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Chapter 21 The Electric Field

Chapter 21 The Electric Field. Topics. Coulomb’s Law Electric Field Motion in an Electric Field Electric Field Lines Electric Dipole Moment. O. Vectors (in 2D): Recap. Magnitude of vector:. Coulomb’s Law. q 1. q 2. Force of charge q 1 on q 2. Net force on charge q 0

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Chapter 21 The Electric Field

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  1. Chapter 21The Electric Field

  2. Topics • Coulomb’s Law • Electric Field • Motion in an Electric Field • Electric Field Lines • Electric Dipole Moment

  3. O Vectors (in 2D): Recap Magnitude of vector:

  4. Coulomb’s Law q1 q2 Force of charge q1 on q2

  5. Net force on charge q0 is sum of forces due to charges q1 and q2 force of q1 on q0 q0 + force of q2 on q0 + q1 - q2

  6. Net force on charge q0 is sum of forces due to charges q1 and q2 force of q1 on q0 q0 + force of q2 on q0 Net force

  7. Net force q0 +

  8. The Electric Field q0 The expression + suggests the concept of the electric field of a point charge where is the unit vector from charge q to charge q0.

  9. The electric field at a given point is just the sum of the electric fields due to every point charge - + + - - + +

  10. Motion in an Electric Field The total electric force on a charge q is which, together with Newton’s 2nd Law, can be used to calculate the motion of an electric charge, of mass m

  11. Motion in an Electric Field Newton’s 2nd Law for an electric charge can be written as If E is constant, both in direction and magnitude, so to is the acceleration Note that the acceleration depends on the charge to mass ratio.

  12. An ink-jet printer uses an electric field to alter the path of charged ink droplets

  13. Electric Field Lines A field line shows the direction of the electric force on a positive point charge

  14. Electric Field Lines

  15. Electric Field Lines

  16. The Electric Dipole a a P -q +q x The electric field at point P far away is

  17. The Electric Dipole a a P -q +q x Dipoles are so common in Nature that it turns out to be useful to define a new quantity called the electric dipole moment L is a vector of length 2a, from –q to +q

  18. Dipole in an Electric Field The +ve charge wants to move right, while the –ve charge wants to move left

  19. +q O Potential Energy of Dipole Electric Field

  20. +q O Potential Energy of Dipole Electric Field

  21. +q O Potential Energy of Dipole Electric Field where we have chosen U0= 0

  22. +q O Potential Energy of Dipole Electric Field For a dipole we must multiply by 2 to get the potential energy of the two charges

  23. H2O has a permanent dipole moment of about 40 e.pm

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