SACE Stage 1 Physics Electrostatics. The Structure of the Atom Modern Atomic Theory Began in 1897 when English Physicist J. J. Thompson discovered the

SACE Stage 1 Physics

Electrostatics

The Structure of the Atom

Modern Atomic Theory

Began in 1897 when English Physicist J. J. Thompson discovered the electron.

All matter contains electrons

All atoms are nuclear in nature – contain a small positively charged nucleas orbited by a negatively charged electron.


Sub-Atomic particles

Atoms consist of three particles

(1) Protons

(2) Neutrons

(3) Electrons


Sub-Atomic particles

•Protons and neutrons are approximately the same in mass and size. Electrons are much smaller.


•Electrons and Protons have a property called electric charge. The charge for both particles are the same but opposite in sign, i.e., electrons are negatively charged and protons are positively charged.

•Neutrons do not carry a charge, they are said to be neutral.

•Like charges repel each other, unlike charges attract each other.


Atomic Structure

•There are 92 different naturally occurring elements from hydrogen (1 proton, 1 electron) the lightest to uranium (92 protons, 92 electrons, 132 neutrons) the heaviest.

•All atoms contain the same number of protons as electrons (elements are neutral in their natural state).

•Atoms of all known elements are created by adding 1 proton, 1 electron and some number of protons to the hydrogen atom.

•Protons and neutrons are found in the nucleus of an atom and the electrons can be found orbiting the nucleus.


Atomic Structure•The nucleus is very much smaller than the orbiting electron.

•For any given atom, the electrons are in pre-determined fixed orbits around the nucleus. These orbits are known as electron shells.

Electrostatic Basics

Electric Charge1. There are two types of electric charge – positive and negative charge.

2. Like charges will repel each other, unlike charges will attract each other.

3. The charge on an electron and a proton are the same in magnitude but opposite in charge. Electrons are negative and protons are positive.

4. All bodies are electrically neutral in their natural state.


Movement of Charge

Charging an object

A body can be charged by adding charged particles to it or removing charged particles from it.

The only charged particles that can be added or removed from a body are electrons.


Movement of Charge

Charging an object

If a neutral body has electrons added to it, it has an excess of negative charge and is said to be negatively charged.

If a neutral body has electrons removed from it, it has a shortage of negatively charged particles and is said to be positively charged.


Movement of Charge

The Law of Conservation of Charge

In an isolated system, the total amount of charge of the system must remain constant. Charge cannot be created or destroyed but can be transferred from one object to another.


Movement of Charge

The Earth is Electrically Neutral

In many electrical experiments, and situations involving the movement of charge, the earth is deemed to be a huge repository of charge, which is always electrically neutral.


Conductors and Insulators

Conductors

Materials which allow the movement of charge (electrons) along them.

Insulators

Materials which do not allow the movement of charge along them.

**We say that charge can flow along a conductor but not along an insulator.



Free Electrons

Electrons that are free to move as they are not bound to the nucleus of its host atom.

Delocalized Electrons

They are not localized in space, they are not attached to any one particular atom.



Conducting Electrons

Electrons that move when an electrical charge is conducted along the metal.

In an insulator, all electrons are tightly bound to the nucleus and therefore are free to move preventing and sort of current.


Charging Conductors and Insulators

Conductors and insulators can be charged by adding electrons to them or by removing electrons from them.

Charging ConductorsWhen a conducting material becomes charged (i.e. the shortage or

excess of electrons), the charge is evenly distributed over the surface of the material.



Charging Conductors(1) If we add electrons to a conductor, electrical repulsion causes these

electrons to spread evenly over the whole surface.

(2) If we remove electrons from a conductor, electrical repulsion causes the remaining delocalized electrons to spread themselves evenly over the whole body. Therefore, the excess positive charge is spread over the whole body.



Charging Conductors

The excess of electrons in the material is moving on to the person through to ground (the earth). It is the repulsion of the electrons on the person and in the material that forces the electrons to ground.



Charging Conductors

The positive charge has caused an attraction of electrons from earth, through the persons body and into the material to neutralize the material.



For a conductor to retain its charge, it must insulated from the Earth.



Charging Insulators

When charging an insulator, the deposited electrons remain where they were added as they cannot move around the insulator.

If we remove electrons from an insulator, the area in which they were removed will have a net positive charge and the remaining material will be neutral.

If a charged insulator is held in someone's hand, it does not lose its charge.

The Process of Electrification

Bodies can be charged in three different ways-

1. Charging by friction – rubbing to bodies together.

2. Charging by conduction – the flow of excess electrons from one body to another.

3. Charging by Induction – bringing a charged object close to another neutral object can induce a charge.


Charging by Friction

When rubbing two neutral objects together, we can charge them. Rubbed electrons can jump from one body to another.

We rate materials by its affinity for electrons. This list of materials is known as the tribo-electric series.

The higher the material is on the list, the greater its affinity for electrons.


The Tribo-Electric Series

Human Hands

Rabbits Fur

Glass

Wool

Cat’s Fur

Silk

Cotton

Amber

Hard Rubber

Nickel, Copper

Brass, Silver

Ebonite

Synthetic Rubber

1. The lower substance will negatively charged.

2. The higher substance will be positively charged.

3. The greater the separation of two materials, the greater the strength of the charge.


Charging by Conduction

If a charged conductor is brought into contact with an uncharged conductor, the electrons will flow from one to another to try and balance out the net charge.


Charging by Conduction

Example – Sphere A is negatively charged and sphere B is uncharged.

If the two spheres are bough into contact: Repulsion between the excess electrons on sphere A push them onto

sphere B. The excess electrons then redistribute themselves evenly over both

spheres.


Charging by Induction (Two Objects Simultaneously)

Example – 2 uncharged spheres insulated form ground:

Step 1 – Bring a negatively charged rod towards the side of sphere A.

Step 2 – Move Sphere B so that it is no longer in contact with sphere A, while keeping the charged rod in place.

Step 3 – Remove the negatively charged rod.


Charging by Induction (One Object)

Example – 1 Sphere connected to ground.

Step 1 – Bring a negatively charged rod towards the side of the sphere.

Step 2 – Touch the side of the sphere opposite the rod with your bare finger while keeping the rod in place.

Step 3 – Remove your finger while keeping the charged rod in place.

Step 4 – Remove the charged rod.


The Electroscope

The electroscope is commonly charged by induction.

An uncharged electroscope is used to determine the presence of a net charge on another body.

A charged electroscope is used to determine the sign of the charge on a charged body.


Charging The Electroscope

Step 1 - Bring a negatively charged rod towards the plate on top of the electroscope while touching the opposite side of the plate with your finger.



Step 2 – Remove your finger while holding the negatively charged rod in place.



Step 3 – Remove the positively charged rod.


Using the Electroscope to detect the presence of a charge.

Bring the object near the top plate of the electroscope in its uncharged state.

If the leaves diverge, the object is charged If the leaves of the electroscope do not diverge, the object is not charged.


Using the Electroscope to determine the nature of the charge.

Assuming we know the charge on the electroscope, bring the charged object towards the plate and,

If the leaves diverge further, the charges on the object and the electroscope are of the same sign,

If the leaves collapse, the charges on the object are of the opposite sign.


The Charge Inside a Conductor – Faradays Ice Pail Experiment

To demonstrate that charge resides around the outside of a conductor.

Step 1 – Charge the sphere by induction (use a glass rod and silk)

Step 2 – Bring the metal sphere to the top of the plate on the electroscope. The leaves separate indicating that the sphere is charged.

Step 3 – Remove the sphere and place the uncharged metal can on the top plate of the uncharged electroscope.

Documents

SACE Stage 1 Physics Electrostatics. The Structure of the Atom Modern Atomic Theory Began in 1897 when English Physicist J. J. Thompson discovered the