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Electric Fields

Electric Fields. What is an Electric Field?. One charged object can influence another charged object without any direct contact.

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Electric Fields

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  1. Electric Fields

  2. What is an Electric Field? • One charged object can influence another charged object without any direct contact. • We say a charged object is surrounded by an electric field, a region of influence. Any other charged object in that space will interact with that field and experience an electrical force.

  3. Electric Field Strength • The electric field strength at a given location is defined as the amount of force per unit of charge at that location. E = F / q (units: N/C)

  4. Electric Field Strength • Consider a sphere with a charge of Q. A second charge q nearby would feel a force from the interaction with Q. From Coulombs Law… F = kQq / r2 E = F/q = kQq/r2 /q E = kQ/r2 q Q r

  5. Electric field Strength • E is a vector quantity • The direction of E is defined as the direction of force that would be felt by a positive test charge at the location in question

  6. Electric Field Lines • In a diagram, the vector nature of E can be represented by arrows that represent “field lines”

  7. Field around a positive charge

  8. Field around a negative charge

  9. Electric Field Lines • When drawing, or interpreting, electric field lines keep in mind the following… • Filed lines originate at + charge or infinity • Field lines terminate at – charge or infinity • Filed lines are always perpendicular to the surface of a charged object • Line density is an indicator of field strength • The number of lines leaving a + and terminating at a – is proportional to the magnitude of charge • Field lines NEVER cross each other (Why?)

  10. Field around two charges (same size)

  11. Field around two charges of different size

  12. Conductors in Electrostatic Equilibrium • The electric field is ZERO everywhere inside the conductor • Excess charge resides on the conductors outer surface • The electric field just outside the conductor is perpendicular to the conductor’s surface • For irregular shapes, charge tends to accumulate where radius of curvature is smaller (ie. sharp points)

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