Properties of Electric field lines - Electric Charges & Fields Video Lecture - Class 12

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charges and fields part 21 is brought to
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please make sure that you have watched
all the videos still part 20 before
going ahead with part 21 so now that we
have talked about electric field
intensity how to calculate electric
fields at a particular point now it is
time to talk about the direction of
electric field or how do we represent
electric field with a pictorial
representation there comes the concept
of electric field lines these are a set
of lines they you do not see those lines
in real life they are all imaginary
concept but these are certain lines
which tells us about the nature of
electric field it tells us how what is
the strength I mean it gives us a rough
idea of the strength of the electric
field it tells us a very precise
information about the direction of the
electric field so let us have a look at
electric field lines
so what electrophoresis
representing electric field intensity so
note here they are imaginary lines so
they are all abstract concept they
represent the electric field intensity
directed radially outwards from positive
charge so electric field lines will
always originate from a positive charge
and they will always terminate to a
negative charge so it is always outwards
from a positive charge and inwards for a
negative charge so it will always start
from positive and end in negative for
example let us suppose we have of some
positive charges if this is a positive
charge then the electric field lines
would be somewhat like this so this is
how the electric field lines would be so
it will be outward similarly if this is
a negative charge then the electric
field lines would be somewhat like this
okay
so this
we'll be somewhat like this so the
electric field lines are always outward
from positive charge and inwards to a
negative charge so they define the
direction of electric field basically so
electric field is always directed from
positive towards negative charge now if
we have two charges let us suppose if we
have two light charges in front of each
other this is a positive charge this is
also a positive charge in this case from
this also the electric field lines
originate from this charge also the
electric field lines originate
but both the lines will try to be away
from each other that is why they exert
some kind of force on each other and
they try to go away from each other so
your figure looks somewhat like this the
charges as the two charges come closure
their lines of force try to be straight
but due to repulsion they get curved so
this is what they look like similarly if
you have opposite charges let us suppose
this is a positive charge and this is a
negative charge so electric lines are
starting from here they are originating
instead of going straight otherwise if
this charge would have not been there
then it would have moved somewhat like
this right but since there is a negative
charge so it gets bent towards the
negative charge and it terminates in the
negative charge because lines of force
go inward to the negative charge so it
originates from positive charge and
terminates in negative charge right so
this is how the electric field lines
look like when you have two like charges
or two unlike charges in front of each
other now let us look at certain
properties of electric field lines so
the first property says that no two
field lines ever intersect that means
two field lines cannot intersect why is
it so just imagine if two field lines
intersect
let us suppose this is one field line
this is another field line and they are
intersecting what are these field lines
representing they are representing the
direction of electric field right so at
the point of intersection we need to see
that
there are two direction of electric
field right because the direction which
is represented by line one is also there
at the point of intersection the
direction represented by line two is
also there that means at the point of
intersection both the directions of the
electric field exists but is that
feasible practically can there be two
direction of electric field at one point
no right at one point of time the
electric field can point to one
direction only therefore no two field
lines can ever intersect okay so the
next property says field lines are
always perpendicular to the conductor
let us suppose this is my conductor this
is a positive conductor so the way the
field lines are originating from the
conductor they will always be
perpendicular to the surface of the
conductor at each point the field line
will be perpendicular to the conductor
similarly when the field line terminates
to a conductor let us suppose if this is
a negative ly charged body so when the
field lines come inside this that time
also the field line will be
perpendicular to the surface of the
conductor number of field lines is
directly proportional to the amount of
charge that means let us suppose if I
represent first scenario with these
field lines this represents some charge
q1 now if the field lines are more more
number of field lines that would mean
that the charge Q 2 is greater than the
charge Q 1 that means more number of
lines means more amount of charge so
number of lines is directly proportional
to the amount of charge field lines
contract longitudinally during
attraction between life charges as I
told you let us suppose this is a
negative charge this is a positive
charge so it originates from the
positive charge and tell me
the negative charge so basically the
felines are contracting because if this
negative charge would have not been
there the field lines would have moved
like this but instead of that it
contracted because of the attraction due
to the negative charge right so they
contract longitudinally in case of
attraction with unlike charges similarly
they exert lateral pressure during
repulsion between life charges so when
they are life charges they repel each
other because of a lateral pressure
because they exert lateral pressure on
each other and that is why there is a
lateral pressure during repulsion
between like charges so these are some
of the properties of electric field
lines now when when we talk about
exerting lateral pressure in case of
light charges there is a point if both
the charges are equal there is a point
which is exactly the midpoint between du
between these two charges where the
electric field intensity is zero because
it completely cancels out the electric
field intensity due to one charge gets
completely canceled out due to the other
charge so this point is the point where
the electric field intensity is zero now
if the two charges are similar like
charges but the magnitude of one is
larger than the magnitude of other then
what will be the position of this point
this point is often known as the neutral
point so this point is known as neutral
point by neutral because the electric
field intensity at this point is zero
now in case both the charges are not
equal in magnitude then this point will
still lie between these two points but
it will be closer to the smaller charge
that means if they are if Q 1 and Q 2
are unequal that means they are like
charges they are still of the same sign
but they are not equal in magnitude then
neutral point will be closer to
smaller charge so it will lie on the
line joining the two charges but it will
be closer to the smaller charge now we
will talk about uniform and non-uniform
electric field how can we see by looking
at the electric field lines whether the
electric field is uniform or non-uniform
what do you before that what do you mean
by uniform electric field the electric
field whose magnitude is same throughout
the electric field remains the same
throughout the region similarly
non-uniform in somewhere the strength is
more somewhere the strength is less that
is known as non-uniform electric field
so basically uniform electric field will
be represented somewhat like this that
is the lines will be equally spaced the
space between the lines will be equal
that means the electric field is uniform
but when the spacing between the line
however the lines will still remain
parallel but the spacing will not be the
same somewhere it will be very close to
each other somewhere it will be very far
away from each other so that will be
non-uniform electric field so this is
uniform and this is non-uniform
similarly in case of curved paths also
if it is uniform then it will be equally
spaced so for uniform it has to be
equally spaced if it is non-uniform it
will not be in be spaced somewhere it is
closer somewhere it is not so closer so
that will be non-uniform electric field
right so what all did you study the
field lines will always be parallel to
each other number of field lines is
directly proportional to the amount of
charge if the field lines are
equidistant from each other that
represents a uniform electric field
otherwise it is a non-uniform electric
field right and you also studied about
the lateral repulsion the lateral
pressure giving repulsion with light
charges and the contraction during
attract
with unlike charges so now that we have
studied about electric field completely
because electric field lines is a part
of electric field right so I think we
have covered almost everything that is
needed at your level about electric
field so it is time that we solve
certain problems on electric field so
let us have a look to the problem one
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