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>> In this video,
we're going to take
a Closer Look at Friction,
Conduction and Induction.
We're going to take a
quantitative look at friction,
conduction and induction
to solve problems
involving charge transfer.
When we talk about friction,
we're talking about one object
rubbing against another.
When the objects are made
of different materials,
one of them will usually be
more likely to take electrons
than the other.
We figure out which material
is more likely to take electrons
by looking at the
Triboelectric Series.
It's a table of materials
that are ranked by how easily
they become charged by friction.
The more charge affinity
a material has,
the more likely it is
to steal electrons when rubbed
against another material.
Think about these three tribo
or rubbing events.
In the first example,
we take neutral pieces
of wool and rubber
and rub them together.
In the second example,
we rub neutral rubber
on a nylon balloon.
Finally,
we rub a neutral nylon balloon
on our hair.
Which materials
do electrons move to
in these friction events?
Electrons tend to go to
whichever material has the
strongest affinity for them.
So when we rub wool
and rubber together,
wool accepts electrons
more easily and takes on
a negative charge.
The Law of Conservation of
Charge tells us that charge
cannot be created or destroyed.
It is just transferred
from one place to another.
This means the rubber ends up
with the same amount of excess
positive charge.
When neutral rubber
and nylon come in contact,
since nylon is higher up
on the list of electron
affinity, it takes electrons
from the rubber.
And, lastly,
rubbing a neutral balloon
on our hair gives our hair
extra negative charge.
Electrons are more drawn
to hair than to nylon.
So you see, we can use
the Triboelectric Series,
this table, to find out
how charges move
when two different materials
are rubbed together.
We know that electric charge
can move just from two objects
coming in contact by conduction.
Conduction between two objects
happens when their charges are
unbalanced and can move easily
from one object to the other.
Specifically, if two conductors,
like these metals of
unequal charge, touch,
electrons will move to balance
out the charge on each metal.
And at any point throughout
the charge transfer,
we always have the same number
of protons and electrons
that we started with.
In other words,
electric charge is conserved.
Here's a practice problem
to test our understanding
about conduction.
Two identical metal spheres
rest on insulating platforms
on a tabletop.
One metal sphere
is electrically neutral.
The other sphere is given
a positive net charge
of 6.4 times 10
to the negative 2 coulombs.
The spheres are brought
into contact with each other.
As a result,
the first metal sphere
becomes charged positively.
What is the charge on the sphere
that started out neutral?
And which of the following
occurred during the process?
Circle or write down
all that apply.
Let's start with the second part
of the problem.
What actions occurred in
the charge transfer process?
Remember how metals
are constructed.
Their atom's nuclei,
where their protons are,
are arranged in a fixed pattern
and held tightly in place.
While each metal atom
in the structure
has an electron
that can move freely
throughout the material.
It's not protons that do
the moving, but electrons.
So, 'A' is out.
'B' is correct,
because the sphere that
starts out charged
is charged positively.
That means negative charged
electrons will flow from
the uncharged sphere
to the charged one
to balance out the charge.
'C' is another way
of saying 'B'.
It's correct.
'D' is true.
Though electrons flow
from one sphere to the other,
the overall electric charge
is always conserved.
'E' assumes protons in metals
are mobile.
But we've seen that the way
atoms are arranged in metals
means they are not.
'F' gets the part about
electron transfer right,
but electrons have a negative
and not a positive charge.
Overall, this statement
is not correct.
Now that we have an idea
how charge moved,
what about the final charge
of the sphere
that started out neutral?
Since the two middle spheres
are identical,
we know that once they touch,
electrons will rearrange
until both spheres
have an equal charge.
And since the charge
is conserved,
we know the total charge of
the two spheres at the end
will equal the total charge
they had at the start.
So when the spheres touch,
the positive charge of 6.4 times
10 to the negative 2 coulombs
will be spread evenly
across them.
Electrons will flow out
from the neutral sphere,
until both spheres have
an equal positive charge.
The charge of each sphere
is half the original charge
of the system,
or positive 3.2 times 10
to the negative 2 coulombs.
Another way a conductor
can become charged is by
the process of induction.
Induction happens when a charged
object is brought near,
but doesn't touch a conductor.
Because of attraction between
opposite charges and repulsion
between like charges,
the electrons in a conductor
will move around when a charged
object comes near.
The charged object will create
an electric field that pushes
or pulls the electrons
within the conductor.
Try out this example.
Two uncharged metal spheres
of insulating stands
are in contact with one another.
A negatively charged rod is
brought near the left sphere,
but does not touch it.
With the rod held in place,
the right sphere is moved.
So it no longer touches
the other sphere.
Finally,
the rod is moved far away
from the spheres.
Sketch the spheres on a piece
of paper for each of the stages,
one through four.
At each step in the diagram,
draw in the charge distribution
of each of the metal spheres.
Use plus signs for positive
charge and minus signs
for negative charge.
At each stage,
show what type of charge exists
and where it is located.
Go ahead.
Press pause.
Give it a try.
And when you think
you have the answer,
come back and check your work.
Did your diagram end up
looking like this?
If so, great job.
If not, let's solve it together.
In our first snapshot,
we have two neutral metal
spheres in contact.
Since neither
has a charge imbalance,
there's no net charge
on either one.
A net charge of zero
means there's an equal number
of protons and electrons
on both spheres to draw in
on the diagram.
In the second scene,
we've brought
a negatively charged rod
near the left sphere.
Since the spheres
are conductors,
electrons can flow easily.
And a response to the
electric field the negatively
charged rod makes,
electrons flee as far away
as they can.
Because the spheres
are in contact.
That [indistinct] distance
is the right side
of the right sphere.
So the charged rod
induces a charge imbalance.
Electrons are pushed
to the far right,
leaving a net positive charge
on the far left.
That's what the plus and minus
signs in the diagram mean.
At step three,
the right sphere has been moved
out of contact
with the left sphere.
We now have two disconnected
systems instead of
one connected one.
And now that they are separated,
the spheres have equal
and opposite charges.
If we add up their charges,
we get zero,
just like at the beginning when
we started with zero net charge.
That's the Law of Conservation
of Charge in action.
The rod continues to draw
the positive charges
on the left sphere.
Those charges come as close
as they can to the
oppositely charged rod.
The right sphere, which,
like we said,
now has an overall charge,
wants to spread that charge
over its entire surface.
The electrons repel each other
as far as they can,
spreading out over the surface
of the sphere.
They are still affected by
the field created by the rod,
but their self repulsion
is now a strong effect.
Finally, in frame four,
we removed the charged rod
from the picture.
The left sphere
has a positive net charge,
and those charges repel
each other and spread out
over the sphere.
The negative charges on the
right sphere will be affected
only slightly by the removal of
the rod and the redistribution
of positive charge
on the left sphere.
We started with two uncharged
metal spheres,
and induced an overall positive
charge on one and a negative
charge on the other,
without ever touching
our charged object,
the rod, to the spheres.
That's a Closer Look
at Friction,
Conduction and Induction.
For more "Physics in Motion",
go to our homepage,
where the entire series
is available to you.