Static Electricity

Layout of an Atom

An atom has a center, called the nucleus, which is made of protons and neutrons Protons are positive Neutrons are neutral

Electrons are flying around the nucleus like planets fly around the sun Electrons are negative Electrons are so small, they have almost no mass

Electric Charge in the Atom

Atom:

Nucleus (small, massive, positive charge)

Electron cloud (large, very low density, negative charge)

Attractions

Opposite charges attract

Like charges repel

The positive nucleus attracts the negative electrons, keeping them from flying off the atom

Forces

Electricity is a force, just like gravity

Gravity F=G(m1m2/d2) F is force of gravity G is gravitational constant m1 and m2 are the masses of the two objects d is distance Force has an inverse square relationship to

distance.

Coulomb’s Law determines the force of attraction or repulsion between two charged objects F=k(q1q2/d2) F is force of attraction/repulsion k is a constant q1 and q2 are the charges of the two objects d is distance Force has an inverse square relationship to

distance.

In evaluating forces in space, gravity is the dominating force because the objects (planets, stars, etc.) have so much mass

Forces on the Earth are both gravitational and electric

When looking at atoms or electrons, the electrical force is dominate because the masses are so small (especially for electrons)

Generating Static

A static charge is generated as electrons are rubbed off one object and collected on a second

This separation of charge creates a collection, or build up, of electrons on one object giving it a negative charge

Objects with higher electron affinity want electrons and pull them from objects with low electron affinity

Hairy Idea

When your body generates a static charge, you feel it – your hair feels like it stands up (because it does)As you generate a charge, all of the parts of your body have the same charge and therefore repel each otherYour arms have too much mass to be moved by this repulsion, but your hair has a small mass and the charge can move itSince all the hair has the same charge, it all repels each other and your scalp – your hair stands up!!

You can gain a negative charge in your body via a Van de Graaf generatorYou can gain a positive charge by using a plastic comb in your hairPlastic has a higher electron affinity than

your hair, so the comb takes electrons from your hair while combing

Keeping a comb wet while combing keeps down static and prevents a “bad hair” day

Static Discharge

When a negatively charged object contacts a neutral or positively charge object, some or all of the negative charge can be transferred to the 2nd object - THIS IS STATIC DISCHARGE

This is why you get a shock when you touch a door knob on a winter day – you build up a charge while walking and release the charge into the door knob when touching it

Transferring part of the charge into the 2nd object simply makes the 2nd object negatively charged as wellIf you rub one balloon on your sweater, then touch it to a 2nd balloon, both balloons will be negatively chargedThese balloons will repel each otherThey will both stick to a wall

Induced Charge; Conduction

Metal objects can be charged by conduction:

Induction

Induction is the charging of an object without direct contact.EXAMPLE: 1) The negative charge of a “rubbed” balloon pushes some of the electrons in the wall farther back (likes repel)2) This creates a positive charge at the front of the wall3) The negative balloon and the positive wall attract and the balloon sticks to the wall4) When the balloon is removed from the wall, the charge on the wall “disappears” as there is no negative charge to push the wall electrons further back5) An induced charge is temporary

Induced Charge; Induction

They can also be charged by induction:

Insulators and Conductors

Conductor:

Charge flows freely

Metals

Insulator:

Almost no charge flows

Most other materials

Some materials are semiconductors.