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Transcript
Electrostatics, Electric Fields CHHS Physics Mr. Puckett Electrical Charges ELECTRICAL CHARGES are natural forces that exist on matter under certain conditions of state. They are the result of an excess or deficiency of electrons on atoms, making them ions. Positive ions are Cations and Negative ions are Anions. Protons are much harder to move because of their mass and nuclear forces. These charges normally move around to mingle with opposite charges and become more stable. Most books show + charges in RED and – charges in GREEN or BLUE LAWS OF ELECTROSTATIC CHARGE INTERACTIONS: A. Like charges repel each other just as similar poles of two different magnets will repel away from each other. B. Opposite charges will attract each other. This is the same as putting a north and south poles of two different magnets together. They attract and stick together. Opposites Attract; Similar Repell Charging by Friction and Conduction Electrostatics ELECTROSTATICS is the study of electrical charges that can be collected and held in one place. The force exerted by charged objects is greater than gravitational force. The electrical effect and the effect of gravity are different because only the effect of gravity is constant. Balanced Atoms and Charges The positive charge of the nucleus is balanced by the negative charge of the electrons, so the balanced atom has no net charge. The removal of electrons by friction or rubbing leaves a positive ion because the positive charge of the nucleus is no longer balanced by the negative electrons. If the removed electrons become attached to another atom, it changes the atom to a negative ion and carries a net negative charge. Conservation of charge and energy are maintained throughout this change because the electrons are not created or destroyed, only put in different places. Conduction and Induction: You can cause charges to move around by either touching them to another object (conduction), or by bringing them near another object (induction). The study of electrical charges is called ELECTROSTATICS. The figure below is charging by conduction With friction on the Plastic wand Charging by Induction Charge Induction by Grounding Charging action by Induction Charging by Induction Electroscope Diagram This device detects electrical charges. If a non-metallic insulator touches the mast, the excess electrons will move to the insulator and out of the conductive mast into the leaves. They will spread due to similar charge repulsion. Induction: Charging by Proximity Charge Attraction Charges are induced by coming close to but NOT touching. It is caused by the electrostatic laws of opposite attraction. Charging by Polarization Conduction: Charging by Touching Charges are transferred by touching. Note that the positive charges spread out evenly Conductors and Insulators If they are in a medium that allows them to move easily, then that would be a conductor. Metals are the normal conductors. Some metals are better conductors than others. Copper, gold and aluminum are good conductors. If the charges are in a medium that does not readily allow for charge movement then that is an insulator. Glass and rubber are normal insulators. Charge Separations and Placements On a sphere the charges will go to the surface and spread out evenly. On an insulator the charges will stay where they are placed. Distribution of Charges on a Surface Critical Angles vs. Smooth surfaces. The shape of an object will be important in determining the surface charge concentrations. Charges will spread out evenly over a smooth surface but will concentrate at sharp angles. This critical angle will promote charge discharge. Safe from Lightning in the Car? Charge Distribution on Different Sizes and Angles Sharp angles increase the charge density like decreasing size. Charge and Conduction Conventions The following symbols and sign conventions are used throughout the unit. Charges are + and – signs and the conductors are shown as charge flow paths. History of Electrical Fields The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses, and like it, extends towards infinity. It shows an inverse square relationship with distance. However, there is an important difference. Gravity always acts in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Electric Fields of Force All electric charges produce a force field around them that interact with other force fields and some types of matter. These force fields produce Potential Energy and produce effects that help us predict interactions in the future. The fields look analogous to gravity and topo maps that have high and low areas. The high areas are + charges and the lows are – charges. Electrical vs. Gravitational Fields As the picture above shows potential energy with the top of the stadium seats would have more potential energy and the track would have less energy. Think of how rain water would flow from the + to the - . That is how electrical fields work. The rows of seats would be lines of equal potential energy. Equipotential energy lines are common in electrical and magnetic force fields. Compare Gravity to Electrical Charge Forces Gravity Force vs. Electrostatics Electrical Fields Visualized Electrical Field Visualization (-) Electrical Field Visualization (+) Test Charges (+) are Used to Test the Strength of Electrical Fields A small positive test charge “q” is placed around the electrical field and the forces are measured that affect it. Electrical Field Force Lines The force lines go from + to – charges. Potential Energy lines (red) are perpendicular to the force lines (purple). Coulomb’s Law of Electric Force COULOMB'S LAW describes the force between two charged objects. The electric force varies inversely with the square of the distance between the two charged objects. The electric force varies directly with the product of the charges on the objects. Coulomb's law can be written as the equation F = K q q' d2 , where F is force, q and q' represent the charges of the objects, d represents the distance between the objects and K is a proportionality constant of charge conduction through free space;. K is equal to 9 x 109 Nm2/C2. The Coulomb Unit: The SI UNIT OF CHARGE is the Coulomb (C) defined as the charge of 6.25 X 1018 electrons. The magnitude of the charge of one electron is called the elementary charge. The charge on one electron is -1.6 x 10 -19 C. The electric force is a vector quantity and has both magnitude and direction. A repulsive force has a positive sign and an attractive force has a negative sign. Coulomb’s Example What is the force of a hydrogen nucleus on its electron. Orbital radius = 0.53 x 10-10m and charge on the electron is 1.6 x 10-19 C. F = K q q‘ d2 = (9.0x109 Nm2/C2)(-1.6x10-19C)(1.6x10-19C) 0.53 x 10-10m2 = - 8.2 x 10-8 N (the negative sign means attractive forces because opposite charges attract) Electron Charge Calculations What is the charge on 5000 electrons? 5000 e- X -1.6 x10-19 Coulombs/electron = 8x10-16 C or 8x10-10 μC How many electrons are in 2 μC ? 2x10-6 C / 1.6x10-19 C / e- = 1.25x1013 e- Calculate the Force on a Charge (+ or -) in an Electrical Field A uniform electric field has a value of 8e5 N/C and a positive test charge of 2e-5C is placed in the field. What force does the charge experience? F = qE (2e-5C)(8e5 N/C) = 16 N repulsive ( attractive forces are negative because opposites attract and similar charges repel) Forces on the Center Charge: -1 μC -3 μC +5 μC q1●-------------q2 ●----------------------- ● q3 |---- 2cm ---------------| ---------- 5 cm-------------------| What is the Force on the center charge? Use Coulombs law to compute the individual electrical forces and then add them. F21 = K q1 q2 / r2 = 68N and F 23 = 54 N so the sum is 122 N to the right. Testing Electric Fields To test electric fields we place a small + charge around the field and measure the interaction. Ex: a positive test charge of 1.85e-5C is placed in a field and experiences a force of 14 N. What is the Electric field strength at that point? E = F/q = 14N/1.85e-5C = 7.5e5 N/C Electric Field of a Single Point Charge. A positive test charge of 2e-5C is placed in an electrical field of 8e5 N/C. What force does the charge experience? F = qE F = 2e-5C X 8e5 N/C = 16 N Electrical Potential Energy = Volts Calculate the electrical potential energy of a 0.05C charge that is 5 m away. V = Kq/r = (9e9 )(0.05C) / 5 m = 9e7 Volts Electrical Fields do WORK !! Electrical fields have the ability to create a force that moves things a given distance. This is how you get electricity out of the wall socket. Ex: The potential energy difference ( Voltage) is 6 V. How much work does it take to move 2 coulombs of charge across a filament? W = qV so 2C (6 J/C or V) = 12 J Voltage and Work Voltage (PEe) is equal to the amount of work it takes to move a charge from the negative side of an electrical field to the positive side, divided by the amount of charge you are moving. W = Fd = qEd V = PE/q = W/q = Ed How Do We Use Electrical Fields and Electrostatics in Daily Life? Storing the Charges of Electrostatics: CAPACITANCE A capacitor is a device that is used to store electrical charges and electrical potential energy. It changes AC to DC current A capacitor is “charged” when a potential difference (voltage) is applied. The capacitance, C, of an object is the amount of charge, Q, it can store for a given voltage. The UNIT is the FARAD, F. Formula for capacitance: C = Q / ΔV Formula for PE = ½ Q ΔV Potential Energy in a Capacitor A charged capacitor stores electrical PE because it requires work to move charges through a circuit to the opposite plates of a capacitor. PE electric capacitor = ½ QV = ½ CV2 = Q2/2C Capacitors Semiconductors Materials that have medium qualities of conduction and insulation. These properties can be changed by coatings or materials processing methods (alloying). They are mostly used in computer chips and electronics. Electric fields allow these chips to process electrical signals very rapidly. Superconductors Superconductivity is an electrical resistance of zero which occurs in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes in 1911. Superconductivity is a quantum mechanical phenomenon. The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, this decrease is limited by impurities and other defects. Superconductors in your Future Van de Graaff Generator A Van de Graaff generator is a charge producing device that has a frictional belt that separates electrons and allows them to collect on the surface of the dome. The voltage is very high but the amperage is very low. Van de Graaff Generator makes hair stand up by static electricity Electrical Fields The Photocopier Charged Images How is Lightening Made? Grounding: Discharging Static When static charges are given a route to discharge; it is called Grounding; like when lightning uses a barn to “strike” and ground out. Ben Franklin first came up with this idea to stop the burning of church steeples by lightening in the early colonial days of the USA. THE END !! Go forth and get a CHARGE out of Life! You have the capacitance to do anything that you put your mind to. May the Volts be with you !