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Electric Fields
• How do we have forces where we don’t have
physical contact ?
• Newton called it action at a distance
Michael Faraday
• Why is there a force
between charges?
• Came with the idea of
electric field
• - a force acting upon a
charge is due to a field
emitted by a second charge
that interacts with the first
charge.
• The field of big (Q)
creates the force on
little q.
• Need at lest two
charges two have a
force.
• The direction of an
electric field is away
form a positive charge
and towards a negative
charge
• Point charge ( test
charge) - is a tiny
charged body and the
charge at a single point
where all the charge is
concentrated .
• If a small positive test charge (
q ) experiences a force of
magnitude F at a given
location, the magnitude of the
electric field E at that location
is :
• Magnitude of electric field =(
force on positive charge ) /
(amount of charge )
• E=F/q
• SI unit : N/C
• The electric field E
points away from
positive charge and
towards negative
charges
• .
• The force on a positive
test charge(+) is in the
direction of the eclectic
field
• The force on a negative
test charge (-) is in the
opposite direction form
the electric field.
The electric force due to an electric field
• A test charge ( q ) in
electric field ( E )
experiences a force F by
:
• F=qE
• SI unit : N
• A positive charge
experiences a force F in
the same direction
• A negative charge
experiences a force F in
the opposite direction
from E
Point charge electric fields decrease
with distance
Point charge electric fields decrease
with distance
• F = ( k Qqt)/ (r2)
• E = ( k Q qt)/ (r2)/ qt
• E= kQ/r2
• E = F/ qt = (k q) / r2
• ( E ) decreases by the
square of the radius
( Radial fields ) and ( line source fields )
• 1-Electric field lines point in
the direction of the electric
field vector E at every point
• 2- Electric field lines start at
positive (+) and terminated
on the negative charges.
• 3- Electric fields lines are
closer together where
vector E has a greater
magnitude
• 4-The number of electric
field lines entering or
leaving a charge is
proportional to the
magnitude of the charge
Field lines never cross