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A flashy presentation on the wonders of electrostatics!

Sean McNamara

on 7 December 2010

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Transcript of Electrostatics

Electrostatics Conduction Polarization Induction Conductors Insulators What is Charge? Electric charge is a fundamental property of all matter.

An object is charged by observing the force between it and another charged object.

Charges are positive and negative.

Charges cannot be made or destroyed but are transferred.

Q=Ne Electrons flow freely throughout the object and are not bound to a particular atom. Electrons bound to nucleus and don't flow freely. Charging an object by bringing charged objects close together to polarize one (but not touching), then grounding an object. It is the transfer of electrons through contact between objects. The object with the greatest electron affinity gains electrons. And how do we use all of this..? Coulomb's Law What is force? Force is a push or a pull and the capacity to do work or cause physical change. It is that which changes or tends to change the state of rest or motion of a body. The density of this charge is equal to the charge (Q) over units, which are represented differently for lines, areas, and volumes. So what holds charge? But how do you charge an object? We can apply this to... Which uses the relationship between 2 charges and the distance between them to find force. We use this in different problems involving point charges, colomb forces, electric fields, etc. But first we have to talk about the basics There are four fundamental forces
Gravitational Force
Electromagnetic Force
Weak nuclear Force
Strong Force Electric Flux is...
Quantitative version of electric field lines.
Represented by the number of electric field lines filling the surface.
It is the number of field lines passing through a surface encolsing a charge is proportional to the net charge and is independent to the shape of the surface. Gauss's Law The net flux through any close surface surrounding a point charge is given by the enclosed charge over the permitivitity of free space. Conductors in Electrostatic Equilibrium no net motion of charge
electric field is zero everywhere inside a conductor
any charge on an isolated conductor resides on its surface Electric Potential work per unit charge that an external force must have to get from point A to point B The electric potential at an arbitrary point equals the work required per unit charge to bring a positive test charge from infinity to that point Electron Volt
The energy that an electron or proton gains or loses by moing through a potential difference of 1 volt
1.602x10^-19 J = 1 eV Equipotential Surface name given to any surface consisting of a continuous distribution of points having the same electric potential Potential of a Charged Conductor The surface of any charged conductor in equilibrium is an equipotential surface. Since the E field is 0 inside the conductor, the potential is constant everywhere inside the conductor and equal to its value at the surface. An Electric Field is a representation of the properties in space based off of the presence of a charge, resulting in attraction or repulsion of other objects that possess charge. The sign conventions for flux
The net flux is proportional to the net numer of lines, which is those leaving the surface minus the number entering the surface.
If more lines enter than leave, then the net flux is negative.
If more lines leave than enter then the net flux is positive.
The intergral is over a closed surface. Capacitance Capacitor Capacitence is the ratio of the magnitude of the charge on either conductorin a set up of two to the magnitude of the potential difference between them. This set up of two conductors for the purpose of storing energy is called a... There are different types of capacitors
Parallel plate
Spherical Insulating material is usually put between the conductors. This is called a dialectric. The purpose of a dielectric is to...
increase capacitance
increase operating voltage
provide a mechanical support Semi-conductors has electric conductivity due to electron flow, and is intermediate in magnitude between conductors and insulators = 9.0 x 10^9 r = distance between charges q = charge Equations Parallel Plate Spherical Cylindrical The fundamental units are the most basic ones that every other unit can be broken down into. We know of three: meters, kilograms, and seconds. For example, a Newton is actually a kilogram meter per second squared. We must use these units for values and use these values in equations, often which are solved through techniques such as derivatives, integrals, dot products, etc. Then these equations and values must be applied to real concepts. such as... For electrostatics, we are dealing with the electromagnetic force. Gravity is far weaker and can be ignored. The presence of this charge creates an effect called an electric field. The equation for the strength of the electric field is: E=kq/r^2, which essentially is the strength of the charge divided by how far away you are from it squared, times a constant k. Example F=k(+2Q)(-Q)/(d^2) The coulomb force is also equal to the electric field multiplied by a test charge that is a distance "r" away. Sometimes dealing with electric fields and forces can be complicated, and there are other ways to go about the same type of problems, using electric flux and Gauss's Law. What is electric flux? Example The Gaussian surface can be drawn anywhere (the red dashed line) and you could find the flux and the E field inside. The current surface yields a flux and electric field of 0. (conductor in equilibrium with point charge in the middle) If the shape is irregular, charge will tend to build up at the small radial curvatures or the points. No work is done moving a test charge between two points on an equipotential surface Potential Difference: the change in potential from point A to B measured in Volts (Joules per Coulomb) Sometimes looking at the phenomenon of electrostatics in a different way is useful by looking at energy as opposed to forces, fields or flux. Potential is defined as the work per unit charge, or U/q, which means the energy is equal to Vq. When q is multiplied by the equation for potential due to a point charge, and multiple charges are all acting on each other, we get: The potential due to a point charge is kq/r. Electric Potential makes an important connection back to some earlier ideas in the way that the Electric Field is the derivative of Electric Potential with respect to seperation. How is this electric potential energy that we have been talking about stored? If dW is set equal to Vdq and then solve for Work, you get: This is the work necessary to move a charge dq from one plate to the other, and more energy equations can be solved using this one for work. From this equation we learn that energy is proportional to the electric field squared As you can see, everything all sort of works together and shows up in the equations for the other topics, and the relationships between each and every part of electrostatics is valuable and important. You are now enlightened on the wonderful world of...
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