Presentation on theme: "SACE Stage 1 Physics Electrostatics. The Structure of the Atom Modern Atomic Theory Began in 1897 when English Physicist J. J. Thompson discovered the."— Presentation transcript:
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.
The Structure of the Atom Sub-Atomic particles Atoms consist of three particles (1)Protons (2)Neutrons (3)Electrons
The Structure of the Atom Sub-Atomic particles Protons and neutrons are approximately the same in mass and size. Electrons are much smaller.
The Structure of the Atom 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.
The Structure of the Atom 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.
The Structure of the Atom 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 Charge 1.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.
Electrostatic Basics 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.
Electrostatic Basics 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.
Electrostatic Basics 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.
Electrostatic Basics 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.
Electrostatic Basics 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.
Electrostatic Basics Conductors and Insulators 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.
Electrostatic Basics Conductors and Insulators 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.
Electrostatic Basics Charging Conductors and Insulators Conductors and insulators can be charged by adding electrons to them or by removing electrons from them. Charging Conductors When a conducting material becomes charged (i.e. the shortage or excess of electrons), the charge is evenly distributed over the surface of the material.
Electrostatic Basics Charging Conductors and Insulators 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.
Electrostatic Basics Charging Conductors and Insulators 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.
Electrostatic Basics Charging Conductors and Insulators 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.
Electrostatic Basics Charging Conductors and Insulators For a conductor to retain its charge, it must insulated from the Earth.
Electrostatic Basics Charging Conductors and Insulators 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.
The Process of Electrification 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 Process of Electrification 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.
The Process of Electrification 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.
The Process of Electrification 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.
The Process of Electrification 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.
The Process of Electrification 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 Process of Electrification 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.
The Process of Electrification 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.
The Process of Electrification Charging The Electroscope Step 2 – Remove your finger while holding the negatively charged rod in place.
The Process of Electrification Charging The Electroscope Step 3 – Remove the positively charged rod.
The Process of Electrification 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.
The Process of Electrification 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 Process of Electrification 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.