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Capacitors

Honors Physics Power-Point 6-5. Capacitors. Capacitors store electric charge and decay through exponential means. They are made out of two parallel plates with a material (typically waxed paper) between them. This material is called a dielectric. What is a capacitor?.

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Capacitors

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  1. Honors Physics Power-Point 6-5 Capacitors

  2. Capacitors store electric charge and decay through exponential means. They are made out of two parallel plates with a material (typically waxed paper) between them. This material is called a dielectric. What is a capacitor?

  3. It all began with the Leyden jar (demonstrated earlier). It is now used in computers, radios, televisions, and camera flashes. Applications of capacitance

  4. It is the charge stored per unit voltage. It is measured in a unit called a Farad. (It is named after Michael Faraday (1791). The definition of capacitance

  5. A 12.0 V battery is connected to a parallel plate capacitor which is rated at 1.0 micro-Farads. The plates are separated by 0.00600 m of dry air. • What is the charge on each plate? • What is the electric field exactly between the plates? Example #1: use of definitions

  6. The relative permittivity (K) indicates the degree to which a capacitor can hold its charge. Factors affecting capacitance

  7. A parallel plate capacitor consists of two metal plates separated by 0.00600 m of dry air. The capacitor is connected to a 100 V source. The area of each plate is 0.0400 m2. Example 2: factors affecting capacitance Where k= relative permittivity (k=1 here) The permittivity constant is 8.85 X 10-12 What is the value of the capacitance? What happens when the area of the plates is increased? What happens when the distance between the plates is increased?

  8. To derive expressions for energy stored in a capacitor, we must appeal to integral calculus. (We only need to use the results here!) Example 3: relationship to energy

  9. A 1.0 pF capacitor is hooked up to a 9.0 V battery. (a) What is the charge stored? (b) What Is the energy stored? EXAMPLE 3: energy in a capacitor

  10. Derive the exponential decay equation for Voltage in a capacitor. Example 4: Capacitor decay

  11. The constant k is the time constant for a circuit and is equal to 1/RC (proven through differential equations—well beyond the scope of the course. Expression for exponential decay

  12. A 9 V battery is connected to a 100 kilo-ohm resistor and a 1 micro-Farad Capacitor. (a) What is the time constant for this circuit? (b) What is the time that half the value of the voltage decays? Example 4: Capacitor decay

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