Figure one shows a polythene rod
and a cloth duster. At first, neither object has a net
electric charge. If the cloth duster is rubbed
against the polythene rod, the polythene rod will accumulate a net negative electric
charge and the cloth duster will become positively charged. Explain how an uncharged object may
become negatively charged.
Okay, so the question is talking
about a generic object here. We’ve been asked to explain how an
uncharged object may become negatively charged. The question doesn’t actually tell
us what object we’re talking about specifically. But we can use the example of the
polythene rod and the cloth duster to help us answer the question, because, in this
case, we’ve been told that if the cloth duster is rubbed against the polythene rod,
then the polythene rod will accumulate a net negative electric charge and the cloth
duster will become positively charged.
This happens even though, at first,
neither object has a net electric charge. In other words, initially, both of
these objects are neutral. They’re neither positively charged
nor negatively charged. But then when we rub them together,
one has a net negative electric charge and the other has a net positive charge.
So let’s consider the situation
before we rub these objects together. Well, both of these objects are
made up of atoms. Now specifically, these atoms can
be thought of as a whole bunch of positively charged ions that cannot move around
and a sea of negatively charged electrons that are free to move around.
Now when these objects are neutral
— they’re not positively charged or negatively charged — then the total negative
charge on all of the electrons cancels out the total positive charge on all of the
ions. And remember, this is true for both
the polythene rod and the cloth duster. In each item, the positive charges
cancel out the negative charges.
However, when we rub the two
objects together, there is a transfer of negatively charged electrons from one
object to the other. In this case, we’ve been told that
the polythene rod accumulates a net negative electric charge. Therefore, what’s happening here is
when we rub the two objects together, some of the electrons from the cloth duster
are transferred to the polythene rod. This means that the cloth duster is
now left with fewer electrons than it had before. And conversely, the polythene rod
now has more electrons than it had before.
In other words, in the cloth
duster, the total charge on the positive ions is now larger than the total negative
charge on the electrons that remain, because there are fewer electrons remaining,
whereas in the polythene rod, the situation is the opposite. The polythene rod now has a lot
more electrons than it had before, which means that it has a net negative
Now it’s important to remember that
this transfer of charge is because of the movement of electrons. The positively charged ions cannot
move anywhere. They’re stuck in place. And so when it comes to explaining
how an uncharged object may become negatively charged, we can say that negatively
charged electrons are transferred to the object, resulting in an accumulation of a
net negative charge. This is how an uncharged object, in
this case the polythene rod, becomes negatively charged.
Figure two shows a metal sphere
that has a net negative electric charge. Draw the lines of the electric
field around the metal sphere when it is isolated from its surroundings. Use arrows to show the direction of
the electric field.
Okay, so in this part of the
question, we’ve been told that we’ve got a negatively charged metal sphere. And we’ve been asked to draw the
electric field around this metal sphere when it is isolated from its
surroundings. In other words, we don’t need to
worry about any other electric fields. If it’s isolated from its
surroundings, then only the sphere’s electric field is what we’re going to have to
So first things first, let’s give
ourselves a little bit of space to work with. And let’s start considering what we
need to think about in order to draw the electric field that we’ve been asked to
So first of all, we’ve been told
that the sphere that we have is negatively charged. This is important because we can
recall that electric field lines start on positive charges and end on negative
charges. This means that electric field
lines point away from positive charges and point towards negative charges. So whatever our field pattern is
going to look like, we now know that the field lines are going to be pointing
towards the negatively charged metal sphere, because it’s negatively charged.
Secondly, we can recall that a
charged sphere has the same electric field as a point charge outside the sphere. What this means is that if we have
a point charge — well, let’s say, in this case, it’s a negative point charge because
we’ve got a negatively charged sphere that we’re talking about — then this negative
point charge has the same electric field lines as this negatively charged sphere,
except this is only true on the outside of the sphere, not inside the sphere.
So next, we can recall what the
electric field of a negatively charged particle looks like. Well, for a negatively charged
particle, the electric field lines point towards the particle, because, remember,
electric field lines end on negative charges. More specifically, the field lines
for a charged particle point towards the particle in a radial direction. What this means is that the field
lines look like the following diagram. The field lines are all pointing
inwards from outside towards the particle.
Now let’s say that, instead of this
particle, we have a negatively charged sphere. Let’s say that this is the
sphere. What we know that, outside the
sphere, the electric field is exactly the same as for the charged particle. And that’s exactly what this
diagram is showing. The electric field lines end at the
surface of the sphere. Inside the negatively charged
sphere, there is no electric field. And also, rather interestingly, the
reason that the electric field lines are known to be radio is because each field
line is a continuation of the radius of the sphere.
Now the radius can be measured by
starting at the center of the sphere and moving outwards until you get to a point on
the surface of the sphere. And this line is the radius of the
sphere. Well, these electric field lines
that we’ve drawn lie along the same line as each radius of the sphere, at which
point we’ve basically answered the question. All that remains is to draw the
field lines on figure two.
So here’s the electric field of the
negatively charged metal sphere. Each line ends at the surface of
the sphere and is a continuation of the shortest line that goes from the surface of
the sphere to the center of the sphere, in other words a radius of the sphere. Also, each line is pointing towards
the sphere because it’s negatively charged, and so this is the electric field
produced by the negatively charged metal sphere.
So having done this, let’s move on
to the final part of the question. Another negatively charged object
is placed in the electric field. Look at figure three. In which position would the object
experience the greatest force? Tick one box: P, R, S, T.
Okay, so here we’ve got the same
negatively charged sphere as the previous part of the question. And what we’re doing is we’re
placing another negatively charged object in positions P, R, S, or T. We’ve been asked to state the
position in which the object would experience the greatest force.
To answer this question, we once
again need to look at the electric field from the negatively charged sphere. So here is the electric field from
before. Now here’s the reason why we need
to consider the electric field in the first place. If we take a charged object and
place it into an electric field, then naturally the electric field will exert a
force on the charged object.
And here’s the important point. Let’s say we’ve got some other
electric field that looks something like this. Here, we can see an electric field
where all the field lines are parallel to each other. But in this region of the field,
the field lines are far apart from each other, whereas in this region, the field
lines are close together. And in fact, as we go from here to
here, the field lines are getting closer and closer to each other.
Well, here’s what we need to
know. In a region where electric field
lines are close together, the force exerted on a charged particle in that electric
field is a large force. So in this case, let’s say we’ve
placed a negatively charged particle in the electric field. Well, a negatively charged particle
moves against the direction of the electric field. So the force on the negatively
charged particle due to the electric field is going to be in this direction, against
the direction of the electric field.
But the important thing is that the
force is going to be a very large force. This is because, in this region,
the electric field lines are very close together. However, if we were to take that
same charged particle and place it over here, for example, then again it would
experience a force in the opposite direction to the electric field. But the strength of that force
would be very weak. This is because the electric field
lines in this region are very far apart from each other.
So basically, here’s the key
point. If we place a charged particle in a
region of an electric field where the field lines are far apart from each other,
then the force on that particle is going to be small. However, if we place that same
charge in a region of the electric field where the field lines are close together,
then the force on that particle is going to be large. The closer the field lines are
together, the larger the force the electric field exerts on a particle in that
So coming back to our question
then, we need to find the position out of positions P, R, S, and T where the
electric field lines are closest together. Well, we can see that as we come
nearer and nearer to the sphere, the electric field lines are getting closer and
Let’s say, for example, we were in
this position here. Well, the electric field lines are
quite far apart from each other. However, in this position here, the
electric field lines are slightly closer to each other. And in this position here, they’re
even closer to each other. This means that the force exerted
by the electric field is weakest here, slightly stronger here, and the strongest
here. In other words, as we get closer to
the sphere, the force exerted gets larger and larger.
So essentially, we just need to
find the point that’s closest to the surface of the sphere. And that point happens to be point
R. Hence, the answer to our question
is position R. When we place a negatively charged
object in the electric field of the negatively charged sphere, it will experience
the greatest force at position R out of the four positions given to us.