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Chapter 16

Chapter 16. Electric Energy and Capacitance. Energy Stored in a Capacitor. Energy stored = ½ Q ΔV From the definition of capacitance, this can be rewritten in different forms. Applications. Defibrillators

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Chapter 16

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  1. Chapter 16 Electric Energy and Capacitance

  2. Energy Stored in a Capacitor • Energy stored = ½ Q ΔV • From the definition of capacitance, this can be rewritten in different forms

  3. Applications • Defibrillators • When cardiac fibrillation occurs (in heart attack victims), the heart produces a rapid, irregular pattern of beats • A fast discharge of electrical energy through the heart can return the organ to its normal beat pattern • In general, capacitors act as energy reservoirs that can slowly charged and then discharged quickly to provide large amounts of energy in a short pulse

  4. Example 16.10 – page 556 • A fully charged defibrillator contains 1.2kJ of energy stored in a 1.1x10-4F capacitor. In a discharge through a patient, 600J of electrical energy are delivered in 2.50ms. • A) find the voltage needed to store 1.2kJ in the unit. • B) what average power is delivered to the patient?

  5. Capacitors with Dielectrics • A dielectric is an insulating material that, when placed between the plates of a capacitor, increases the capacitance • Dielectrics include rubber, plastic, or waxed paper • C = κCo = κεo(A/d)=KQ0/∆V0= Q0/∆V • Relative voltage is lower: ∆V= ∆V0/K; • Q0=does not change  store larger amounts of charge C=KC0 • The capacitance is multiplied by the dielectric constant κ (K > 1) when the dielectric completely fills the region between the plates

  6. Capacitors with Dielectrics

  7. Dielectric Strength • For any given plate separation, there is a maximum electric field that can be produced in the dielectric before it breaks down and begins to conduct • This maximum electric field is called the dielectric strength • Table 16.1 – page 558 – gives dielectric constants and dielectric strngths of various materials at room temperature.

  8. An Atomic Description of Dielectrics • Polarization occurs when there is a separation between the “centers of gravity” of its negative charge and its positive charge • In a capacitor, the dielectric becomes polarized because it is in an electric field that exists between the plates

  9. More Atomic Description • The presence of the positive charge on the dielectric effectively reduces some of the negative charge on the metal • This allows more negative charge on the plates for a given applied voltage • The capacitance increases

  10. Example 16.11 – page 560 • A parallel-plate capacitor has plates 2cm by 3cm. The plates are separated by a 1mm thickness of paper. Find • A) the capacitance of this device • B) the maximum charge that can be placed on the capacitor (use dielectric strength from table 16.1) • C) remove dielectric: find electric field across the capacitor.

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