Summary Revision on Electrostatics according to the exam guideline with examples Grade 12
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Course
Physical Sciences
Institution
12th Grade
This document is based on the topic on Electrostatics. It is according to the examination guideline as given out by the Department of Education. It has examples included
Electrostatics
Definitions to be studied:
• State Coulomb's law: The magnitude of the electrostatic force exerted by one point
charge (Q1) on another point charge (Q2) is directly proportional to the product of the
magnitudes of the charges and inversely proportional to the square of the distance (r)
between them:
• Electric field: a region of space in which an electric charge experiences a force. The
direction of the electric field at a point is the direction that a positive test charge would
move if placed at that point.
• Electric field at a point: The electric field at a point is the electrostatic force
experienced per unit positive charge placed at that point
Electric field patterns ☺
From exam guideline
• Draw electric field lines for the following configurations:
o A single point charge
o Two point charges (one negative, one positive OR both positive OR both
negative)
o A charged sphere
NOTE: Restrict to situations in which the charges are identical in magnitude.
Easy to score marks:
How marks are allocated:
• A mark for the direction of field lines (direction in which a positive point charge would
move when placed in the field).
• A mark for the shape.
• Lines should not cross each other, and they should touch the sphere (negative marking
is applied).
Force diagrams, free-body diagrams: ◊
• Use past examination papers to practice as many different scenarios as possible.
Formulae, substitutions, and calculations.
From exam guideline
Coulomb's law
𝑘𝑄1 𝑄2
• Solve problems using the equation 𝐹 = for charges in one dimension (1D) (restrict
𝑟2
to three charges). ♥
𝑘𝑄 𝑄
• Solve problems using the equation 𝐹 = 12 2 for charges in two dimensions (2D) for
𝑟
three charges in a right-angled formation (limit to charges at the 'vertices of a right-
angled triangle).
Electric field
.♦
𝐹
• Solve problems using the equation 𝐸 = 𝑞
• Calculate the electric field at a point due to a number of point charges, using the
𝑘𝑄
equation 𝐸 = 𝑟 2 to determine the contribution to the field due to each charge. Restrict
to three charges in a straight line. ♦
Easy to score marks:
• The formulae are in table 2 under ELECTROSTATICS.
• Make sure you can differentiate between the formulae for ‘F’ and that for ‘E’ and use
them appropriately.
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