Transcript Electric Fields

Electric Fields
Force over a distance
 Both
gravity and electric force act over a
distance without touching (unlike other
forces)
 Very difficult for early scientists to
accept
 Michael Faraday proposed the Electric
Field to explain
Electric Field
 Force
that extends outward from any
charged object and permeates through
space
 Any second charged object placed
around the first charge will feel a force
of attraction or repulsion due to this field
 Tested with an imaginary positive test
charge (q)
 Magnitude
of the force acting on the test
charge can be measured
Definition
 Electric
field (E) is defined as the
amount of force (F) per unit of charge
(q)
E = F/ q
units N/C
Another Equation
E = F/ q


F = k qQ/ r2
E
= k Q/ r2
Example
 Find
the magnitude and direction of the
electric field on a particle P which is
located 30 cm right of a point charge of
–3.0 x 10-6 C.
Copying machines work by (+) charges
on a drum attracting (-) charged ink
particles. When a paper runs over the
drum, the ink sticks to paper. Each ink
particle has a mass of 9.0 x 10-16 kg and
carries 20 electrons. In order to stick,
the force of attraction must be at least
2x greater than its weight. What is the
field strength at the drum’s surface?
 If
the field is due to more than one
charge, the total field is the sum of the
two individuals
Example
 Find
the total field acting on point P. If P
is a proton initially at rest, what will its
acceleration be?
Q1 = -25μC
P
Q2 = +50μC
Field Lines
 Lines
of force can be drawn to visualize
the force acting on a single point charge
 Stronger fields have more lines
 On point charges
 On double points
 Start
 On
on + and go to -
plates
Fields and Conductors
 The
electric field inside any good
conductor is zero
- charge distributes itself evenly over the
surface of a conductor making the net
field inside zero
 Electric field is always perpendicular to
the surface of a conductor
 Excess charge tends to accumulate on
sharp points or areas of greatest
curvature
What will happen?
+