ELECTROSTATIC FORCES AND THE STRUCTURE OF THE ATOM.

ELECTROSTATIC FORCES AND THE STRUCTURE OF THE ATOM

Electrostatic forces

Have you ever walked across a nylon carpet then touched something metallic and felt a tingle?

Or hopped out of a car and touched the metal door handle only to be zapped?

These things happen because of electric charges.

When objects are rubbed together, the friction between them can cause the build-up of electric charge.

Electric Charges

Early History

Around 600 BC Greeks found

– by rubbing a hard fossilized resin Amber– against a fur cloth,– it would attract particles of straw.

This strange effect remained a mystery

for over 2000 years.

Two Thousand Years Later

Around 1600,– William Gilbert, a physician – Studied magnetic phenomena and,– demonstrated that the Earth itself, was a huge

magnet, by means of his "terrella" experiment. He also studied the attraction produced when

materials were rubbed, and named it the "electric" attraction.

From that came the word "electricity".

This build-up of charge is called static electricity because the charge remains stationary.

These are non-contact forces and can affect objects from a distance.

Static Electricity

Static Electricity

Two objects may become oppositely charged when surface electrons are transferred from one to another by rubbing

Static Electricity

• Take a comb or plastic pen and rub it on almost any piece of fabric, it will acquire a small electric charge

• If placed near small pieces of paper, they will be attracted.

(B) after (B) after combing.combing.

Arbitrary numbers Arbitrary numbers of protons (+) and of protons (+) and electrons (-) on a electrons (-) on a comb and in haircomb and in hair(A) before (A) before combingcombing

Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair

If charge is transferred from or to an atom it is now called an ion.

An object becomes electrostatically charged by– Friction ,which transfers electrons between two objects in

contact– Contact with a charged body which results in the transfer of

electrons– Induction which produces a charge redistribution of

electrons in a material

Static Electricity

Charging by induction. The comb has become charged by friction, acquiring an excess of electrons.

The paper (A) normally has a random distribution of (+) and (-) charges.

(B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges.

This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb

Identify which objects are charged

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Which objects would be attracted to each other

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Questions

1. Why does your hair stand on end when you place your hands on a van der graaf?

2. What happens if you rub a balloon on your jumper?

3. How can a balloon stick to a wall?

4. Why does water bend towards a charged comb?

A neutral atom has no net charge because the numbers of electrons and protons are equal.

In the Atom

Removing an electron produces a net positive charge; the charged atom is called a positive ion.

The addition of an electron produces a net negative charge and a negative ion.

There is an invisible area around charged objects called a field. Anything entering this field is affected.

Charged objects behave in certain ways when they are brought together.

Fields

Electric Fields

Around a charge is an electric field

Fields are easier to see for magnetism

Remember the iron filling experiment from y8

Electric and Magnetic Fields

When a pith ball is rubbed, it attracts small

bits of paper, or another pith ball The first pith ball was charged and the

second uncharged. Therefore, charged objects can attract

uncharged objects

If two balloons are both rubbed with the same cloth or charged in the same way and brought together, they don’t attract each other.

Instead, they push one another away.

So objects that have the same charge repel each other.

Questions

1. What is charging by induction?

2. What is an electroscope?

3. How do atoms become negatively ions?

4. What causes lightning?

5. Can you name benefits of charging objects?

Is charge a good thing?

Advantages of charge– Photocopiers– Spray painting– Cleaning chimneys– Identifying atoms

PHOTOCOPIERS

Photocopiers rely on negative and positive charges to produce an image.

The paper is positively charged and the toner is negatively charged

The toner is therefore attracted to the positive paper, forming an image.

Spray painting

Video clip

Spray painting works by making the metallic body oppositely charged to the paint (the paint is charged as it is passed through the fine nozzle)

Spectroscopy

The identity of atoms can be determined by flame tests

If we put atoms in a large electric field (like an electric arc) or feed a flame, we can cause rearrangements of the electrons in the atoms

As the atoms return to their resting states they emit light

Is charge a bad thing?

Disadvantages of charge– Lightning– Sparks

In operating theatres and at petrol pumps, electrostatic sparks can ignite the gases in the air

Lightning

Positive and negative charges can build up in thunderclouds.

If these charges become large enough electrons can suddenly move from one part of the cloud to another, or to the ground, causing a spark that heats the air and causes lightning.

Earthing

Earthing is the discharging of an object This is done by giving the static electricity a

path to the ground, so that it leaks away and does not build up and cause problems.

In operating theatres, doctors and nurses wear

Charges

All matter is made up of atoms and molecules– contain charged particles,– the proton and electron.

The charges on each are equal– but opposite in sign.

The Role of the Electrostatic Force

The electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus, keeps the electrons in orbit.

Just like gravity keeps the Earth in orbit around the Sun

The Atom and charge

The atom consists of a positively charged nucleus

The nucleus is made up of protons and neutrons

Electrons orbit the nucleus If the atom was 10 metres in diameter, the

nucleus would be 1 mm wide.

Structure of the atom

nucleusprotonneutron electrons

Electrons have a negative electrical charge

Protons have a positive electrical charge

These charges interact to create an electrical force.– Like charges produce repulsive forces.– Unlike charges produce attractive forces.

Electric Charge and Electrical Forces

Size of nucleus vs. electron cloud

If an atom was the size of a sports stadium, the nucleus would be the size of a marble!

Subatomic Particles: Protons : + charge, relative mass = 1.007 atomic mass units

(amu); round to 1 Neutrons: = charge, relative mass = 1.009 atomic mass

units (amu); round to 1 Electrons: - charge, relative mass = 0.0005 atomic mass

units (amu); round to 0 (not factored in when figuring total mass of an atom)

How big is the nucleus

If the atom was the size of the MCG, the nucleus would be the size of a pea in the middle of the oval and electrons would move over the entire field

Charges

The quantity of charge is measured in coulombs (C).

1C = charge carried by 6.25 x 1018 electrons The charge on one electron = -1.6 x 10-19 C The charge on one proton = +1.6 x 10-19C

How many electrons?

If an electron has a charge of -1.6 x10-19C In a coulomb In a sodium ion In a thousand coulombs In 25 coulombs

Coulomb’s Law (not assessed)

Charles Augustin de Coulomb in 1785, Investigated the force acting between any two

charged objects. He discovered that there was a relationship between

– force (F),

– two charges on the objects q1and q2.

Coulomb’s Law

This is similar to Newton’s law of universal gravitation:

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What is the force when?

Two electrons are placed 3m apart A proton and an electron are placed 2mm

apart An electron is orbiting a calcium nucleus at a

distance of 3 x 10 -11 m An electron is the outer shell of hydrogen atom

(radius is 5 x 10-9m) 10μC charge is placed 5cm from a -2 μC

charge