Charging and Discharging Objects

9.3 Charging by FrictionFriction is when two objects are rubbed together.

This is the main cause of static electricity.

When two objects are charged by friction (rubbed together), electrons move and:

- One object is positively charged (lost electrons)

- The other is negatively charged (got electrons)

+ + +- - -

+ + +- - -

+ + +- -

+ + +- - - -

POSITIVE NEGATIVE NEUTRAL NEUTRAL

Charging by friction happens because:

The positively charged nuclei of one of the objects pulls the electrons from the other object.

Remember!

Some types of matter have a stronger hold on their electrons than others.

We can see this in a diagram called:

The Electrostatic Series

This diagram tells us the charge of each object when they’rerubbed together.

Why Does It Happen?

Acetate Weak hold on electronsGlassWoolCat’s fur, human hairCalcium, leadSilkAluminum, zincCottonParaffin waxEbonitePlasticCarbon, copper, nickelRubberSulfurPlatinum, gold Strong hold on electrons

POSITIVE CHARGE

NEGATIVE CHARGE

Examples of Charging By Friction

Some examples we know:

- hair and balloon

- walking on a carpet

- clothes in a dryer

Other examples:

- Pumping gasoline through a hose at a gas station

- Air rushing over the body of a car or an airplane

9.5 Charging by Conduction (contact)

Objects don’t need to rub together to transfer a charge.

In conduction two objects just need to touch.

One object is already charged, while the other may or may not be.

Important:

The two objects must have different amounts of charge.

One object already has a charge.

If the other object has a different charge, then electrons will transfer from one object to another.

This creates the shock.

The objects don’t even need to touch. The electrons can jump across the gap. This creates a spark.

How It Works

- - - - - -

- - - - - -- - - - - -

- - - - - - - -

When you walk across the carpet you pick up electrons through the process of:

Friction.

This means you have a ______ charge.

The doorknob is usually uncharged.

Before your hand touches the doorknob, electrons jump creating a spark.

The doorknob shares your charge and is now negatively charged.

An Example of Conduction

Negative

9.8 Charging by InductionRemember that charges can be transferred by:

1. Rubbing two objects together (friction)

2. Touching two differently charged objects together (contact or conduction)

There is a third way to transfer a charge between objects:Induction

Induction means:Something happens without direct contact.

When an uncharged object comes close to a charged object, it can become charged even without touching.

Induced Charge Separation

Dust floating in the air is usually uncharged, meaning that it is neutral.

Electronic devices like computer monitors and TV screens have a charge.

When the dust floats close to the screen what happens?

The dust sticks. Why?

The charge on the screen makes the electrons in the dust change position. The side of the dust facing the screen now has an opposite charge from the screen.

These opposite charges attract the dust to the screen.

The charged object can have a positive or a negative charge.

A neutral object always has an opposite charge induced on the part closest to the charged object.

Dust

Dust

This charging effect is known as induced charge separation.

TV Screen

9.7 Discharging Electrically Charged Objects

When an object has the extra charges removed it is:

discharged or neutralized

All matter wants to be in a neutral state.

This means if an object has a positive or negative charge it wants to lose the charge and become neutral.

Methods for Discharging Charged Objects

Grounding

Grounding is a method for discharging an object safely.

An object is connected to a conductor that leads into the ground.

This grounded conductor takes the extra charge out of the object and into the ground where the charge is shared with the Earth.

Now when the object becomes charged, the charge is directed safely into the Earth.

Example: gas stations are carefully grounded. Why?

Methods for Discharging Charged Objects

Discharge at a Point

In some cases, a grounding wire is not a practical idea. When?

Round shapes hold charges very easily. A conductor that is pointed at the end loses charges very quickly.

Why?

As the electrons move along the conductor they reach the pointed end. The electrons come close together and repel each other so strongly they are pushed off the point.

This is called discharge at a point.

Airplanes will use pointed rods called “static wicks” on their wings and tails for discharging the accumulated charge. Page 283

How a Static Wick Works

Other ways to Discharge Objects

1. Exposure to humid air- Charge is absorbed by water molecules.

2. Exposure to light- Photon bombardment dislodges electrons.

3. Exposure to radioactivity- Electrons neutralized or repelled by charged particles.

9.6 Insulators and ConductorsIn order to use electricity we need both insulators and

conductors.

Insulators

An insulator is a substance which slows the free movement of electrons from atom to atom.

Since insulators do not allow the free movement of electrons, they cause the build up of static charge on the surface.

The electrostatic series shows us which insulators can build up a charge on their surface. Things like paint and wax cause charges to build up on our cars.

Insulators are very important to use on electrical devices to keep us from being shocked.

A conductor is a substance in which electrons can move freely from one atom to another.

Since conductors (like copper) allow the free movement (conduction) of electrons, a static charge can never build up.

This is why when you polish a metal tap it doesn’t build up a static charge.

Where does the charge go?

Remember that taps are grounded so the charge is taken away through the pipes into the ground.

Conductors

Good Fair GoodConductors Conductor Insulator

Copper Carbon FurGold Human body RubberMercury Earth WoodIron Water Vapour Paper

Common Insulators and Conductors

Lightning

Did you know lightning is static electricity moving between two clouds or a cloud and the ground?

Lightning is a spark in the sky during a thunderstorm.

It is five times hotter than the surface of the sun.

Even though lightning may discharge an enormous amount of electricity, its energy can’t be used because it lasts for such a short amount of time.

Clouds are objects that can hold a charge.

Lightning happens when the negative charges concentrated near the bottom of the cloud transfer to the ground in a huge spark.

What “shape” do we usually think lightning has?

Lightning appears jagged because it follows concentrations of positive ions in the air.

Why does it do this?

The negative lightning is attracted to the positive ions. It’s the easiest way for lightning to move through the air.

How Does it Happen?

What kind of objects are going to conduct the lightning to the ground?

Tall objects such as trees and skyscrapers are most likely to become conductors.

It can even be you if you are the tallest object around.

Lightning rods are used on tall buildings to help neutralize lightning so they don’t get hit. If they do get hit, the electricity is transferred into the ground.

What is this called? How does this happen?

This is grounding. The lightning travels down the rod, through a wire and into a metal plate buried in the ground.

Lightning Conductors

How To Stay Safe from Lightning?

If You Are Outside

Stay away from water

Go indoors or into a car

Stay away from telephone poles and trees

Stay low to the ground to avoid being the tallest object

Stay away from anything metal, such as a bicycle

If You Are Inside

Stay away from windows and doors because of metal parts

Stay away from water taps that might conduct electricity

Don't use and unplug any electric appliances

Do not use a telephone