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Potential Difference & Capacitance

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  • 1. Potential Difference & Capacitance March 9th, 2009
  • 2. Electrical Potential
    • The electrical potential energy associated with a charge particle, divided by the charge of the particle
    • V = PE electric / q
  • 3. Electrical Potential vs. Gravitational Potential
    • It might help you understand electrical potential to think about it like gravitational potential
    • When does an object have high gravitational potential?
    • Where do objects like to come to rest? With high potential energy or low potential energy?
    • It requires energy to move an object from an area of low potential energy to an area of high potential energy
  • 4. Electrical Potential
    • The same is true for charged particles
    • Think about a positive field, where will a positive test charge have have the highest potential energy? Lowest potential energy?
    • HINT: Is this force attractive like gravity???
  • 5. Think you got it? Try these!
    • For each picture determine:
      • If work is being done on the charge
      • Where the Electric potential is greatest
  • 6. Potential Difference
    • Think about this picture:
    • As the charge moves from point A to point B work is done, and the charge now has more potential energy
    • This can be represented by the equation below
    • The unit of potential difference is the Volt (V) which is a J/C
  • 7. Potential Difference in a Uniform Electric Field
    •  V = -Ed
    • 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?
  • 8. Potential Difference & Batteries
    • The role of a battery is to provide that potential difference
  • 9. Capacitors and Charge Storage
    • C = q/  V
    • Capacitor: ability
    • of a conductors
    • to store energy in the
    • form of electrically
    • separated charges.
  • 10. Capacitors & Batteries
    • Capacitors are similar to batteries because they store electrical energy.
    • A battery can produce electrons, but a capacitor just stores them
    • Think of it like a water tower.
      • A water tower "stores" water pressure -- when the water system pumps produce more water than a town needs, the excess is stored in the water tower.
      • At times of high demand, the excess water flows out of the tower to keep the pressure up.
      • A capacitor stores electrons in the same way and can then release them later.
  • 11.
    • 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.
    • 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.
    • 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.
  • 12. Capacitive Touch Screens
    • 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.
    • 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.
  • 13. Keeping all your vocab straight…
    • In a 12 V car battery the
    • POTENTIAL DIFFERENCE
    • between the terminals is 12 V
      • 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)
      • Since a Volt is J/C a 1 C charge would now have 12J of electrical potential energy
    • The ELECTRICAL POTENTIAL ENERGY of a charge changes as it moves through the circuit
      • Increases as it moves through the battery
      • 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.
    Battery Stereo