Cambridge A Levels A2 Physics Chapter 18 Electric Fields
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Course
Cambridge A Levels A2 Physics
Institution
Chapter 18 Electric Fields
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Chapter 18 Electric Fields
Electric Field & Field Lines
Electric Field
A region of space in which a charge experiences a force.
Field Lines:
Separation of the lines
▪ Shows strength of the electric field.
▪ Closeness of lines , electric field strength
Direction of the lines
▪ Shows the direction of the electric field.
▪ The arrows go from positive to negative.
▪ The direction of the electric field lines indicates the direction of the force on a positive
charge in the field.
Uniform Electric Field
Radial Electric Field
,Coulomb’s Law
Coulomb’s Law
The electrostatic force acting between two point charges is directly proportional to the
product of the charges and inversely proportional to the square of their separation.
𝑄1 𝑄2
𝐹∝
𝑟2
𝑘𝑄1 𝑄2
𝐹=
𝑟2
Constant of proportionality, k = 8.99 × 109 mF-1
1
𝑘=
4𝜋𝜀0
value of k depends on the medium around the charges
Permittivity of free space, 𝜀0 = 8.85 × 10-12 Fm-1
only applies to charges in vacuum (free space)
Coulomb’s Law
𝑄1 𝑄2
𝐹=
4𝜋𝜀0 𝑟 2
, 𝟏
𝑭∝
𝒓𝟐
Difference between electric forces and gravitational forces
Gravitational Forces Electric Forces
Acts on masses Acts on charges
Attractive Attractive / Repulsive
(Depending on the sign of the interacting charges)
𝐺𝑚1 𝑚2 𝑘𝑄1 𝑄2
𝐹= 𝐹=
𝑟2 𝑟2
G = universal gravitational constant k = constant of proportionality
G is medium independent (irrespective to k is medium dependent (depends on the
the location of the 2 masses) medium in which the changes are in)
Electric Field Strength, E
Electric Field Strength
Force per unit charge acting on a stationary small positive test charge at that point.
Electric Field Strength
𝐹 Vector quantity
𝐸=
𝑞
Unit: NC-1 / Vm-1
E = Electric Field Strength
F = Force acting on the charge Direction of E is the direction of
the force acting on a positive test
q = charge charge at a given point.
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