Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric potential energy Electric potential Conservation of energy Capacitors and Capacitance Chapter 21 Electric Potential Topics: Sample question: Shown is the electric potential measured on the surface of a patient. This potential is caused by electrical signals originating in the beating heart. Why does the potential have this pattern, and what do these measurements tell us about the heart’s condition?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Discussion of other units for Energy and E-field Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. eV – electron Volts => Unit of energy for particle accelerators The energy gained by an electron that goes through a potential difference of one volt 1 eV = 1.60 x J V/m – Volts per meter => Unit of Electric Field |Delta V| = |E||Delta r| => |E| = |Delta V| / |Delta r| [E] = V / m

Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. Slide ∆ V bat = =  W chem q ____

Parallel Plate Capacitor Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.

Parallel Plate Capacitor Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. (a) Parallel-plate capacitor connected to battery. (b) Battery and Capacitor in a circuit diagram. Relationship of E-field & Delta V? Delta V

Define Capacitance Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Capacitance is a measure of how much charge can be stored in a capacitor for a given amount of voltage

The Capacitance of a Parallel-Plate Capacitor Slide 21-31

Capacitance and Capacitors The charge ±Q on each electrode is proportional to the potential difference ΔV C between the electrodes: Slide 21-29

Charging a Capacitor Slide 21-30

Capacitors Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Note: Battery is a source of constant potential What happens when you pull the plates of a capacitor apart? With a Battery connected With no Battery connected Do the following quantities (a) increase, (b) decrease, or (c) remain the same: Charge E-Field Delta V

Dielectrics and Capacitors

The molecules in a dielectric tend to become oriented in a way that reduces the external field. This means that the electric field within the dielectric is less than it would be in air, allowing more charge to be stored for the same potential.

Dielectric Constant With a dielectric between its plates, the capacitance of a parallel-plate capacitor is increased by a factor of the dielectric constant κ: Dielectric strength is the maximum field a dielectric can experience without breaking down.

Energy stored in Capacitor – Storing Energy in E-field Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Energy stored in Capacitor – Storing Energy in E-field Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A charged capacitor stores electric energy; the energy stored is equal to the work done to charge the capacitor.

Storage of Electric Energy The energy density, defined as the energy per unit volume, is the same no matter the origin of the electric field: (17-11) The sudden discharge of electric energy can be harmful or fatal. Capacitors can retain their charge indefinitely even when disconnected from a voltage source – be careful!

Capacitors and Capacitance (Key Equations) Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Capacitance C = |Q| / |Delta V| Property of the conductors and the dielectric Special Case - Parallel Plate Capacitor C = Kappa * Epsilon 0 *A / d Energy Pe e = 1/2 |Q| |Delta V| |Delta V| = Ed

Properties of a Current Slide 22-8

Light the Bulb Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Can you light a bulb when you have 1 battery 1 Bulb 1 wire A - yes B - no

Definition of a Current Slide 22-9

Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. Slide ∆ V bat = =  W chem q ____

Storage of Electric Energy Heart defibrillators use electric discharge to “jump-start” the heart, and can save lives.

The Electrocardiogram (ECG or EKG) The electrocardiogram detects heart defects by measuring changes in potential on the surface of the heart.

Capacitors Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Note: Battery is a source of constant potential What happens when you insert a dielectric? With a Battery connected With no Battery connected Do the following quantities (a) increase, (b) decrease, or (c) remain the same: Charge E-Field Delta V Energy stored