STAGE 2 PHYSICS
READING
Electricity and Magnetism
Key Ideas pg
pg
Electric Fields A
Key Ideas
Intended Student Learning
Coulomb’s Law
Any two stationary point charges experience mutual
forces along the line joining them.
Solve problems involving the use of F 
1 q1q2
4 0 r 2
where F is the magnitude of the electric forces, q1
and q2 are the charges, r is the distance between
them, and 1 4  0 is the proportionality constant.
These forces are attractive if the charges are unlike
and repulsive if they are alike.
Using proportionality, discuss changes in the
magnitude of the force on each of the charges as a
result of a change in one or both of the charges
and/or a change in the distance between them.
The magnitude of these forces is directly proportional
to the product of the two charges and inversely
proportional to the square of the distance between
them.
Explain that the electric forces are consistent with
Newton’s third law.
Principle of Superposition
When more than two point charges are present, the
force on any one of them is equal to the vector sum
of the forces due to each of the other point charges.
Using vector addition, calculate the magnitude and
direction of the force on a point charge due to two
other point charges.
Electric Field
Electric charges establish an electric field E in the
surrounding space. The electric field at any point
produces a force on an electric charge placed at that
point.
Describe how the concept of the electric field
replaces the concept of action at a distance (inherent
in Coulomb’s law) with the localised action of the field
of one charge on the other charge.
The electric field at a point is defined as the electric
force F per unit charge on a small positive test
charge q placed at that point, provided that all
Solve problems involving the use of E  F q.
charges remain undisturbed: E  F q.
The direction of the electric force on a charge is
parallel to the electric field if the charge is positive
and antiparallel if the charge is negative.
Determine the direction of the electric field at any
point due to a point charge.
Using Coulomb’s law, derive the expression
1 q
for the magnitude of the electric field at
E
4 0 r 2
a distance r from a point charge q.
1 q
Solve problems involving the use of E 
.
4 0 r 2
Pictorial Representation of Electric Fields
An electric field can be represented by field lines such
that the direction of the field is at a tangent to each
line, and the magnitude of the field at any point is
represented by the number of lines crossing a unit
area perpendicular to the field in the vicinity of the
point.
Sketch the electric field lines for an isolated positive
or negative point charge.
Superposition of Electric Fields
The principle of superposition applies also to electric
fields, as the electric field is the force per unit charge.
Calculate the magnitude and direction of the electric
field at a point due to two charges with the same or
opposite sign.
Sketch the electric field lines for two point charges of
equal magnitude with the same or opposite sign.
Coulomb’s Law
There is a mutual force between any two point charges which is directly
proportional to the product of their charges and inversely proportional
to the square of the distance between them.
“This force acts along the line joining them and is repulsive if the charges
are alike and attractive if the charges are unlike" (Required definition)
This law can be represented by the formula:
𝐹=
1
4𝜋𝜀0
1 𝑞1 𝑞2
4𝜋𝜀0 𝑟 2
is called Coulomb’s Law constant and is equal to 9.00 x 109 Nm-2C-2 for a vacuum or air
If the two charges are alike, the force will have a positive value and be repulsive, while if the two
charges are unlike, the force will have a negative value and be attractive.
Newton's Third Law applies to the case of electric forces. This means that each charge will exert an
equal and opposite force on the other charge, even if the two charges have different magnitudes.
These forces must act along the line connecting the two
point charges.
Changes in the Electric Force: Due to a change in charge
The electric force is proportional to the product of the two charges.
Therefore, if one of two charges that experience an electric force is altered by a certain proportion,
the electric force between them will change by the same proportion.
For example, if one of the two charges is doubled, the electric force will also be doubled. i.e. if the
two initial charges are Q and q (at a distance r apart), the electric force will be
Changes in the Electric Force: Due to a change in distance
The electric force is inversely proportional to the square of the distance between the two point
charges.
Therefore, if the distance between two point charges that experience an electric force is altered by a
certain proportion, the electric force between them will change by the inverse square of that
proportion.
For example, if the distance between the two charges is doubled, the electric force will be one
quarter of the original force.
i.e. if the two charges (Q and q) are initially a distance r apart, the electric force will be
The Principle of Superposition
Coulomb's Law can also be applied to situations where there are more than two point charges.
In this case, the overall force acting on any one charge is equal to the vector sum of the forces due to
each of the other charges.
For example, given three point charges A, B and C, the force on charge A is the vector sum of the
force that B exerts on A and the force that C exerts on A.
Electric Fields
Any electric charge will create an electric field in the space around it.
Whenever another charge is placed in this space, it will experience an electric force due to the
electric field.
"The electric field at a point is defined as the electric force F per unit charge on a small positive test
charge q placed at that point, provided that all charges remain undisturbed.
It is given the symbol E and has the units of Newton's per coulomb (N.C-1)
When a charge is placed in an electric field, it will experience a force in the same direction as the
field if the charge is positive, while it will experience a force in the opposite direction to the field if it
is negative.
Action at a distance
The concept of an electric field can replace the concept of "action at a distance". Instead of one
charge exerting a force on another charge that is some distance away, it is the electric field of the
first charge that exerts a force on the second, and correspondingly, the electric field of the second
charge exerts a reaction force on the first.
Electric Field Strength
Derive the formula for the strength of an electric field:
Pictorial Representation of Electric Fields
An electric field can be represented by drawing "electric lines of force". These lines indicate the
direction of the field at any point (i.e. the direction of the force that a small positive charge would
experience if placed at that point).
Electric lines of force are directed away from
points of positive charge and towards points of
negative charge.
An electric field will be strongest in regions
where the lines of force are close together and
weakest in regions where the lines of force are
far apart.
Superposition of Electric Fields
The total electric field strength at a point due to more than one electric field is equal to the vector
sum of each electric field at that point.