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# Potential Difference & Capacitance

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### Potential Difference & Capacitance

1. 1. Potential Difference & Capacitance March 9th, 2009
2. 2. Electrical Potential <ul><li>The electrical potential energy associated with a charge particle, divided by the charge of the particle </li></ul><ul><li>V = PE electric / q </li></ul>
3. 3. Electrical Potential vs. Gravitational Potential <ul><li>It might help you understand electrical potential to think about it like gravitational potential </li></ul><ul><li>When does an object have high gravitational potential? </li></ul><ul><li>Where do objects like to come to rest? With high potential energy or low potential energy? </li></ul><ul><li>It requires energy to move an object from an area of low potential energy to an area of high potential energy </li></ul>
4. 4. Electrical Potential <ul><li>The same is true for charged particles </li></ul><ul><li>Think about a positive field, where will a positive test charge have have the highest potential energy? Lowest potential energy? </li></ul><ul><li>HINT: Is this force attractive like gravity??? </li></ul>
5. 5. Think you got it? Try these! <ul><li>For each picture determine: </li></ul><ul><ul><li>If work is being done on the charge </li></ul></ul><ul><ul><li>Where the Electric potential is greatest </li></ul></ul>
6. 6. Potential Difference <ul><li>Think about this picture: </li></ul><ul><li>As the charge moves from point A to point B work is done, and the charge now has more potential energy </li></ul><ul><li>This can be represented by the equation below </li></ul><ul><li>The unit of potential difference is the Volt (V) which is a J/C </li></ul>
7. 7. Potential Difference in a Uniform Electric Field <ul><li> V = -Ed </li></ul><ul><li>The gap between electrodes in a spark plug is 0.060 cm. To produce an electric spark there must be an electric field of 3.0 x 10 6 V/m. What minimum potential difference must be applied to start a car? </li></ul>
8. 8. Potential Difference & Batteries <ul><li>The role of a battery is to provide that potential difference </li></ul>
9. 9. Capacitors and Charge Storage <ul><li>C = q/  V </li></ul><ul><li>Capacitor: ability </li></ul><ul><li>of a conductors </li></ul><ul><li>to store energy in the </li></ul><ul><li>form of electrically </li></ul><ul><li>separated charges. </li></ul>
10. 10. Capacitors & Batteries <ul><li>Capacitors are similar to batteries because they store electrical energy. </li></ul><ul><li>A battery can produce electrons, but a capacitor just stores them </li></ul><ul><li>Think of it like a water tower. </li></ul><ul><ul><li>A water tower &quot;stores&quot; water pressure -- when the water system pumps produce more water than a town needs, the excess is stored in the water tower. </li></ul></ul><ul><ul><li>At times of high demand, the excess water flows out of the tower to keep the pressure up. </li></ul></ul><ul><ul><li>A capacitor stores electrons in the same way and can then release them later. </li></ul></ul>
11. 11. <ul><li>The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge. </li></ul><ul><li>That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. </li></ul><ul><li>This can make a large, charged capacitor extremely dangerous -- flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can, potentially, kill you with the charge they contain. </li></ul>
12. 12. Capacitive Touch Screens <ul><li>One of the more futuristic applications of capacitors is the capacitive touch screen. These are glass screens that have a very thin, transparent metallic coating. </li></ul><ul><li>A built-in electrode pattern charges the screen so when touched, a current is drawn to the finger and creates a voltage drop. This exact location of the voltage drop is picked up by a controller and transmitted to a computer. </li></ul>
13. 13. Keeping all your vocab straight… <ul><li>In a 12 V car battery the </li></ul><ul><li>POTENTIAL DIFFERENCE </li></ul><ul><li>between the terminals is 12 V </li></ul><ul><ul><li>As a charge moves through the battery (from negative to positive) the field inside the battery does work to move the charge and the potential energy of the charge increases (from 0 to 12 volts) </li></ul></ul><ul><ul><li>Since a Volt is J/C a 1 C charge would now have 12J of electrical potential energy </li></ul></ul><ul><li>The ELECTRICAL POTENTIAL ENERGY of a charge changes as it moves through the circuit </li></ul><ul><ul><li>Increases as it moves through the battery </li></ul></ul><ul><ul><li>Decreases as it moves through external devices, they consume the 12 J of energy so when the charge returns to the battery it now has 0 J of potential energy. </li></ul></ul>Battery Stereo