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Electric Fields
Electric fields are created by electric charges. Any object with a charge has
an electric field around it. Opposite charges attract each other; like charges
repel. An electric field is a region in which a charge experiences a force.
We can represent electric fields by using electric field lines. As with gravitational
fields, the lines will indicate 2 things about the field:
1. Its direction - from the direction of the field lines
2. How strong the field is – from how close the field lines are to each other
Uniform electric fields have the same strength field
at all points. An electric field between oppositely
charged plates is a uniform magnetic field.
A radial electric field spreads outwards
in all directions. The electric field
around a point charge is a radial field.
The electric field strength at a point
is the force per unit charge exerted
on a positive charge at that point.
𝑭
𝑬=
𝑸
Units for electric field
strength are NC-1
Uniform Electric Fields
In a uniform field, the
field strength is the
same at every point. The
field lines are parallel
and evenly spaced.
𝑽
𝑬=
𝒅
Electric field strength is directly
proportional to the voltage of
the plates and inversely
proportional to the separation
between the plates.
The force on a
charge in a uniform
electric field:
𝑭 𝑽
=
𝑸 𝒅
hence
An electric field between oppositely
charged plates is a uniform field.
F=
𝑸𝑽
𝒅
Coulomb’s Law
Any two point charges exert an electrical
force on each other that is proportional to
the product of their charges and inversely
proportional to the square of the distance
between them.
𝑄𝑞
𝐹=
2
4𝜋𝜀0 𝑟
Radial Electric Fields
Radial magnetic fields spread
outwards in all directions.
For a radial field, the electric field
strength depends on the distance r from
the point charge Q. There is an inverse
square relationship – field strength
decreases as you get further from the
point charge.
Electric field
strength for
a radial field:
This is shown by the fields lines:
they get further apart from each
other as you move away from
the point charge.
𝑄𝑞
𝐸𝑞 =
4𝜋𝜀0 𝑟 2
𝑸
𝑬=
𝟒𝝅𝜺𝟎 𝒓𝟐
The effect of a uniform magnetic field on the motion of
charged particles
The electron is moving to
the right. Its initial
horizontal velocity will not
be affected because there
is no component of the
eelectric field in this
direction. It will therefore
accelerate upwards, curving
towards the positive plate,
following a parabolic
trajectory.
+
+ +
+
- - - -
Comparing Gravitational and Electric Fields
Gravitational Fields
Electric Fields
Arises from masses
Arise from electric charges
Only gravitational attraction; no Both electrical repulsion and electrical
repulsion
attraction are possible as there are
both positive and negative charges
Field strength is force per unit
mass: g= f/m
Field strength is force per unit
positive charge: E = F/Q
Uniform gravitational fields have Uniform electric fields have parallel
parallel gravitational field lines electric field lines
Radial gravitational fields : force Radial fields have radial electric field
is given by F = –GMm/r2
lines diverging outwards. The force is
given by Coulomb’s law: F= Qq/4πεor2
For a radial field: Force and
field strength obey an inverse
square law with distance
For a radial field: force and field
strength obey an inverse square law
with distance