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# 1.1: Background Material

## Producing Static Charge

For this lab, you will need a means for producing large amounts of static charge. All of the most effective methods with common household items consist of cloth-rubbing insulating materials such as plastic, acrylic, rubber, and latex. Various materials can also be used as the rubbing cloth, including natural fibers like wool or cotton, as well as synthetic materials like polyester and nylon, or blends between both. Different rubbing materials may work better with certain insulating materials, and you may want to experiment to determine what works best.

Generally a rod-shaped insulator works best, both because it is easier to rub with cloth, and because it is easier to direct the effects of the static charge produced. Fast rubbing of a short piece of pvc piping with a wool or polyester sock is known to be particularly effective. A well-inflated latex balloon (which can be rubbed with just about any cloth, human hair, dog fur, etc.) also works well, though it is a little tougher to deliver the charge to a specific location than when using something rod-shaped.

## Removing Static Charge

In order to keep controls on your experiments, you may wish to remove charge from something you have already charged. There are a couple of useful methods for this. The first involves humidity. It turns out that water molecules in the air are quite effective at picking up static charge off surfaces, and carrying them away. This explains why, in colder northern climates, static discharges like shocks to fingers when touching doorknobs are more common in the winter, when the humidity is significantly lower. If you are doing this lab in a humid climate, you may find it challenging to keep an object charged for any significant period of time. Anyway, this does provide a means for removing charge – breathing hot breath on the object a few times often does the trick. Touching the object on a grounded metal like a water faucet can also help, though since the rod is an insulator, you will want to make contact with as many places on the rod as possible, as the charge will not flow across the insulator to a single point of contact.

## Controls

There are only a few things to keep in mind in order to keep your experiment under control. Remembering to neutralize charge between tests, as mentioned above is one of them. Another has to do with when the experiment turns to conductors. As these materials will allow charge to flow through them, a single point of contact is enough to discharge the entire object, so if you wish to keep a conductor charged, you will want to insulate it from its surroundings. Also, don't forget that your own body can collect and distribute charge. It is not a great conductor, but if in one part of the experiment you collected charge on your fingertips, this charge could have an adverse effect on a later part of the experiment.

Also keep in mind that after a cloth is used to rub the insulator, the cloth is charged (charge is conserved, so if there is net charge on the insulator, there is the opposite net charge on the cloth). If that cloth is reused, it may deposit charge and have the opposite of the desired effect. Humidity should help with this as well.