ECE747 Chapter 2

ECE747 Chapter 2 - 1/15/2010 Chapter 2 Electric and...

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1/15/2010 1 J. Wang 2010-1-15 1 Chapter 2 • Electric field basics and examples • Magnetic field examples • Electric field distribution • Electric field measurement Electric and Magnetic Fields Electric Field J. Wang 2010-1-15 2 Coulomb’s Law (1785) Inspired by Newton, derived from experimental results, foundation of electric engineering 12 3 12 0 2 1 12 4 r r q q F r r πε = Electric field: a vector field defined by Coulomb Newton r r Q r q r Qq q F E r r r r 3 0 3 0 4 4 = = = i n i i i n r r Q E E E E r r L r r r = = + + + = 1 3 0 2 1 4 1
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1/15/2010 2 Definitions J. Wang 2010-1-15 3 Electric field: A vector field of electric strength, , or of electric flux density, Electric field strength: At a given point in space, the ratio of force on a positive test charge placed at the point, P, to the magnitude of the test charge. E r D r IEEE STD 539 Electric Potential J. Wang 2010-1-15 4 The work is done when a unit charge is moved from point ‘1’ to point ‘2’ by electric force. ) 1 1 ( 4 2 1 0 1 1 1 2 1 2 r r Q q r d E q r d F W r r r r - = - = - = πε r r r r Electric potential difference is defined as: ) 1 1 ( 4 2 1 0 1 12 1 2 r r Q r d E q W V r r - = - = = r r Potential rise is always in the opposite direction of electric field strength. E V r r - = General equation for relationship between voltage and electric field strength ) ( z k y j x i + + =
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1/15/2010 3 Dielectric Stress and Strength J. Wang 2010-1-15 5 Dielectric Stress: Electric field strength (or voltage gradient) applied Dielectric Strength: It is the maximum stress a material can withstand without disruptive discharge Breakdown types: Spark over: between electrodes in a gas or liquid Flashover: over surface of a solid insulation in a gas or liquid Puncture: through solid insulation (permanent damage) Disruptive discharge: Completely bridges the insulation under test, reducing the voltage between electrodes to practically zero Disruptive discharge IEEE STD 4-1995 Facts Sheet J. Wang 2010-1-15 6 Permittivity of free space: 12 9 0 10 85 . 8 36 10 - - × = = π ε F/m Permittivity: 0 = r Relative permittivity: Air: 1 Styrofoam: 1.03 Wood: 2 Acrylic: 3.4 Glass, mica: 6 Distilled Water: 80 Air: 30 Oil: 150 Glass: 300 SF 6 : 75 Kapton ®: 2120 Mica: 2000 Dielectric strength (kV/cm peak):
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1/15/2010 4 Electric Field Strength Calculation: Simple Cases J. Wang 2010-1-15 7 Methods: 1. Method of images A charge (+Q) and a charged surface can be substituted by the original charge and its image charge, with respect to the surface in
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ECE747 Chapter 2 - 1/15/2010 Chapter 2 Electric and...

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