Chapter 18: Electrical Energy and Capacitance Physics Chapter 18: Electrical Energy and Capacitance

Electrical Energy and Capacitance Electric Field Surrounds All Charged Particles Electric Force Applied by an Electric Field

Electrical Energy and Capacitance Electrical Potential Energy (PE) Electrical Force Can Move Charged Particles Movement of a Mass Over a Distance is Work Electrical Force Does Work in the Form of Electrical Potential Energy

Electrical Energy and Capacitance Electrical Potential Energy (PE) Uniform Electric Field How about something other than particles?

Electrical Energy and Capacitance Electrical Potential Energy (PE) Uniform Electric Field + + + + + + + + + + + + + + + + Charged Plates - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electrical Potential Energy (PE) Uniform Electric Field + + + + + + + + + + + + + + + + Charged Plates Direction of E Field - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electrical Potential Energy (PE) Uniform Electric Field + + + + + + + + + + + + + + + + Charged Plates +q0i E Field - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electrical Potential Energy (PE) Uniform Electric Field + + + + + + + + + + + + + + + + Charged Plates +q0i Displacement Due to Applied Force E Field +q0f - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electrical Potential Energy (PE) In the Same Way that Work is done by Gravitational Force…Wg=DPEg Electric Force does Work as a Force Vector on a Test Charge (q0)

Electrical Energy and Capacitance Electrical Potential Energy (PE) PE Decreases for a + Charge PE Increases for a - Charge

Electrical Energy and Capacitance Electrical Potential Energy (PE) Also Applies to Displacement of Charged particles Due to Other Charged Particles

Electrical Energy and Capacitance Electrical Potential Energy (PE) Units Energy, so… Joules

Electrical Energy and Capacitance Electric Potential Difference (V) As the Positive Charge Moves from the Positive Plate to the Negative Plate the PE Decreases At Any Point in the Field, the PE will Increase with Increased Charge + + + + + + + + + + + + + + + + +q0i +q0f - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference (V) This Potential Difference is Measured in Joules/Coulomb (Energy/Charge) The Units of Potential Difference are Volts (V) + + + + + + + + + + + + + + + + +q0i +q0f - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference (V) + + + + + + + + + + + + + + + + +q0i +q0f - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference (V) + + + + + + + + + + + + + + + + +q0i +q0f - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Problem “PE Difference” q0 = 3x10-9C Wa-b = 6x10-8J What is DEPE? What is DV? ++++++++++++++++ q0a E q0b - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Solution “PE Difference” q0 = 3x10-9C Wa-b = 6x10-8J What is DEPE? DEPE = -Wa-b = -6x10-8J ++++++++++++++++ q0a E q0b - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Solution “PE Difference” q0 = 3x10-9C Wa-b = 6x10-8J What is DV? ++++++++++++++++ q0a E q0b - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Note in the Previous Problem q0 had a Positive Charge A Positive Charge Moves from Higher PE to Lower PE A Negative Charge Moves from Lower PE to Higher PE ++++++++++++++++ ++++++++++++++++ -q0b q0a E E -q0a q0b - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Problem “The Light Will Come On” Electron Particle: q1 = 1.6x10-19C System Voltage: V = 12 J/C System Power (Light) = 40 W Time = 1 minute How Many Electrons Will Pass, and in Which Direction? ++++++++++++++++ -q0 E -q0 - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Solution “The Light Will Burn” Electron Particle: q1 = 1.6x10-19C System Voltage: V = 12 J/C System Power (Light) = 40 W Time = 1 minute Energy = Power x Time DEPE = 40W x 60s = 2400 w/s (J) q0 = DEPE/DV = 2400J/12V = 2x102C ++++++++++++++++ -q0 E -q0 - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Solution “The Light Will Burn” q0 = DEPE/DV = 2400J/12V = 2x102C Total Particles: q0 / q (-e) = (2x102C) / (1.6x10-19C) = 1.3x1021 Direction? “A Negative Charge Moves from Lower PE to Higher PE” ++++++++++++++++ -q0b E -q0a - - - - - - - - - - - - - - - -

Electrical Energy and Capacitance Electric Potential Difference Energy, The Big Picture Translational KE Rotational KE Gravitational PE Elastic PE Electric PE

Electrical Energy and Capacitance Electric Potential Difference Problem: “Energy Conserved?” Particle Moves from Point “a” to Point “b” by Electric Force Mass = 3.6x10-3kg Charge (q0) = 6x10-3C DV = 25V V0 = 0 What is the Particle Speed at Point “b”? + - a b

Electrical Energy and Capacitance Electric Potential Difference Solution: “Energy Conserved?” Mass = 3.6x10-3kg Charge (q0) = 6x10-3C DV = 25V V0 = 0 Rotation = 0 Elasticity = 0 Enet = Translational KE + Gravitational PE + EPE

Electrical Energy and Capacitance Electric Potential Difference Solution: “Energy Conserved?” Mass = 3.6x10-3kg Charge (q0) = 6x10-3C DV = 25V V0 = 0 ha = hb (Horizontal Movement Only) EPEa – EPEb = q0(va-vb) So…

Electrical Energy and Capacitance Electric Potential Difference Solution: “Energy Conserved?” Mass = 3.6x10-3kg Charge (q0) = 6x10-3C DV = 25V V0 = 0

Electrical Energy and Capacitance Electric Potential Difference Solution: “Energy Conserved?” Mass = 3.6x10-3kg Charge (q0) = 6x10-3C DV = 25V V0 = 0

Electrical Energy and Capacitance Homework Pages 683-685 Problems: 12 (2.6x104V) 13 (-154V) 32 (4000V/m)

Electrical Energy and Capacitance Capacitors Two Conductors Placed Near Each Other but Not Touching The Area Between the Conductors is Filled With an Insulating Material (Dielectric)

Electrical Energy and Capacitance Capacitors Dielectric Contains Dipolar Molecules + Charge on One End Charge on the Other End E + + + + + + + Dielectric Material -

Electrical Energy and Capacitance Capacitors Dielectric Each Molecule Accepts Charge (q) E + + + + + + + Dielectric Material - - + - +

Electrical Energy and Capacitance Capacitors Dielectric Each Molecule Accepts Charge (q) Entire Dielectric Material Becomes Charged E + + + + + + + Dielectric Material - - +

Electrical Energy and Capacitance Capacitors Store Electric Charge Positive and Negative Terminals Carry the Same Charge Magnitude

Electrical Energy and Capacitance Capacitors Capacitance (C) The Ability of the Capacitor to Store Charge E + + + + + + + Dielectric Material -

Electrical Energy and Capacitance Capacitors Units coulomb/volt Farad (after Michael Faraday) E + + + + + + + Dielectric Material -

Electrical Energy and Capacitance Varies with Type, Size, and Shape Parallel Plate Permittivity of Free Space 8.85x10-12C2/Nm2 E + + + + + + + Dielectric Material -

Electrical Energy and Capacitance Capacitors Due to the Charged Molecules of the Dielectric, the Electric Field (E) is Reduced E + + + + + + + Dielectric Material - - +

Electrical Energy and Capacitance Capacitors and Electric PE Stored Charge is Stored Electric Potential Energy E + + + + + + + Dielectric Material - - +

Electrical Energy and Capacitance Capacitors and Electric PE Stored Charge is Stored Electric Potential Energy

Electrical Energy and Capacitance Homework Pages 684-686 Problems: 26 (7.2x10-11C) 27 (a, 1.3x10-3C b, 4.2J) 43 (4x10-6F)