Science Puzzle
Why Does a Rubbed Balloon Stick to the Wall?
Rub a balloon on your hair or on a woolly jumper, hold it against a wall and let go. It stays there, sometimes for hours on a dry day. Your hair might stand on end too.
The wall isn't sticky and there are no magnets involved. What holds the balloon in place?
The Answer
Static electricity. Rubbing gives the balloon an electric charge, and that charge pulls on the wall even though the wall itself has no overall charge.
Everything is made of atoms, which contain positively charged protons and negatively charged electrons. When rubber rubs against hair, some electrons are scraped off the hair and onto the balloon. The balloon ends up with extra electrons, a negative charge; your hair is left positive. Each hair now carries the same charge as its neighbours, and like charges push apart, so they stand up and spread out.
The wall starts neutral, but its charges can shift a tiny amount. Near the balloon, the negative charge pushes the wall's electrons slightly away, leaving the wall's surface a little positive. Opposite charges attract and the force gets stronger the closer they are, so the pull from the nearby positive charge wins over the push from the slightly further negative charge. The balloon clings on.
Water in the air slowly carries the extra charge away, which is why the trick works best on a dry winter's day and may barely work on a damp one. The same effect makes clothes cling together in a tumble dryer and lets a charged comb bend a thin stream of water from a tap.
The principle: Static electricity and induced charge. A charged object can attract a neutral one by nudging the charges inside it, so opposite charges sit closer than like charges.