# ELECTROSTATIC FORCES AND THE STRUCTURE OF THE ATOM

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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?

Electric Charges These things happen because of electric charges.
When objects are rubbed together, the friction between them can cause the build-up of electric charge.

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".

Static 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 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.

Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before combing
(B) after combing.

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

Static Electricity 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

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
_ _ + _ _ + _ _ + _ _ + _ _ + _ + _ + + _ + + _ + _ + _ + _ + + _ _ _ + + + _ _ _ + +

Which objects would be attracted to each other
_ _ + _ _ + _ _ + _ _ + _ _ + _ + _ + _ + _ + _ + _ _ + + + _ _ + + + + _ _ + +

Questions Why does your hair stand on end when you place your hands on a van der graaf? What happens if you rub a balloon on your jumper? How can a balloon stick to a wall? Why does water bend towards a charged comb?

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

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.

Fields 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.

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 What is charging by induction? What is an electroscope?
How do atoms become negatively ions? What causes lightning? 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

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 nucleus proton neutron electrons

Electric Charge and Electrical Forces
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.

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

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 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