Video Transcript
A polythene rod is rubbed with
a cloth duster, giving it a net negative charge. A second polythene rod is
rubbed with a duster and the two rods are brought close together. What will happen?
So here we have a polythene rod
— that’s a plastic rod — and a cloth duster. And at the outset, we can
assume that both of these objects are electrically neutral; that is, they have
no net electric charge. It’s important to realize
though that this doesn’t mean that the rod and the cloth don’t have positive and
negative charges in them. All it means is that they
balance one another out. In each object, there are the
same number of positive charges as negative charges. So on a net or overall level,
they’re neutral.
Okay, we’re then told that the
rod and cloth are brought in contact and they’re rubbed together. And once that has done, we’re
told that overall the rod now has a net negative electric charge. What is that mean? That means that while the cloth
and rod were rubbed together, electrons were moving from the cloth onto the
rod. Since electrons have a negative
electric charge, when they build up on the rod they give it a net negative
charge as well. This also means by the way that
after this interaction, the cloth has a net positive charge. That’s because it’s lost some
electrons to the rod.
But anyway, our focus is on
this negatively charged polythene rod. We’re told that with a second
originally neutral polythene rod and a second originally neutral cloth duster, a
similar process occurs. The two are rubbed
together. And we know that because this
is the same type of charge transfer process as before, this second polythene rod
will also end up with a net negative charge. We’re then told that these two
rods are brought close together. And the question is what will
happen.
We can see that both of these
objects have a net negative electric charge. In other words, they have the
same charge, in this case negative. We can recall that when it
comes to electric charges interacting together, opposite charges attract and
like charges repel one another. We know that both our polythene
rods have a net negative electric charge. In other words, they both have
like charges.
That tells us that the rods
will experience a repulsive force. They’ll try to push one another
apart when we try to bring them together. That then is our answer to the
question of what will happen when these rods are brought close together. We can say that the two
polythene rods will repel each other.
