Micheal Faraday
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Michael Faraday developed a concept called the
field which we can use to explain action at a distance
forces
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According to Faraday each charge creates an
electric field which extends out and through all
space. Other charges feel forces because they
interact directly with the field.
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Fields are a concept which was invented to help
describe action at a distance forces.
Mathematical Description of Fields
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place a small positive test charge in a field and
measuring the force it experiences due to that field.
use a small test charge because we don't want it to
exert enough force to move the charges which have
created the field.
The test charge is positive simply for historical
reasons.
Testing a Electrical Field
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strength and direction of the
electric field at the various
points is indicated by
drawing an arrow.
When the test charge is
close to the positive charge
the electric field is stronger
so we draw a longer arrow.
The field would also repel
the test charge so we draw
the arrow accordingly.
Drawing Fields
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To visualize fields around electric charges,
we draw a series of lines to indicate the
direction of the field at various points.
These lines are called electric field lines and
we draw them so that they indicate the
direction of the force that the field would
apply to a positive test charge.
Electric field lines for a single positive
charge and a single negative charge
Notice that where the field is stronger (near the charge) the lines
are closer together
A little more complicated when more
that one charge is involved
Unlike
Charges
Imagine placing a positive test charge in the field
This helps with direction
Like Charges
Common Characteristics of Every
Electric Field
1.
Electric field lines start on positive charges
and end on negative charges
2.
Electric Field lines do not cross
3.
The density of the electric field lines is
proportional to electric field strength
Field Strength
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There is one problem with simply measuring the force on test
charge and using that as a measure of the electric field
strength. The problem is that there is no standard for exactly
how large the test charge should be.
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For example I might use a 1 µC test charge but you may use a
2 µC test charge. Your test charge would experience twice the
force as mine.
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However there is one quantity that will be the same for each of
our charges, that is the ratio of the force to the actual charge on
the test charge.
This leads us to the following formula
for electric field strength.
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E = F/q
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E = electric field strength (N/C)
F = force experienced by test charge (n)
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q = charge of the test charge ( C)
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Comparison – electric field near a charged rubber
rod is approx 1000 N/C
Examples
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A positive test charge of 2.0x10-5 C is placed in an
electric field where it experiences a net force of
0.0010 N. What is the electric field strength at that
point?
A charge of -200 µC is placed at a point in an
electric field where the strength is 5000 N/C . What
force will it experience?
Electrical Field Strength and Coulombs
Law
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By using (1) E = F/q and (2) F = kq1q2/ d2
You can create a formula that calculates the
electric field strength at any distance from a
point charge
Rearange (1) F = Eq and sub. Into (2)
Field Strength
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E = kQ
d2
Q – charge that creates the field
d – distance between field charge and test charge
Here you go
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7.
What is the electric field strength at a point 30 cm from a charge of
5.0 µC
The electric field strength at a particular point near a 50 µC charge is
found to be 4.5x104 N/C. What is the distance from this charge to the
point in question?
A positive charge of 2.0 x 10-8 C experiences a force of 0.060 N
when placed in an electric field. What is the magnitude of the electric
field?
What charge exists on a test charge that experiences a force of 1.4 x
10-16 N at a point where the electric field intensity is 2.0 x 10-4 N/C?
What is the electric field strength 24 cm from a point charge of 6.0
mC?
What is the charge on a point charge if the electric field strength is
3.8 x 105 N/C at a point of 59 cm from the point charge?
How far from a 4.2 mC charge is the electric field intensity 2.6 x 105
N/C?