This content highlights physics of grade 12 (Ethiopian) note.

1. Electrostatics
Lesson :one
Electric charge and Coulomb’s law
By Deriba Fufa

2. 4.1 Electric charge and Coulomb’s law
➢ Electrostatic phenomena arise from the
forces that electric charges exert on each
other.
➢ These forces are described by Coulomb’s
law – this bears several similarities to
Newton’s laws of gravitation.
➢ This unit explores the forces between static
charges and the associated energy changes
involved when charges are moved from one
place to another.

3. What are electric fields?
An electric field is the region around a
charged object where another charged object
will experience a force.
The force between charges may be attractive or
repulsive depending on the nature of the
charges.
Whenever we represent a region of space
containing an electric field we draw a number
of electric field lines
Lines of electric flux have a direction. They
move away from positive and towards
negative.
❖ When drawing field lines they must not cross
over each other.
❖ The spacing of the field lines represents the
strength of the field. The closer the lines are
together the stronger the field.
❖ It is possible for two (or more) electric fields
to cancel each other out and create a neutral
point. Here the resultant field strength is zero.
See video from next slides

4. Video lesson……………..

5. Electric field strength
The strength of the electric field is defined as:
The force per unit positive charge
acting on a positive test charge placed
in the field.
Mathematically this may be expressed as:
𝐸 =
𝐹
𝑞
The equation for electric field strength is
often used to determine the force acting on
a charged particle due to an electric field:
𝐹 = 𝑞𝐸
Hint: Electric field strength is a vector quantity.
It is important to include the direction of the
lines of force/ flux in any field diagrams..

6. Charge movement in the direction of the field
A particle with charge q starts from rest at the
point shown. It moves a distance d through the
field of electrical field strength E. While it is
in the field, a force 𝐸𝑞 is acting on it in the
direction of the field and therefore work is
done on it equal to 𝐸𝑞𝑑.
Depending on what other information is
available (its mass, for example), it would be
possible to find its acceleration, its velocity
after travelling any particular distance and the
time it took to complete that distance.
These ideas have considerable practical
applications in various types of electronic
equipment, for example, in X-ray tubes, particle
accelerators, and etc

7. Charge movement initially at right angles to the
direction of the electric field
▪ Consider a positively charged particle q travelling
horizontally with velocity v in a vacuum and entering a
uniform electric field of magnitude E for a distance d.
▪ The force that the field exerts on the charge will always
be in the direction of the field, so there cannot be any
alteration to its horizontal velocity.
▪ It will therefore spend a time t =
𝑑
𝑣
in the field.
▪ During this time the constant force on it is F = qE
downwards, giving it a constant acceleration downwards
of 𝑎 =
𝑞𝐸
𝑚

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