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VOLTAGE, ELECTRIC ENERGY AND CAPACITANCE Engr. Mehran Mamonai Applied Physics 1 st Term 1 st Year SUPPOSE THIS TECHNOLOGY DEFIBRILLATOR WHAT IS CAPACITOR FLOW OF ENERGY WHY FLOW OF ENERGY IN PARTICULAR DIRECTION If we follow Potential Energy in Gravity The object which is raised from the surface will follow the downward path In order to get equilibrium state WHAT IS WORK PERFORMED HOW MUCH AN ELECTRIC ENERGY SYSTEM HAVE? Everything sense energy Question arises, How much an Electric Energy a system have And How much an Work it can perform DID YOU KNOW ?? DEFIBRILLATOR PRINCIPLE The device which we discuss before Defibrillator, Provides enough charge to heart to make it stop But energy is not so strong that it damages it permanently So do we know, how much energy is required to charge up POINT CHARGE IN CAPACITOR WORK DONE CONCEPT MOVING POINT CHARGE ACROSS THE CAPACITOR Now we can apply equal and opposite force to move the point charge across the capacitor slowly as we like So the Kinetic Energy of particle remains negligible From Law of Conversation of Energy and Work Energy Theorem Change in PE is equal to negative work done (Lets understand Work Energy Theorem first) WORK-ENERGY THEOREM The principle of work and kinetic energy (also known as the work-energy theorem) states that the work done by the sum of all forces acting on a particle equals the change in the kinetic energy of the particle. Work can change the potential energy of a mechanical device, the heat energy in a thermal system, or the electrical energy in an electrical device. The work-energy theorem can be derived from Newton's second law. KE = WORK DONE BY EXTERNAL FORCE ELECTRIC FIELD ON POINT CHARGE So We conclude that Potential Energy Decreases on a point charge in Uniform Electric Field POTENTIAL ENERGY VS MAGNITUDE OF POINT CHARGE REARRANGING THE EQUATION ELECTRIC POTENTIAL VS ELECTRIC ENERGY • Electric Potential depends on Electric Field difference and the distance at which Point charge is Placed • It doesn’t depends on magnitude of point charge UNIT OF ELECTRIC POTENTIAL EFFECT OF POINT CHARGE Electric Potential decreases as the point move away in distances MATHEMATICAL EXPRESSION OF VOLTAGE HYPNOTICAL POINT CHARGE VS REAL LIFE We discussed all the phenomenon is happen on a point test charge We calculated the Electric Potential on this charge across a charged Capacitor. Let us formulate, how can we measure Electric potential in real world components SUPPOSE A CAPACITOR ELECTRIC FIELD CAN BE FOUND BY SO WE CAN EXPRESS EQUATION AS ELECTRICAL POTENTIAL AND DISTANCE Since a Capacitor creates an uniform electric field We can assume Electric Field is Constant Acts in the direction, where the test charge moves So As charge moves, Electric Potential decreases more and more with distance EFFECT DUE TO EQUIPOTENTIAL LINES IN CAPACITOR The Equipotential line are parallel in Capacitor These Lines are always perpendicular to Electric Field Lines ELECTRIC POTENTIAL IN CAPACITOR VS ELECTRIC POTENTIAL IN POINT CHARGES Now we have calculated the voltage in Capacitor Can we apply the same principal on point charges? As there is no uniform Electric Field available on point charge, so we can’t use these equations on point charge Because we can’t measure the ef fect on the point charge infinitely far away CONCLUSION We find that point charge with same charge (positive) is place near to charge Q it will experience high Electric potential energy WE CAN CALCULATE AS EQUIPOTENTIAL LINES BY POINT CHARGE EQUIPOTENTIAL LINE IN ELECTRICAL DIPOLE POINT CHARGE ON DIFFERENT POSITIONS LETS DISCUSS WHAT WE HAVE STUDIED Now let’s go back to potential energy in capacitors Why Capacitor? These are very useful – from defibrillators to electronic component When a capacitor’s plates store electric charge, they’re actually storing energy LETS APPLY CURRENT IN ONE DIRECTION WILL CAPACITOR GET ANY CHARGE? Capacitor hasn’t gained any net charge There’s just as much positive charge on the positive plate as there is negative charge on the negative one So How does Capacitor gets charged up?? Answer: Battery uses its own electric potential to generate a current that transfers voltage in the capacitor, giving the capacitor a certain amount of potential energy. ENERGY OUT OF THE CAPACITOR Now if you’re trying to save a life, you want to make sure you’re getting just the right amount. how much charge a capacitor can store? We can use a battery in our circuit to create a voltage between the plates, and then divide the charge in each plate by that voltage. This value is known as capacitance UNIT OF CAPACITANCE Capacitance uses units of Farads, One Farad equal to one Coulomb per one Volt SMALL VALUES OF CAPACITANCE Typically, capacitance values are very small (Why ??) so we often talk about capacitors in terms of microFarads or nanoFarads. Capacitance is actually determined by the size and shape of a capacitor ANOTHER WAY TO REPRESENT CAPACITANCE HOW CAN WE CHANGE CAPACITANCE Make the plates larger or Move them closer together This way there’s room to fit more charge, creating a stronger electric field once you’ve established the geometry of the plates, the capacitance does not change unless you stick something between them, which actually increases the capacitance ADDING SOMETHING TO CAPACITOR This something is called a dielectric WHAT IS DIALECTIC? Typically an insulating material like plastic or glass A dielectric is used to increase capacitance while preventing any charge from jumping from plate to plate. But why would charges jump? Sometimes, as the plates get hotter or the voltage gets higher, some electrons naturally jump between the plates, decreasing the amount of stored charge PURPOSE OF DIELECTRIC So an insulator prevents electrons from crossing the gap you usually want the plates to be as close together as possible without touching Because a smaller distance equals a larger capacitance, By using a very thin dielectric, the distance is decreased while the plates remain separate. Dielectrics are polar, which means one side of the molecule is slightly positive while the other side is slightly negative EFFECT OF DIELECTRIC SO EQUATION OF CAPACITANCE WILL REFORM AS POTENTIAL ENERGY STORED IN THIS FIELD By integrating the voltage over the charge in the plates, which reduces down to one half charge time voltage ENERGY DENSIT Y IN CAPACITOR It is useful to know how much energy is stored in an electric field per unit volume To know how much energy is in a certain location, like between capacitor plates The amount of energy stored in the electric field per unit volume is called Energy Density We can calculate the energy density associated with an electric field (at any point space not only in Capacitor) POTENTIAL ENERGY BY THE VOLUME With some algebra CONCLUSION Now that we know all this, a medic can make sure those defibrillator paddles have the right capacitance, have been given enough charge to create a high voltage we learned a lot things (dif ferently) We talked about electric potential energy and how it dif fers from electric potential, or voltage. We discussed how capacitors function and the factors that determine how much charge they hold. We also learned how to maximize energy storage and how to calculate the potential energy held by any capacitor.