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Electric Field Lines

Electric Field Lines. Drawing electric field lines Motion of charged particles in an electric field Electric flux. Electric Field Lines. Electric field lines are a way of visualizing the field. Rules for Drawing field lines:

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Electric Field Lines

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  1. Electric Field Lines • Drawing electric field lines • Motion of charged particles in an electric field • Electric flux

  2. Electric Field Lines Electric field lines are a way of visualizing the field. • Rules for Drawing field lines: • Lines start on (+) charges, end on (-) charges, or go to infinity • (# of lines)  charge • Lines never cross • Strength of field is proportional to the density • of field lines Interpreting the picture: is parallel to the field line at each point.

  3. Electric Field lines for an isolated Charge +Q E E – magnitude of field E + Q

  4. Field lines + Area S, Where is the density of field lines greatest?

  5. 2 Point Charges • Note: • number of lines on -2Q is twice as many as on +Q +Q -2Q

  6. Quiz: Which way will the dipole start to move in the electric field? -q +q • up • down • left • right • nowhere – there is no net force.

  7. Quiz: Does this dipole feel atorque ? -q +q • Yes - clockwise • Yes – counter clockwise • No • Depends on the strength of E

  8. Parallel Charged Plates + + + + + + + + - - - - - - - E approx. uniform, between the plates, except near the edges.

  9. Electric Force Therefore we can solve for motion as easily as projectile motion!

  10. Example: Uniform E An electron enters a uniform field of E = -200N/C j with an initial velocity of vo = 3x106 m/s i. Find: a) The acceleration of the electronb) The time it takes to travel through the region of the fieldc) The vertical displacement of the electron while in the field 0.1m -

  11. Solution:

  12. Electric Flux Electric flux is the measure of the “number of field lines passing through a surface S ” For uniform : Define: Electric Flux Units: N•m2/C S A is the surfacearea perpendicularto S, so Φ=EAcos(θ)

  13. Notes: • is a scalar called electric flux • Units: N•m2/C • represents the “number of field lines through surface S.” • For a closed surface, the area vector points • in the outward direction. • 5) Flux is zero for a surface parallel to the field(normal is at 90o to E)

  14. S2 S1 S3 30° (rectangle, 1m x 2m) (rectangle, 1m x 2m) (hemisphere, radius 1m) Example: Find: flux through S1, S2, S3.

  15. solution

  16. If E is not uniform, or S is not flat, then: For a small surface For the whole surface,

  17. Summary • Electric field lines help show the direction of E • Electric flux is defined as the magnitude of the field times the area (maybe negative if the angle between the vectors is more than 90 degrees) • Electric flux is a quantitative equivalent to “the number of field lines through a surface”.

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