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

The Dangers of Transferring Charges by Contact

One spark from a charge transferred by contact can cause huge explosions.

Places like:

Coal mines Hospital Operating Rooms Oil Refineries

Coal dust explosions have killed thousands of miners.

People who work in these places have to wear special clothing and shoes to prevent sparks.

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

Charging Conductors by Induction

Computers, cell phones, TVs, i-pods use charging by induction.

A conductor is a material that can move an electric current.

When induced charge separation happens in a conductor, the electrons don’t just change position. They can move from atom to atom.

When a negatively charged object is placed near an uncharged object, the electrons are forced to move.

If the object is connected to a conductor, the electrons will move out of the object. The object now has a positive charge.

Look at Figures 2 and 3 on page 286.

Using Charging by Induction

The process of charging by induction has many important uses.

Dust and pollution can be removed from air by making the particles have a positive charge.

As the dirty air moves through the air cleaner, the positive dirt particles are attracted by negatively charged plates.

What happens to the dirt particles?

They stick to the plates, and are now cleaned out of the air.