8.2A
What is an electric field?
Reminder of what the rules
are on bringing electrically charged objects together?
An electric field is created around any
particle or object that has an overall positive or negative electric charge.
Any pocket of static charge on an object,
when brought near another object that is also carrying a static electrical charge then one
of two things can happen.
When an electrically charged object is placed in an electric
field e.g. from another charged object, it will experience a force due to the
interaction of two electric fields.
The magnitude and direction of this force is determined by
three factors, which are all to do with the strength of the electric field.
Also note that both charged objects experience a force
e.g. both the objects Q and q move apart or come closer together.
1. Direction of the force
If the static charges are alike, two
positive or two negative charges, then a force of repulsion occurs
between the objects, pushing them apart.
This is called electrostatic repulsion,
a non-contact force.
If the static charges are different, a
positive and a negative charge, then a force of attraction occurs between
the objects, attracting them together.
This is called electrostatic
attraction, a non-contact force.
This non-contact force acts due to the presence of an
electric field around each object which do NOT have to be in contact with each
other.
You can demonstrate rule 1. by suspending
two different plastic rods of different static charges and bring them
towards each other e.g. two of the same charge brought near each other OR
two plastic rods of opposite charge.
Rule 2. The size of the charge
For oppositely charged objects, the greater the two electrical charges e.g.
Q+ and q-, the greater the force of attraction between the charged objects. The
strength of the electric field is increased by increase in the magnitude of
the charges.
If the two charges are of the same sign
(+ + or - -) then the greater the magnitude of the charges, the greater the
force of repulsion pushing them away from each other.
Rule 3. The distance between two charged
objects
For oppositely charged objects (+ve and
-ve), the smaller the distance between the two
static charges, Q+ and q-, the greater the force of attraction between the charged objects.
The strength of the electric field is increased by decreasing the distance
between the charges.
If the two charges are of the same sign
(+ + or - -) then the closer they are, the greater the force of repulsion
pushing them away from each other.
Footnote:
(at higher level i.e. A level, the force of
attraction is proportional to charge Q+ x charge q- / d2)
Electric charge is measured in coulomb
units, denoted by C
See
Calculation
of the charge passing through a point in a circuit Q = It
8.2B
More on electric fields - 'maps' of the electric field lines
Can we map out the field lines of a static
electric field (as with magnets!)?
An electrical field is created around any
electrically charged object.
The electric field of an object is the
region that will experience a force if a second charged object is moved into it.
The nearer you are to a charged object AND
the bigger its charge, the stronger
the electric field effect is.
The orientation of field lines around a charged object
The diagram above shows the direction of the
field lines in an electric field emanating ('spreading out') from two isolated
spherical objects, one positively charged Q, and one negatively charged
object q.
The lines of the electric force field go
from positive to negative and at right angles to the surface of the
charged object.
The closer together the field lines
are, the stronger the electric field - you can tell from the diagram the
field is strongest the closer to the centre of charge you get.
The diagram above forms the basis for showing
the electrical field around an object for an isolated positively charge object
and an isolated negatively charged object.
By convention the field lines run
from positive to negative as indicated by the added arrowheads.
Electrostatic force field diagrams when
charges interact
The above diagrams show the maps of field
lines for electrically charged objects when you get:
Note the electrostatic charge convention that
the field direction is from positive to negative.
(i)
ATTRACTION: when two oppositely
charged objects approach each other (+ve and -ve)
Here the field lines join up between
the two charged particles.
If the charged objects are free to
move towards each other, the field lines will strengthen, become closer together, as the
attractive force increases.
(ii)
REPULSION: when two objects of the
same charge approach each other (two +ve or two -ve).
Here the field lines do not join up,
but are pushed away from each other.
If the charged objects are free to
move apart, the field lines will weaken as they become further apart, and the attractive
force decreases.
The closer the field lines are together
the stronger the electrical field effect.
This happens as you get closer to any
charged object OR if the charge is increased, which also increases the
electric field strength.
You can see this as the field lines get
closer to the source of the static charge.
See also the three rules in the previous
section.
Note: The greater the potential
difference between the charges, and the closer the charge fields are, the
more likely you are to see the spontaneous discharge of the electrical
energy and a visible spark.
Parallel charged plates
The electric field between parallel plates
The electric field between two oppositely
charged parallel plates is quite uniform with all the field lines parallel to
each other and all exactly at right angles to the surfaces of the plates.
Therefore the strength of the electric field is the same at any point between
the plates and will only differ at the ends of the plates where the field lines
become curved.
INDEX physics notes on static
electricity - electrostatics & electric fields
Keywords, phrases and learning objectives
on static electricity
Be able to describe and explain what is an electric field of static electricity
is.
Be able to describe 'maps' of the field around an
static electric charge e.g. between two spheres or two plates.
Know the rules of repulsion and attraction of static charges
e.g. what happens when two charged objects interact.
Know the rules on force of repulsion or attraction
in terms of the size of
the static charge and the distance between the centres of the two
statically charged objects.
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INDEX physics notes on static
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