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.