Physics 30: Unit B - Chapter 10.1 (THEORY)

Electrostatics

the study of static electricity

The Law of Charges

• LIKE charges REPEL

• OPPOSITES ATTRACT

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• Two types of charges: positive & negative

  • charge is SCALAR: “+” or “-” just communicates they are opposite types!

• Objects that have no overall charge are neutral

Law of Charges (pictures)

Ionized

assumed to mean an electron has been removed

Atomic Theory (all matter contains electric charge)

• all matter is made of atoms

• atoms are made of protons (+), electrons (-) and neutrons (0)

• protons and neutrons are fixed in the nucleus

• electrons are loosely held around the nucleus in energy levels

  • check formula sheet for charges and masses of each!

Neutral Charges DO contain charges

they just have equal amounts of protons (positive) and electrons (negative)!

Movement of Charges

• protons do NOT move within a solid → only the electrons can

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• free positive charges that are NOT CONFINED within a solid are of course capable of moving

• if electrostatic forces are strong enough, the entire object itself can be moved, bringing the positive charges with it.

Why can neutral objects be attracted and only attracted, never repelled to charged objects?

• bring (-) near → e- “flee” → leaving p+ closer

• bring (+) near → e- moves towards, e- closer

  • (like repel, opposites attract)

Charge

• charge of an object is determined by its excess (more) or deficit (fewer) of electrons compared to protons

  • about how many more/fewer electrons it has than protons

Conductors

materials that ALLOW charge to move freely

Insulators

materials that DO NOT allow charge to move freely

Semiconductors

materials that are:

• good conductors under certain conditions

• good insulators under other conditions

Superconductors

material that has zero electrical resistance

General Rule of Conductors and Insulators

• metals are good conductors

• non-metals are good insulators

  • the more freely that electrons are able to move through a material → the more conductive it is

Electroscope

device used to detect the presence of electrical charge

• does NOT indicate the type of change

Law of Conservation of Charge

• net charge of an isolated system is conserved

  • charge may be transferred, total charge stays constant

• any transfers of charge are due entirely to the exchange of electrons, protons dont transfer

Static Discharge

electrons can move spontaneously if the potential difference between two objects is great enough

Three Ways to Change an Objects Net Charge

1. Conduction

2. Friction

3. Induction

Conduction (conductors, gain like charges)

• When multiple charged objects are in contact, their electrons are free to move from one to the other.

• The electrons will begin to move to the object that is more attractive (or less repulsive) to them.

  • As this happens, the charges of the two objects “even out”

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• The objects the electrons are leaving is becoming more attractive/less repulsive to the electrons

  • By losing negative electrons, it is becoming more positive → (opposites attract)

• The object the electrons are going to is becoming less attractive/more repulsive to the electrons

  • By gaining negative electrons, it is becoming more negative → (like repel)

Friction (insulators, gain equal but opposite charges)

• Not all nuclei are equally attractive to the electrons bound to them - some hold on to their electrons more tightly than others.

• Rubbing objects “shakes up” the electrons

  • The energy put into the rubbing does work to rip off electrons from the nuclei.

  • They will then return to the one with a tighter hold on electrons.

• This results in one material “stealing” electrons from the other

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• The final charges of two neutral materials charged by friction will always end up with opposite charges!

  • One gains a negative electron

  • One loses a negative electron

Induction (gain opposite charges)

• charged object is brought near a neutral object

• the neutral object is grounded: connected to unlimited storage reservoir for charge

• electrons have a path to enter or leave the initially neutral object when the charged object is brought near

  • charged object (+) → electrons from the ground will be attracted and flood into the neutral object

  • charged object (-) → electrons in neutral object will be repelled and exit through the ground

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• ground must be removed before the charged object is taken away

  • cuts off path for electrons to enter/exit the neutral object (now charged)

  • if not removed, electrons will move through it and return the system into its initial charge distribution