Electric force is an
action-at-a-distance force.
How can objects at a distance exert
forces on each other?
The best explanation to this question
Action-at-a-distance forces are
sometimes referred to as FIELD forces.
The concept of a field force is utilized
Another way to say is to simply
suggest that there is something rather strange about the SPACE surrounding a charged object.
Any other charged object that is in that
space feels the effect of the charge.
A charged object creates an electric
Other charges in that field would feel
the unusual alteration of the space.
Whether a charged object enters
that space or not, the electric field exists.
Space is altered by the presence of a
charged object; other objects in that space experience the strange and
The strength of the electric field is
dependent upon how charged the object CREATING the field is and upon the DISTANCE of separation from the charged object.
The electric field (E) is a vector
How can we tell if there is an electric
field?
The only way we can tell if a field
The magnitude of the electric field is
defined in terms of how it is measured.
Imagine an electric charge (Q) creates
an electric field…
Since Q is the source of the electric field, we will refer to it as the SOURCE charge.
The strength of the source charge’s electric field could be measured by any other
The charge that is used to measure
the electric field strength is referred to as a TEST charge (q) since it is
used to test the field strength.
When place within the electric field,
A FORCE field that fills the SPACE around
every ELECTRIC charge or group of charges.
The magnitude of the electric field is simply
defined as the force per charge of the test charge.
Electric Field Strength = Force/Charge
Unit: Newton/Coulomb or N/C
The electric field strength is not
Another very useful hybrid equation can be
created by combining the formula for
COULOMB’S Law and the formula for FIELD intensity.
E = F/q = kqQ/d2/q = kQ/d2
E = kQ/d2
Note that the derivation above shows that the
test charge q was canceled from the equation. The new formula for electric field strength
E = kQ/d2
The strength of an electric field as
created by source charge Q is
inversely related to the square of the distance from the source. This is
The worldwide convention which is
Several generalities can be made
about the direction of the electric field vector.
1. The electric field vector would always
be directed AWAY from positively-charged objects.
2. Electric field vectors are always
Rather than drawing countless vector
arrows in the space surrounding a
source charge, it is perhaps more useful to draw a PATTERN of several lines
which extend between infinity and the source charge.
These lines are called ELECTRIC FIELD
1. Lines EMANATE from positive charges 2. Lines TERMINATE at negative charges 3. Large charges have MORE field lines
beginning/terminating at the charges. More lines indicate a stronger field.
4. The closer the lines, the STRONGER the field. The greater the line density the
5. The electric field is always directed PERPENDICULAR to the surface of an
object. If there were ever any component of force parallel to the surface, then any
excess charge residing upon the surface of a source charge would begin to accelerate. This would lead to the occurrence of an
electric current within the object.
6. Electric field lines should never CROSS. Every single location in space has its own electric field strength and direction
Several electric field line patterns
are shown in the diagrams below. Which of these patterns are
Several electric field line patterns
are shown in the diagrams below. Which of these patterns are
incorrect? Explain what is wrong with all incorrect diagrams.
In order to bring two like charges near each
other WORK must be done on the objects.
In order to separate two opposite charges,
WORK must be done on the objects.
Whenever WORK is done on a charged
object by moving it through an electric field, the amount of position or potential energy CHANGES.
This stored electrical energy is known as
A positive test charge will naturally move in the
direction of the field without the need for work being done on it; this movement will result in the loss of
Since the electrical potential energy can
change depending on the amount of
CHARGE you are moving, it is helpful to
describe the electrical potential energy per unit of charge.
This is known as ELECTRIC POTENTIAL.
Electrical Potential = PE/Q
While electric potential energy has a
dependency upon the CHARGE of the object experiencing the electric field, electric
Electric potential difference (V or ΔV):
The difference in POTENTIAL (V) between the
final and the initial location when work is done upon a charge to change its POTENTIAL energy.
Unit: VOLT (V) and was named in honor of
Allesandro Volta. One Volt is equivalent to one Joule per Coulomb.
ΔV = Vb – Va = Work/Charge = ΔPE/Charge
Because electric potential difference is
Definition: Two or more points that have an
electric potential difference of 0 V.
The work done to move a charge from any
1. Equipotential surfaces are
PERPENDICULAR to electric field lines
2. Electric field lines have a defined
direction, the direction of the force on a POSITIVE test charge. The
The image in your notes shows what
the electric field lines and the
equipotential lines look like for two oppositely charged parallel plates.
Notice that the field lines are EVENLY spaced (except at the very ends).
E = V/d
According to the field intensity
formula, the SMALLER you make
the distance between the plates,
the STRONGER the electric field
becomes if the potential
The electric field is concentrated where
the tree is taller than it’s surroundings.
Remember that the CLOSER the field
lines are together, the STRONGER the electric field strength.
The stronger the field, the more likely
Capacitor: A device used to store
CHARGE
Capacitance (C): The RATIO of
charge stored to electric potential difference
Unit: FARAD or F Equation:
