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Understanding Electric Fields: Definitions, Forces, and Energy

Learn about electric fields, forces in space, Coulomb's law, charges and capacitors, energy units in coulombs, volts, and joules, and how to calculate voltage, potential energy, and work in electrostatics.

marjean-granahan
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Understanding Electric Fields: Definitions, Forces, and Energy

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  1. Stream lines

  2. front plate slightly chargedinduces opposite charge on back plate.Brushes pull off chargescharges collected in leyden jar (capacitor) Wimshurst Machine

  3. Electric Field

  4. Definition • Electric field is the strength & direction of the forces in space surrounding a positive test charge

  5. Electric field as coulomb’s law • F = q1 * (k q2 /d2) rewrite the force equation F = q1 * E q1 is the positive test charge E is the field created by charge(s) q2

  6. Top tips on electric fieldE=0 inside a metal (faraday) cageField lines go from + to – chargesCloser field lines are stronger

  7. A B - C + Electric Fields Electric Field between 2 unlike charges Note that the distance between electric field lines at C is shorter than that at B. Distance between electric field lines at B is shorter than at A. Electrostatics

  8. Electric Fields Electric Field between 2 charged plates positive negative Electrostatics

  9. + + + + + + + + - - - - - - - - Electric Fields Electric Field between 2 charged plates Electrostatics

  10. coulombs, joules, volts • Coulomb= unit of charge • Joule = unit of energy • Volt= unit of energy per charge

  11. coulomb • How many charges are in 1C? coulomb? • 1C is about 6 billion billion charges)(1/1.6 x 10-19 = 6.25 x 1018) • The man, the law Charles Coulomb~1750

  12. Joules of energy • Charges have a type of energy called electron potential energy (PEe or U) High energy: + is close to +: ++ or + is far from –: + -

  13. It takes work to move charges against (opposite) an electric field High energy work needed to move + charge (W = F*d) E F + low energy

  14. (a) When a positive charge moves in the direction of an electric field, the field does positive work and the potential energy decreases. Work = qo E d POSITIVE charge moving in an E field.

  15. Volts is an energy density • Voltage is also called potential • 1 volt = 1 joule / 1 coulomb • Example: 12 V battery: every coulomb of charge has 12 joules of energy

  16. Examples • Static balloon 9 V battery (1hr,1A) 1 joule of energy 9x104 joule energy 0.001 C of charge 1x104C of charge 1/.0001 = 1000 volts 9/1 = 9 Volts

  17. Another c,j,v example • Van de graaf static generator has.. 1,000,00 volts (high) 1 joule of energy (low) .00001 coulombs of charge 1,000,000 v = 1 j/ .00001c

  18. Uniform electric field 2 situations Point Charges Force: F=q*E Force= k q1*q2 / d2 Electric potential energy Electric potential energy PE= F*d = q*E*d PE = F*d = k q1*q2 / d Voltage (electric potential)Voltage (electric potential) V= PE/q1 = q*E*d /q = E*d V= PE/q1 = k q2 / d2 Voltage difference: Voltage difference: DVe = D PE/q1 = E* Dd DVe = D PE/q1 = k q2 / d2 - kq2/ d1

  19. It takes work to move + charge against static forces like it takes work to lift against gravity • PEfinal • +++++ • + • Peiniitial work = gain in PE = F*d work

  20. Electrostaticsgravitational Force: F= kq1 q2 / d2 F= Gm1m2/d2 Field E= F/q1 = kq2/d2 g = F/m1 = Gm1/d2 potential energy PE= F*d = kq1 q2 / d PE = F*d= Gm1m2/d (using field) =qE*d = mg*h Potential V= PE/q1 = kq2 / d U= PE/m1 = G m2 / d (Using field) = E*d = g*h

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