Electric Fields,
Electric Potential Energy
&
Electric Field Intensity
Electric Fields
A charged object is any object in which there is an imbalance of
electrical charges (there are more electrons than protons or more
protons than electrons).
An electric field:
is invisible and found around (in 3-dimensions) a charged object
is a vector quantity (it has magnitude and direction)
has a direction that a proton would move in if the proton were placed in the field.
The field lines point toward negative charges & away from positive ones
has a magnitude that is the force in newtons per coulomb of charge
obeys the inverse-square law.
+
-
Electric Fields
Field lines:
Are used to indicate the direction of electric fields. An arrow
on the end opposite the more positive area.
They show how strong the field is:
Many field lines, close together indicates a strong field.
Only a few lines, far apart, indicate a weak field.
Fields exist around single points or even from one flat or
curved plate toward another plate.
Electric Fields
Electric potential energy is energy that a charged object
has due to its location in an electric field.
The stronger the field, the greater the electric potential
energy.
The greater the charge, the greater the electric potential
energy.
In the same electric field, a pair of electrons would have
twice the electric potential as a single electron.
In a field with strength 10 newtons per coulomb (10 N/C),
an electron would have twice the electric potential
energy as it would in a field of strength 5 N/C.
Draw the field lines between the proton on the
left and the electron on the right…
+
-
Draw the field lines between the proton on the
left and the proton on the right…
+
+
Electric Fields
If a charge is to move with an electric field, the field does
work on the charge.
If the charge is to move against the field, work must be done
on the charge.
This is somewhat like a toy dart gun. To give the gun/dart
system some elastic potential energy, some work must be
done in pushing the dart against the spring inside the gun.
When the dart is released, the spring does work on the dart,
changing electric potential energy, EPE, into kinetic energy,
KE.
Electric Fields
In the following slide, the field direction is from left to right…
That means that a positive charge placed in the field would
be pushed to the right by the field (the field does the work
on the positive charge)…
It also means that moving a positive charge to the left would
require work from an outside source.
Electric Fields
Electric Potential Energy: It requires work to push a charged
particle against the electric field…
Note how the EPE increases
Electric
as the positive charge is
Potential
Energy
pushed against the field…
Monitor
+
Electric Fields
A released particle accelerates away from the sphere as
electric PE is changed to KE…
Note how the EPE decreases
Electric
as the positive charge is
Potential
Energy
pushed by the field.
Monitor
+
Electric Potential Energy
Electric Potential Energy
Batteries and generators pull negative charges away
from positive ones, doing work to overcome electrical
attraction.
The amount of work depends on number of charges and
separation distance.
Work done by a battery or generator is then available to a
circuit as electrical PE.
Electric Potential
Electric potential is electric potential energy per charge;
the energy that a source provides to each unit of charge:
Electric potential =
electric potential energy
charge
W
V = -----Q
(OR)
W = VQ
(OR)
Units: V = joules/coulomb,
Q = V/W
W = joules,
Q = coulombs.
Electric Potential
Electric potential and voltage are the same.
The unit of measurement is the volt…
How much work is done when a 9 volt battery
moves 2 coulombs of charge through it?
Energy = Work = Voltage x Current…
W = VQ
W = (9 v)(2 C)
W = (9 J/C)(2 C)
W =
18 J
Electric Potential
How much charge can be moved through a
potential difference of 36 volts using 9 joules of
work?
W = VQ
Q = W/V
Q = (9 J)/(36 V)
Q = (9 J)/(36 J/C)
Q =
0.25 C
How much work can be done by a charge of 6
coulombs if it has 12 volts of electric potential as a
source of energy?
W = VQ
W = (12 V)(6 C)
W = (12 J/C)(6 C)
W =
72 joules