Download The Electric Field

The Electric Field
What do you think?
 Electric forces and gravitational forces are both field
forces. Two charged particles would feel the effects
of both fields. Imagine two electrons attracting each
other due to the gravitational force and repelling
each other due to the electrostatic force.
 Which force is greater?
 Is one slightly greater or much greater than the
other, or are they about the same?
Drawing
Field Lines
• In the chapter “Circular
Motion and Gravitation,” you
learned about the
gravitational field (g). The
diagram to the right shows the
“g” field around Earth.
• In this section, we will study
the electric field “E” around
charged particles.
• On the next slide are three
different diagrams. Make a
sketch of the “E” field for each
charge or combination of
charges.
Drawing Field Lines:
What do you think?
• Make a sketch of the
“E” field for each
charge or
combination of
charges.
– Field lines have
direction
– What direction do you
think the field lines go
around a + or –
charge?
1
3
2
Electric Field Strength
 Electric fields (E) have magnitude and direction.
 The direction is defined as the direction of the force on a small,
positive test charge (q0) placed in the field caused by Q.
 The magnitude of the field is defined as the force per unit
charge on q0.
Fel
E
qo
Test Charges
 A small test charge will
not significantly affect
the field
 If the test charge (q0)
is large, it will affect the
way the charges are
distributed on the
charged conductor
This would change
the field around
the conductor.
• Test charges will always be considered small
enough to have no effect on the field.
Electric Field Strength
 Combine Coulomb’s law with
the definition of electric field
to derive an equation for E
due to a point charge.
qo q2
kc 2
Fel
r
E

qo
qo
 SI unit: N/C
 The field strength does not depend on the test charge.
Example Electric Field
Strengths
Practice Problem
Use your ninja physics skills!!!
 An electric field around a charged object is 5.95 
106 N/C at a distance of 0.100 m. Find the charge on
the object.
 Answer: 6.62  10-6 C or 6.62 C
 Suppose a small test charge of 0.200 C was placed
at the point that is 0.100 m from the charged object.
What force would be exerted on the test charge and
on the object?
 Answer: 1.19 N for both test charge and object
Electric Field Line Rules
Apply the above rules
and sketch the E field
around the charge
shown.
Electric Field Line Rules
Apply the above rules
and sketch the E field
around the charge
shown.
Electric Field Line Rules
Apply the above rules
and sketch the E field
around the charge
shown.
Electric Field Lines Rules
Electric Field Line Rules
Apply the above rules
and sketch the E field
around the charge
combination shown.
Electric Field Lines Rules
Electrostatic Equilibrium
 Electrostatic equilibrium occurs in conductors when no net motion
of charges exists within the conductor.
 Charges in a conductor are free to move, but are not moving
when equilibrium exists. The rules below result from this fact
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