All physics to date has led to one primary conclusion: • There are four fundamental forces: But why four? Why not just one master force? The quest for a single unified master force is on!!! Idea: There is one force only which manifest itself differently in different situations • Strong nuclear • Electromagnetic • Weak nuclear • Gravitational GUT - grand unified theory : Higgs boson ~250 yrs or so since we first learned what electricity is “Electricity” – from the Greek word electron(elektron) - meaning “amber”. The ancients knew that if you rub an amber rod with a piece of cloth, it attracts small pieces of leaves or dust. “amber effect”– the object becomes electrically charged
Electricity & Magnetism • static electricity (Electrostatics) • Why do I get a shock when I walk across the rug and touch the door knob? • Why do socks stick to my pants in the dryer? • Why does my hair stick to my comb, and I hear a crackling sound ? • Why does a piece of plastic refuse to leave my hand when I peel it off a package? • What is lightning? What is that all about? It’s the CHARGE
named by No one has ever seen electric charge; it has no weight, color, smell, flavor, length, or width. Charge is an intrinsic property of matter electron has it,proton has it,neutron doesn’t have it – and that’s all • Electric charge is defined by the effect (force) it produces. • positive charge • negative charge Benjamin Franklin 1706 - 1790, American statesman, philosopher and scientist)
10-15 m 10-10 m Electricity has origin within the atom itself. mnucleon≈ 2000 x melectron ratom≈ 100000 x rnucleus Atom is electrically neutral = has no net charge, since it contains equal numbers of protons and electrons.
Electric forces + + + • charges exert electric forces on other charges • two positive charges repel each other • two negative charges repel each other • a positive and negative charge attract each other The repulsive electric force between 2 protons is 1,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than the attractive gravitational force!
French physicist Charles A. de Coulomb 1736 - 1806 Attractive force between protons and electrons cause them to form atoms. Electrical force is behind all of how atoms are formed and…chemistry… • charge is measured in Coulombs [C] Every electron has charge -1.6 x 10-19 C, and every proton 1.6 x 10-19 C 1C represents the charge of 6.25 billion billion (6.25x1018) electrons ! Yet 1C is the amount of charge passing through a 100-W light bulb in just over a second. A lot of electrons!
The smallest amount of the free positive chargeis the charge on the proton. quarks have 1/3, but they come in triplets The smallest amount of the free negative chargeis the charge on the electron. let e = 1.6 x 10-19 C Chargeof the single proton is qproton = e Charge of the single electron is qelectron = - e • Charge is quantized: cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole-number multiple of charge of the smallest amount (a single e). • The net charge is the algebraic sum of the individual charges (+ 5 - 3 = 2).
Everyday objects - electronically neutral – balance of charge – no net charge. Objects can be charged – there can be net charge on an object. How? The only type of charge that can move around is the negative charge, or electrons. The positive charge stays in the nuclei. So, we can put a NET CHARGE on different objects in two ways Remove electrons and make the object positively charged. Add electrons and make the object negatively charged.
Some materials have atoms that have outer electrons (farthest from nucleus) loosely bound. They can be attracted and can actually move into an outer orbit of another type of atom. The atom that has lost an electron has a net charge+e (positive ion). An atom that gains an extra electron has a net charge of– e (negative ion). This type of charge transfer often occurs when two different materials (different types of atoms) come into contact. • Which object gains the electrons depends on their electron affinity:
Conclusion: • electrons can be transferred from one object to another • During that process, the net charge produced is zero. The charges are separated, but the sum is zero. The amount of charge in the universe remains constant (we think!!) It isCONSERVED! • Another Law of Conservation: Charge is always conserved: charge cannot be created or destroyed, but can be transferred from one object to another.
Electrical conductors, insulators, semiconductors and superconductors - distinction based on their ability to conduct electric charge. Any material that allow charges to move about more or less freely is called conductor. So, if you transfer some electrons to the metal rod, that excess of charge will distribute itself all around rod. Tap water, human body and metals are generally good conductors. That’s all very nice, but why is that so?
What makes conductors conduct? • Atoms have equal numbers of positive and negative charges, so that a chunk of stuff usually has no net charge the plusses and minuses cancel each other. • However, in metal atoms the valence electrons – the electrons in the outermost orbits - are loosely bound, so when you put a bunch of metal atoms together (to form a metal) an amazing thing happens valence electrons from each atom get confused and forget which atom they belong to. • They now belong to the metal as the whole. As the result, positive ions which are tightly bound and can only oscillate around their equilibrium positions, form a positive background. All the homeless electrons - “Free electrons” • wander around freely keeping ions • from falling apart – metallic bond!!
Electrons in insulators are tightly bound to atomic nuclei and so cannot be easily made to drift from one atom to the next. Only if a very strong electric field is applied,the breakthrough (molecules become ionized resulting in a flow of freed electrons) could result in destruction of the material. The markings caused by electrical breakdown in this material – look similar to the lightening bolts produced when air undergoes electrical breakdown. Materials like amber, pure water, plastic, glass, rubber, wood… are called insulators. They do not let electricity flow through them. Electrons are tightly bound to nuclei, so it is hard to make them flow. Hence, poor conductors of current and of heat.
Conductors and Insulators REMEMBER: Electrons are free to move in a conductor Electrons stay with their atom in an insulator Most things are in between perfect conductor/ insulator
Semiconductors • Materials that can be made to behave sometimes as insulators, sometimes as conductors. Eg. Silicon, germanium. In pure crystalline form, are insulators. But if replace even one atom in 10 million with an impurity atom (ie a different type of atom that has a different # of electrons in their outer shell), it becomes an excellent conductor. • Transistors: thin layers of semiconducting materials joined together. Used to control flow of currents, detect and amplify radio signals, act as digital switches…An integrated circuit contains many transistors.
The movement of electrons in semiconductors is impossible to describe without the aid of quantum mechanics. As the conductivity of semiconductors can be adjusted by adding certain types of atomic impurities in varying concentrations, you can control how much resistance the product will have. ADVANTAGE – A HUGE ONE
Superconductors • Have zero resistance, infinite conductivity • Not common! Have to cool to very, very low temperatures. • Current passes without losing energy, no heat loss. • Discovered in 1911 in metals near absolute zero (recall this is 0oK, -273oC) • Discovered in 1987 in non-metallic compound (ceramic) at “high” temperature around 100 K, (-173oC) • Under intense research! Many useful applications eg. transmission of power without loss, magnetically-levitated trains… • http://science.nasa.gov/science-news/science-at-nasa/2003/05feb_superconductor/ • http://www.scicymru.info/sciwales/indexphpsectionchoose_scienceuser_typePupilpage_id11696languageEnglish.htm
Van de Graaff Example: • The sphere gives the girl a large negative charge. Each strand of hair is trying to: • Get away from the charged sphere. • Get away from the ground. • Get near the ceiling. • Get away from the other strands of hair. • Get near the wall outlet. Like charges attached to the hair strands repel, causing them to get away from each other.
Seeing the effects of charge: the electroscope ++ ++ • the electroscope is a simple device for observing the presence of electric charge • it consists of a small piece of metal foil (gold if possible) suspended from a rod with a metal ball at its top • If a negatively charged rod is placed near the ball, • the electrons move away because of the repulsion. • The two sides of the metal foil then separate.
Charge polarization is why a charged object can attract a neutral one : • DEMO: Rub balloon on your hair – it will then stick to the wall ! Why? Balloon becomes charged by friction when rub on hair, picking up electrons. It then polarize molecules on the surface, induces + charge layer on the wall’s surface closest to it , and next negative furthest away. So balloon is attracted to +charges and repelled by –charges in wall, but the – charges are further away so repulsive force is weaker and attraction wins. • Charge a comb by rubbing it through your hair, and then see it attracts bits of paper and fluff…
You can bend water with charge! The water molecule has a positive end and a negative end. When a negative rod is brought near the stream of water, all the positive ends of the water molecules turn to the right and are attracted to the negative rod. charged rod What happens if the rod is charged positively? stream of water
As we said Like charges repel, and opposite charges attract. This is the fundamental cause of almost ALL electromagnetic behavior. But how much? How Strong is the Electric Force between two charges?
ELECTROSTATIC – ELECTRIC - COULOMB FORCE The force between two point charges is proportional to the product of the amount of the charge on each one, and inversely proportional to the square of the distance between them. Force is a vector, therefore it must always have a direction.
SHE accumulates a charge q1 of 2.0 x 10-5 C (sliding out of the seat of a car). HE has accumulated a charge q2of – 8.0 x 10-5 C while waiting in the wind. What is the force between them when she opens the door 6.0 m from him and when their separation is reduced by a factor of 0.5? a) They exert equal forces on each other only in opposite direction (“-“ = attractive force) b) r’ = 0.5 r Strong force at very small separation spark How many electrons is 2.0 x 10-5 C ?
When you comb your hair with a plastic comb, some electrons from your hair can jump onto it making it negatively charged. Your body contains more than 1028 electrons. Suppose that you could borrow all the electrons from a friend’s body and put them into your pocket. The mass of electrons would be about 10 grams (a small sweet). With no electrons your friend would have a huge positive charge. You, on the other hand, would have a huge negative charge in your pocket. If you stood 10 m from your friend the attractive force would be equal to the force that 1023 tons would exert sitting on your shoulders – more 100,000 times greater than the gravitational force between the earth and the Sun. Luckilyonly smaller charge imbalances occur, so huge electrical forces like the one described simply do not occur.
Three point charges : q1= +8.00 mC; q2= -5.00 mC and q3= +5.00 mC. • Determine the net force (magnitude and direction) exerted on q1 by the other two charges. • If q1 had a mass of 1.50 g and it were free to move, what would be its acceleration? Force diagram q2 1.30 m 230 q1 230 F2 F3 1.30 m q1 q3 ; x-components will cancel each other = F = = 0.166 N electric force is very-very strong force, and resulting acceleration can be huge
A positive and negative charge with equal magnitude are connected by a rigid rod, and placed near a large negative charge. In which direction is the net force on the two connected charges? 1) Left 2) Zero3) Right Positive charge is attracted (force to left) Negative charge is repelled (force to right) Positive charge is closer so force to left is larger. - + -
F7 • Calculate force on +2mC charge due to other two charges • Calculate force from +7mC charge • Calculate force from –3.5mC charge • Add (VECTORS!) Q=+2.0mC F3 5 m 4 m Fx = F7cosq + F3cosq = F7(3/5) + F3(3/5) = 3 10-3 N + 1.5 10-3 N = 4.510-3 N 6 m Q=+7.0mC Q=-3.5 mC Fy = F7 sin q + F3 sin q = F7(4/5) + F3(4/5) = 4 10-3 N – 2.0 10-3 N= 2.010-3 N
F r P q Q Electric Field Let's take a single electric charge, Q, and put it somewhere. The space around it is different from the space without charge. We have created a situation in which we could have an electric force. All we have to do is bring in a second charge, q, to feel the force. Without q, there is no force ....but we still have the condition that we could have a force. We say that the space around charge contains ELECTRIC FIELD. How to measure/find the strength (magnitude and direction) of electric field at particular location P due to chargeQ? A test charge, q, placed at P will experience an electric force,F- either attractive or repulsive.
F E = q F r P q Q Definition of electric field, E, at a point P distance r away from Q. The magnitude of the electric field is defined as the force per unit charge. As F contains q, EDOESN’T depend on q at all, only on Q. Electric field at any point P in space is always in the direction of the force on a positive test charge if it were placed at the point P.
E E q F F The other way around: If you know electric field E at a point where you place a charge q, that charge will experience the force F: F = q E q
Q F E = k r2 q q E r = 1x10-10 m 1.610-19 (10-10)2 • E = 9109 = 2.91011 N/C Electric field of a charged particle/point charge A charged particle Q creates an electric field. • E Field independent of test charge ◊ magnitude the same value on the sphere of radius r around ◊ direction – radially outward or inward example: Q=1.6x10-19 C + • q positive test charge (to the right)
Question Say the electric field from an isolated point charge has a certain value at a distance of 1m. How will the electric field strength compare at a distance of 2 m from the charge? It will be ¼ as much – inverse square law for force between two charges carries over to the electric field from a point charge.
We use “Electric Field Lines” to visualize el. field. Convention / agreement Direction indicates direction in which apositivetest charge would be pushed – direction of the force!!!.
Electric Field of a Point Charge 0.81011 N/C 321011 N/C 251011 N/C 2.91011 N/C + E This is becoming a mess!!!
Electric Field Lines • Density gives strength • # lines proportional to Q • lines never cross! • Arrow gives direction • Start on +, end on -
negative charge positive charge So always point away from +charges, towards – charges… Denser lines - stronger field el. field decreases with distance more lines revels stronger field due to greater charge
Electric field lines can never cross. If they crossed, that would mean that a charge placed at the intersection, would be accelerated in TWO directions at once! This is impossible! If two sources are creating electric fields in the same place, we have to add the two vectors and get a resultant vector representing the NET ELECTRIC FIELD.
Question? • What is the direction of the electric field at point C? • Left • Right • Zero Away from positive charge (right) Towards negative charge (right) y Net E field is to right. C x
Question? • What is the direction of the electric field at point A? • Up • Down • Left • Right • Zero A x
Question? • What is the direction of the electric field at point B? • Up • Down • Left • Right • Zero y B x
Question? • What is the direction of the electric field at point A, if the two positive charges have equal magnitude? • Up • Down • Left • Right • Zero A x
Electrical Energy and Electrical Potential Two different things that sound alike! Recall Work: W = F d cos(q) In order to bring two like charges together work must be done. In order to separate two opposite charges, work must be done. As the monkey does + work on the positive charge against electric force, he increases the energy of that charge. The closer he brings it, the more electrical potential energy it has. This work done by external force against electrical force is stored as electrical PE, U. When he let it go, the charge will gain kinetic energy and can do a work. Try the same thing with grav. force. It is the same!!!! charge → mass
So essentially, potential energy is capacity for doing work which arises from position or configuration. Greater amount of charge → greater force needed → greater work done → greater stored potential energy U. → introducing the electrical potential energyper unit charge, known as electrical potential, which doesn’tdepend on the amount of charge. If a charge q at point P (in electric field E) has electric potential energy U, the electric potential V at that point is: The SI unit of electric potential is the volt.
+ + + + + + + + + + + + + + + + + + + + • Note important difference between energy and potential: • A point has potential, charge placed there has electric potential energy Two points that are at the same distance from the charged object have the same potential. So, when two charged object are placed there, they are at the same potential, but the one with more charge on it has higher electric potential energy – could do more work. + + + + + + + +
Potential Difference Between Two Points (ΔV = VB – VA) The difference between the potentials at two different points, A & B, is equal tothe change in electric potential energy between these two points or the work done per unit of positive charge in order to move it from one point to the other. Law of conservation of energy: change in potential energy = change in kinetic energy To place a charge in electric field a work has to be done on the charge. That work is stored as potential energy, U, of the charge. The point where the charge is, has potential V (charge q placed there has potential energy U = qV). Charge placed in electric field E will experience electric force, F= qE. If the charge is free to move it will accelerate, it will gain kinetic energy and can do a work.
surprise, surprise ! We use the same name for different things, and even worse we use couple of different names to express the same thing, like: • The variable we use for potential, potential difference, and the unit for potential difference (volts) is V. Cute!!!!! • Don't let that confuse you when you see V = 1.5V • Electric potential energy is not the same as electrical potential. • The electron volt is not a smaller unit of the volt, it's a smaller unit of the Joule. • Electrical potential can also be described by the terms, potential difference, voltage, potential drop, potential rise, electromotive force, and EMF. These terms may differ slightly in meaning depending on the situation.
Electric Current And Ohm’s law