Presentation on theme: "ELECTROSTATIC FORCES AND THE STRUCTURE OF THE ATOM"— Presentation transcript:
1 ELECTROSTATIC FORCES AND THE STRUCTURE OF THE ATOM
2 Electrostatic forcesHave 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?
3 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.
4 Early History Around 600 BC Greeks found by rubbing a hard fossilized resin Amberagainst a fur cloth,it would attract particles of straw.This strange effect remained a mysteryfor over 2000 years.
5 Two Thousand Years Later Around 1600,William Gilbert, a physicianStudied 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".
6 Static ElectricityThis 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.
7 Static ElectricityTwo objects may become oppositely charged when surface electrons are transferred from one to another by rubbing
8 Static ElectricityTake a comb or plastic pen and rub it on almost any piece of fabric, it will acquire a small electric chargeIf placed near small pieces of paper, they will be attracted.
9 Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before combing (B) after combing.
10 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
11 Static ElectricityIf charge is transferred from or to an atom it is now called an ion.An object becomes electrostatically charged byFriction ,which transfers electrons between two objects in contactContact with a charged body which results in the transfer of electronsInduction which produces a charge redistribution of electrons in a material
12 Charging by induction. The comb has become charged by friction, acquiring an excess of electrons.
13 The paper (A) normally has a random distribution of (+) and (-) charges.
14 (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges.
15 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
16 Identify which objects are charged __+__+__+__+__+_+_++_++_+_+_+_++___+++___++
17 Which objects would be attracted to each other __+__+__+__+__+_+_+_+_+_+__+++__++++__++
18 QuestionsWhy 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?
19 In the AtomA neutral atom has no net charge because the numbers of electrons and protons are equal.
20 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.
21 FieldsThere 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.
22 Electric Fields Around a charge is an electric field Fields are easier to see for magnetismRemember the iron filling experiment from y8
24 When a pith ball is rubbed, it attracts small bits of paper, or another pith ballThe first pith ball was charged and the second uncharged.Therefore, charged objects can attract uncharged objects
25 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.
27 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?
28 Is charge a good thing? Advantages of charge Photocopiers Spray paintingCleaning chimneysIdentifying atoms
29 PHOTOCOPIERSPhotocopiers rely on negative and positive charges to produce an image.The paper is positively charged and the toner is negatively chargedThe toner is therefore attracted to the positive paper, forming an image.
30 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)
31 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 atomsAs the atoms return to their resting states they emit light
32 Is charge a bad thing? Disadvantages of charge LightningSparksIn operating theatres and at petrol pumps, electrostatic sparks can ignite the gases in the air
33 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.
34 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
35 Charges All matter is made up of atoms and molecules contain charged particles,the proton and electron.The charges on each are equalbut opposite in sign.
36 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
37 The Atom and charge The atom consists of a positively charged nucleus The nucleus is made up of protons and neutronsElectrons orbit the nucleusIf the atom was 10 metres in diameter, the nucleus would be 1 mm wide.
38 Structure of the atomnucleusprotonneutronelectrons
40 Electric Charge and Electrical Forces Electrons have a negative electrical chargeProtons have a positive electrical chargeThese charges interact to create an electrical force.Like charges produce repulsive forces.Unlike charges produce attractive forces.
41 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 1Neutrons: = charge, relative mass = atomic mass units (amu); round to 1Electrons: - charge, relative mass = atomic mass units (amu); round to 0 (not factored in when figuring total mass of an atom)
42 How big is the nucleusIf 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
43 Charges The quantity of charge is measured in coulombs (C). 1C = charge carried by 6.25 x 1018 electronsThe charge on one electron = -1.6 x CThe charge on one proton = +1.6 x 10-19C
44 How many electrons? If an electron has a charge of -1.6 x10-19C In a coulombIn a sodium ionIn a thousand coulombsIn 25 coulombs
45 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 betweenforce (F),two charges on the objects q1and q2.
46 Coulomb’s LawThis is similar to Newton’s law of universal gravitation:
47 What is the force when? Two electrons are placed 3m apart A proton and an electron are placed 2mm apartAn electron is orbiting a calcium nucleus at a distance of 3 x mAn electron is the outer shell of hydrogen atom (radius is 5 x 10-9m)10μC charge is placed 5cm from a -2 μC charge