Download Electric Potential Difference

J. Pulickeel
April 2009
SPH 4U1
Electric Forces
 An Electric Force is a non-contact force which can act
at a distance. For instance a (+) charged ebonite rod
will attract a (-) pith ball
 An electric force can be created by Friction, Contact or
Induction (Grounding)
Electric Force Fields
 A field force is sometimes referred to as an action at a
distance
 The space surrounding a charged object is affected by
the presence of the charge.
 An Electric Force Field is an alteration of the space
surrounding the charge
Electric Force Fields
 Electric Force Fields are vector quantities. Each field
line represents a net force acting on a charged particle
at that particular point in space.
Electric and Gravitational Forces
Gravitational Forces
Electric Forces
 A Force of attraction between
 A force between two charged
two objects.
particles. It can be attractive
or repulsive.
Gravitational Fields
It is often useful to relate the Force experienced to
a specific location.
F = mg
For instance a 30 kg person would experience
296 N of Force. A 70 kg person would experience
686 N of force.
We could say that at this particular spot, the
Gravitational Field Strength is 9.8 N/kg
What force would a 95 kg person experience at this
location?
Is it valid for other locations?
Electric Fields
 Similarly, a test charge will exert a force around itself
that attracts or repels other charges
Lower Electric Field
4 N/kg
At this location, a charge
would feel 2N/C
A 4 C charge would feel
8 N force
7.5 N/kg
9.8 N/kg
A 10 C charge would
feel 20 N Force
At this location, a
charge would feel 1J/C
High Electric Field
Electric and Gravitational Fields
Similarly, we could define an Electric Field as the electric force per unit
charge at a specific location
Gravitational Fields
 Exists in the space surround
an object in which the force
of gravity exists
 A field illustrating the force
of attraction between two
objects
Electric Fields
 The region in which a force is
exerted on an electric charge
by another electrical source
Electric and Gravitational Fields
Gravitational Fields
Electrical Fields
 Depending on our location
 The space around an
on the earth we experience a
different gravitational field
strength
 We approximately 9.8 N/kg
electrical charge in which an
electric force acts.
 This is similar to
Gravitational Fields on a
microscopic planet…
Potential Energy
 Potential Energy is the stored energy of position of an
object an it is related to the location of the object
within a field.
 Gravitational Potential Energy (GPE) – the stored
energy due to attractive forces of large masses and
their relative distances.
 Electrical Potential Energy (EPE) – the stored
energy due to attractive or repulsive forces of charged
particles and their relative distances.
Potential Energy
Gravitational Potential Energy
Electrical Potential Energy
 When gravity does work on an
 When the electrical charge
object, it GPE is lowered.
does work, the EPE is reduced
High GPE
High EPE
Lower EPE
GPE  KE
EPE  KE
EPE  KE
Lower GPE
Lower EPE
High EPE
Gravitational and Electrical Potential Energy
 It is natural for objects to move from high energy to
low energy.
 Work is required to move an object from low to high
energy
Moving a (+) charge from A to B
will require work and increase the
EPE
Moving a (+) charge from B to A
will naturally occur. Work is not
required. The EPE will decrease
Gravitational Potential
1 kg
2m
GPE depends on the mass of an
object and its height.
2 kg
1m
An object with twice the mass will
have twice the potential energy
and an object with twice the
distance will have twice the
potential energy.
An object an a high position does
not necessarily mean high
potential energy and low
positions don’t mean low
potential energy
Gravitational Potential
Gravitational Potential Energy depends on mass and height
Gravitational Potential tells us the amount of potential
energy per kilogram at a particular spot.
Gravitational Potential
1 kg, 2m
20J PE
1 kg, 1m
10J PE
20J/kg
10J/kg
0 J/kg
2 kg, 2m
40J PE
2 kg, 1m
20J PE
Gravitational Potential
 Gravitational Potential is a means of rating
various locations within a gravitational field in
terms of the amount of potential energy per unit of
mass.
 Gravitational Potential is
 Mass independent and
 Location dependant
Electric Potential (V)
 Similar to GP, Electrical Potential is dependant on the


Charge of the particles
Distance between the particle and the source
10 Joule PE
1C
20 Joule PE
2C
Both (+) charges require work to push them closer
to the Van de Graff Sphere.
The larger charge requires twice the work because it
is twice the charge.
If a (3+) charge were placed at the same location it
would require 30 units of PE
The Electric Potential for this location is 10J/C
Electric Potential (V)
 A (+) charge would be at a high V when held close
to a (+) source charge and at a lower V when held
further away.
 Suppose that the electric potential at a given location is 9
J/C


A 2 C object would have 18 J of Potential Energy at that location
A 0.5 C object would have 4.5 J of Potential Energy at that location
Electric Potential (V)
 A (+) charge would be at a high V when held close
to a (+) source charge and at a lower V when held
further away.
This battery powered electric circuit has areas of high
and low potential.
As a charge moves through the wire it will experience
changes in electric potential.
Movement of (+) charge from (A) will occur naturally
because it does not require work. The charge looses PE.
Chemical Energy is transformed into EPE within the
battery. The (+) charge will move through the circuit
and do work on the light bulb. It will return to the (-)
terminal with low EPE and low Potential.
What is Voltage?
 Voltage is a change in Electric Potential or the Electric
Potential Difference
If the (+) charge were moved from (A) to (B), it
would require work to increase its Potential
Energy.
B
A
As a result of this Potential Energy, there is also an
increase in Electric Potential.
This change in Electrical Potential is called
Voltage
7.4 Q#5
The electric potential at a point is 0 J/C. Is it possible for the
electric field at that point to be non-zero.
 Let’s consider Gravitational Potential. Is it possible to
have 0 J/kg and a Gravitational Field of 0 N/kg?
High GP
If I removed the table, would the
ball continue to fall?
0 GP
Is there a gravitational field
acting on the ball?
7.4 Q#5
The electric potential at a point is 0 J/C. Is it possible for the
electric field at that point to be non-zero.
 If there is no charge, then there is no electric field.
 In order for an Electric Potential of 0 J/C, we need charges of
equal and opposite magnitude. If this were so, then there would
be a force of attraction from the (+) to the (-) charge.
 Therefore it is not possible.
 Click here for animation
0 J/C
Electric
field
vector