CH - 15
Why Does a Balloon Stick to a Wall?
Part of the magic of a
birthday party, back
before you were ten, was
making a balloon stick to
the wall. You rubbed it on
your head and then you
stuck it on the wall. Before
you rubbed the balloon, it
would not stick; after you
rubbed it, it stuck. Magic!
The source of this “magic”
lies in electrical forces.
You may have seen a
demonstration of a rubber
rod being rubbed on a cat’s fur, and then being
brought near a pith ball (a light-weight, nonconducting ball). Initially, the pith ball is attracted
by the rod, and then it is repelled. The initial force
of this attraction is caused by electrostatic
induction. Balloons stick to the wall because of a
similar force.
The initial attraction of the pith ball seems a bit
strange – a neutral object, the pith ball, is
attracted to a charged object. The electrical force
is given as:
F k
Q1Q2
r2
where F is the electric force, k is a constant which
is 9 x 109N m2/C2; Q1 and Q2 are charges in
coulombs, and r is the distance between the 2
bodies in meters. The equation indicates that if
the charge on one object is zero, the force is zero.
If the pith ball has no charge, why is there any
attraction? Attraction occurs
because charges are “induced,”
or forced to move from their
normal positions. This
movement is caused by an
electric field which exerts a
force on a charged particle.
A rod rubbed on cat’s fur has a
negative charge. The negative
charge in the pith ball will tend
to be repelled from the rod and
to move away.
Figure 1.
(On the atomic level, the atoms do not actually lose
electrons, but the
electrons spend more
time on the side
of the atom away from
the side towards the
rod and a negative
charge on the side
away from the
charged rod. This
motion happens
throughout the pith
ball, so the result is
a positive charge on
Figure 2.
the side towards the rod and a negative charge on
the side away from the rod. The motion of charges in
the pith ball is caused – or induced – by the electric
field on the rubber rod; this motion is called
induction.
The electric force equation indicates that less force is
exerted when charges are farther apart. Because the
positive charge is closer to the rod than the negative
charge, the force of attraction is greater than the
force of repulsion, so the net force is attractive. The
ball moves towards the rod even though the ball has
no net charge. Generally, when then pith ball
touches the rod, a negative charge is transferred, or
conducted, from the rod to the pith ball. The pith ball
will become negatively charged. Like charges will
repel each other, so after the pith ball hits, the ball
will then be repelled from the rod.
A balloon sticking on a wall is also caused by
induction. The balloon is made from rubber and it is
rubbed on human hair instead of cat’s fur. The
balloon gains negative charge. When it is brought
near the wall, the charges in the wall will be induced
to move. Negative charge moves away from the
balloon leaving positive charge closer to the balloon.
The attractive force is larger than the repulsive force,
so there is a net attractive force, and the balloon
sticks to the wall. The attractive force between the
balloon and the wall is exerted perpendicular to the
wall. Gravity pulls straight down, so the balloon
would accelerate straight down were it not for an
opposing force. Friction provides that opposing force.
Questions and Problems
The force of friction is given as:
F   fn
where F is the force of friction in newtons, µ is the
coefficient of friction, and f n is the normal force.
The normal force is the force between the
surfaces. This is the attractive force caused by
induction. The coefficient of friction between
rubber and most surfaces is quite high (larger
than 1), so friction will easily oppose gravity.
1. A negatively charged rod will create an
electric field. What does this field do to the
charges in a neutral pith ball?
2. How does the electric field in problem 1
cause a net attraction in the pith ball?
3. After the pith ball is attracted to the rod in
problem 1, it is then repelled. Why does this
repulsion occur?
4. Why is a neutral pith ball which is attracted to
a charged rod similar to a balloon sticking to
a wall?
5. What force balances gravity as the balloon
sticks to a wall?
Figure 3.
Induction can also be observed with a plastic or
rubber comb. If a clean comb is run through clean
hair, it will become negatively charged (it can also
be rubbed on a wool sweater which is sheep’s
fleece). Brought near a small stream of water
falling from a faucet, the water drops will be
affected quite dramatically. Try this experiment
and see what happens. You must keep the comb
dry to get the best results.
6. Induction also causes plastic wrap to stick to
dry surfaces, although it will not stick to wet
ones. Explain why this is so.
7. A small stream of water will be broken and
small drops will fly off it when a charged
object like a comb is brought near it. Some
drops are attracted and others are repelled.
Why does this occur?
8. When you bring a comb that has been
rubbed through your hair near a small stream
of water, about how far can you bend the
stream of water?
9. In problem 8, you bent a stream of water
about 14 centimeters. Estimate the size of
the force required to cause this bending.