MYP Physics 3.1 Static Electricity Notes

What Is Static Electricity?

1. Static electricity is the set of effects you observe when electric charge builds up on objects and then stays in place (instead of flowing continuously as a current).
2. It explains everyday experiences such as a small shock after walking on a carpet, sparks when removing a synthetic sweater, or a balloon sticking to a wall.
3. Static electricity is usually noticed when objects attract, repel, or suddenly discharge charge.

Static Electricity Is Charge Imbalance, Not "Extra Electricity"

1. All matter is made of atoms containing positively charged protons and negatively charged electrons.
2. Most objects are overall neutral, meaning they have equal total positive and negative charge.
3. Static electricity occurs when this balance changes locally: one object ends up with an excess of electrons (becoming negatively charged) and another ends up with a deficit of electrons (becoming positively charged).
4. Importantly, the object that gains electrons and the object that loses electrons gain equal and opposite charge because each transferred electron leaves one surface and arrives at the other.

Rubbing Materials Transfers Electrons

1. Many static effects start when two different materials come into contact and separate, especially when they are rubbed together.
2. At the surfaces, some electrons can be pulled away from atoms and transferred to the other material.
3. Rubbing increases the interaction between surfaces, so it usually increases the number of electrons transferred.
4. A classic example is rubbing a balloon on hair or a sweater:
   1. The balloon typically gains electrons and becomes negative.
   2. Hair or wool typically loses electrons and becomes positive.

Why Your Hair Stands Up

1. After rubbing a balloon on hair, each hair strand can be left with a small positive charge.
2. Because like charges repel, the hairs push away from each other, so they separate and can stand up.

The Triboelectric Series Predicts Which Material Becomes Positive Or Negative

1. Different materials have different tendencies to gain or lose electrons when in contact.
2. The triboelectric series ranks materials by how strongly they tend to take electrons from another material.
   1. Materials that easily lose electrons tend to become positively charged.
   2. Materials that easily gain electrons tend to become negatively charged.

Conductors Versus Insulators Matter

1. In a conductor (for example, metals), electrons can move easily.
   1. If charge is transferred, it can spread out and may quickly flow away, especially if there is a path to ground.
2. In an insulator (for example, many plastics and rubbers), electrons do not move easily through the material.
   1. Any transferred electrons tend to stay where they land, so charge can build up on the surface.

Charged Objects Exert Electrostatic Forces

1. Once objects are charged, they interact through the electrostatic force:
   1. Unlike charges attract (positive and negative).
   2. Like charges repel (positive with positive, negative with negative).
2. The force becomes stronger when:
   1. objects are closer together, and
   2. objects carry more charge

Charged Objects Can Attract Neutral Objects By Induction

1. It can feel puzzling that a charged balloon can stick to a neutral ceiling or wall.
2. The explanation is electrostatic induction (often described at this level as polarization): a nearby charged object causes charges inside a neutral object to shift slightly.
3. For a negatively charged balloon near a neutral wall:
   1. Electrons in the wall are repelled slightly away from the balloon.
   2. The side of the wall closest to the balloon becomes slightly positive (because electrons have shifted away).
   3. The balloon is attracted to this closer positive region.
4. Even if the wall is not a good conductor, electrons can still move a small distance within atoms and between nearby atoms, enough to create a slight separation of charge.

Sparks And Shocks Are Rapid Discharges Of Built-Up Charge

1. When you feel a small shock after walking across a carpet, you have typically built up a charge by repeated contact and separation between the shoes and the carpet.
2. When you touch a metal door handle, the charge can suddenly move, producing:
   1. a brief current through the air (or through you),
   2. heating and light in the air, sometimes visible as a tiny spark.
3. Dry air increases these effects because moisture on surfaces can provide a leakage path for charge to slowly escape, reducing build-up.

Van de Graaff Generators Build Up Large Static Charges

1. Electrostatic generators are devices designed to build up static charge.
2. A Van de Graaff generator is a device designed to produce a large static charge. A moving belt transfers charge from one region to another.
   1. A rubber belt runs over two rollers (for example, a nylon roller at the bottom and a polythene roller at the top).
   2. As the belt moves, it becomes charged by contact with the rollers and carries charge upward.
   3. At the top, charge is transferred to the upper roller and then to a metal dome.
   4. Because the dome is a conductor, electrons spread out and accumulate on the outside surface.

What Is Grounding (Earthing)?

1. When a charged object is connected to the Earth, excess charge can flow away.
2. The Earth acts as a very large reservoir of charge.
3. Grounding prevents dangerous build-ups of static charge.