When you rub part of a rubber balloon against wool, electrons will leave the wool, which is slightly conductive, and go onto the balloon. The rubber on the balloon is much less conductive (rubber is more of an insulator than wool), and the electrons will not readily leave the balloon. As a result, the balloon becomes negatively charged. Imagine you bring the negatively charged balloon up to a wall, and it sticks (This is possible! A quick internet search will yield many explanations and demonstrations. You can also try it yourself). Draw a force diagram for the balloon.

• The balloon is negatively charged.
• The balloon is stuck to the wall.

• The specific charge distribution on the balloon.
• The electric properties of the wall.
• The force diagram.

• The balloon is not very conductive, so the electrons from the wool are stuck where they are. We'll say they are distributed near the part of the balloon close to the wall.
• The wall is a perfect insulator, and is neutral.
• The balloon is motionless.
• The balloon is touching the wall at exactly one point.
• The wall is perfectly vertical (parallel to the gravitational force).
• The interaction between the wall and the extra negative charges on the balloon is much stronger than the interaction between the wall and any polarized atoms in the balloon. For this reason, we choose to ignore polarized atoms in the balloon. This seems reasonable, since a balloon will not stick to a wall without having rubbed the balloon with something that will transfer some of its electrons to the balloon.

• It will help to draw a diagram of the balloon and the wall and the interactions going on. This is shown below in the solution.
• As usual, we represent polarized atoms in an insulator with little ovals.
• We can represent the forces on the balloon with a force diagram. This is also given in the solution.

Based on our Approximations and Assumptions, we can draw the following diagram of the (motionless) situation: The atoms in the wall near to the balloon are polarized in this way because of the negative charge in the balloon. See the notes on Charges and Matter for more information on polarization.

We know the balloon is motionless, so air resistance is not a factor here, as it often is with balloons. Also, the net force is zero, so our forces must cancel out. We know we have a gravitational force from the earth, a normal force from the wall, and an attractive electric force from the wall. That is all! Notice that the electric force needs to point both to the left and upward in order for the net force to be zero. If you were to try this experiment out yourself, you may notice the balloon rolling slightly up and down (oscillating) before settling to a motionless state. As the balloon rolls, the charge distribution on the balloon moves with respect to the wall, which changes the direction of the electric force on the balloon from the wall. When the balloon settles, we know it has come to a place where the direction and magnitude of the electric force results in a net force of zero. A force diagram on the motionless balloon is shown below.