Download Electric Force Fields

Electric Force Fields
Of Dreams!
An Electric Charge Changes the space
around it so that…
• Other electrical charges feel an electric force
– Measured by electric force (FE) and electric field (E)
– Force field
• Other electrical charges have an electrical
potential energy
– Energy Field
– Measured by electric potential energy (PEE) and
electric potential or potential or voltage (V)
• Assume a positive test charge when measuring
above.
Electric Force Field Variables
Electric Force (FE)
• Force of attraction or
repulsion between source
and test charge
• Vector quantity
• Measured in Newtons
• FE = QtestE
• Varies with source charge,
distance and test charge
Electric Field (E)
• Force per charge between
source and test charge
• Vector quantity
• Measured in
Newtons/Coulomb
• E=FE / Qtest
• Varies w/ source charge and
distance only
Check Questions
1. A 2 C test charge lays in an uniform electric
field of 20 N/C. What is the force the charge
feels?
2. A 5 C test charge feels a force of 2 N due to
an electric field. What is the magnitude of
the electric field?
3. What force would a 10 C charge feel in the
field described in question 2?
Representations of Force Fields: Force
Line Diagrams
• Assume a positive test charge
• Line spacing indicates strength of field ie
closer the spacing the stronger the field
• Arrows indicate direction of force on positive
test charge.
• Protons will travel in same direction as arrows,
electrons will travel in opposite directions.
Examples of Electric Force Fields
• Single Point Charges
– FE=Kc Qsource Qtest / d2
– E = Kc Qsource / d2
– Force line diagrams
• Opposite point charges
– Vector sums of above formulas
– Strongest field in between
charges
• Opposite Plate Charges
– FE = constant between plates
– E = constant between plates
– Force line diagram
Check Question
•
An electron and proton lie
between two parallel plates of
opposite charge as shown.
1. Which direction does each charge
move?
2. Compare the forces on each
charge.
3. Compare the accelerations of
each charge
4. For the proton, how does the
magnitude of the force change as
it moves?
Electric Shielding
• The electric force and field on
charges inside a conductor is zero.
• To shield an object from electric
fields simply enclose it inside a
conductor.
• Examples
– Lightning striking car or airplane
– Isolating power supplies
– Coaxial cable
Check Question
• Explain why the light bulb lights up when it is
perpendicular to the plasma sphere but not
parallel?
a) When perpendicular, the ends are at different
distances resulting in a difference in electrical
force
b) When perpendicular, the ends are at different
distances resulting in a difference in electrical
field
c) Both A and B are true