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Electrostatics

Electrostatics. Fields Refresher Electrical Potential Potential Difference Potential Blame it on the old folks. Electrical Field. Maxwell developed fields Electric fields exist in the space around charged objects

jeanine-sheehan
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Electrostatics

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  1. Electrostatics Fields Refresher Electrical Potential Potential Difference Potential Blame it on the old folks.

  2. Electrical Field • Maxwell developed fields • Electric fields exist in the space around charged objects • When other charged object enters this electric field, the field exerts a force on the second charged object

  3. Problem Solving Strategy • Draw a diagram of the charges in the problem • Identify the charge of interest • You may want to circle it • Units – Convert all units to SI • Need to be consistent with ke

  4. Electric Field Lines • Electric Field patterns - draw lines in direction of field vector at any point • These are called electric field lines and were introduced by Michael Faraday

  5. Rules for Drawing Electric Field Lines • The lines for a group of charges must begin on positive charges and end on negative charges • In the case of an excess of charge, some lines will begin or end infinitely far away • The number of lines drawn leaving a positive charge or ending on a negative charge is proportional to the magnitude of the charge • No two field lines can cross each other

  6. E Field Lines • Draw E field for a Large Positive Charge • Draw E field for small Positive Charge • Draw E field for Large Neg Charge • Draw E field for small Neg Charge. • Draw E Field for a Dipole (1 pos near 1 neg)

  7. Electric Field Line Patterns • Point charge • The lines radiate equally in all directions • For a positive source charge, the lines will radiate outward

  8. Electric Field Line Patterns • For a negative source charge, the lines will point inward

  9. Electric Field Line Patterns • An electric dipole consists of two equal and opposite charges • The high density of lines between the charges indicates the strong electric field in this region

  10. E Fields • Draw E Field for two + Charges

  11. Electric Field Line Patterns • Two equal but like point charges • At a great distance from the charges, the field would be approximately that of a single charge of 2q • The bulging out of the field lines between the charges indicates the repulsion between the charges • The low field lines between the charges indicates a weak field in this region

  12. E Fields • Draw E Fields for Large +Q and small -q

  13. Electric Field Patterns • Unequal and unlike charges • Note that two lines leave the +2q charge for each line that terminates on -q

  14. Fields Refresher • Start on + or inifinity • End on – or infinity • # field linesmagnitude of charge or field • Dipole = two opposite charges • Fields are everywhere • Fields do not affect everthing.

  15. Fields In Conductors Refresher • Equilibrium Conditions: • ALL excess charge moves to outer surface • E is zero within the conductor • E on surface MUST be  to surface

  16. E Field in Conductor Shielding:

  17. Equipotential Surfaces • Electric Potential is the same at all pts. on surface • WAD=? • WAB =? • E field  Equipotentials

  18. Electric Potential Energy • Fe is a conservative force (?) • Fe can make electrical potential energy • Fe  Work is Independent of Path • WFe = - PE

  19. PE from Fields • Compare to Gravity • PEg=magdy • PE of earth & mass system • PEe=qEd • PE of q & E field System • PEg = PEgo +magdy • often choose PEgo = 0 • PEe=PEeo + qEd

  20. PE from pt Charges Important Note: This relationship for PE is ONLY for PE due to point charges. THIS DOES NOT WORK FOR FIELDS. VanDeGraff & Fluorescent Bulb

  21. Potential Energy

  22. Potential Energy & Pt Charges • Sketch the E field vectors inside the capacitor • Sketch the F acting on each charge • Choose a spot for PEe=0 & Label it. • Is the PE of the + charge +, -, 0

  23. Work and Potential Energy • E is uniform btn plates • q moves from A to B • work is done on q • Won q = Fd=qEx x • ΔPE = - W = - q Exx • only for a uniform field

  24. Electric Potential & Pt Charge • In which direction (rt, lft, up, down) does the PE of the + charge decrease? Explain. • In which direction will the + charge move if released from rest? Explain. • Does your last answer agree with the F drawn earlier?

  25. Potential Difference • Voltage = Potential = Electrical Potential • V=PE/q • V measured in ---? • Within E, different PE at Different Pts. • V=VB-VA Potential Difference • V= PE/q • V = qE  d/q • V = E  d A + B Think about the VanDeGraff demo

  26. Electric Potential of a Point Charge • PEe=0 as r • The potential created by a point charge q at any distance r from the charge is • A potential exists w/ or w/o a test charge at that point

  27. Electric Potential of Multiple Point Charges • Superposition principle applies • Is PEe a vector or a scalar? • The total electric potential at some point P due to several point charges is the algebraic/vectoric? sum of the electric potentials due to the individual charges

  28. Energy and Charge Movements, cont • When the electric field is directed downward, point B is at a higher or lower potential? than point A • A positive test charge that moves from A to B gains/loses? electric potential energy • It will gain/lose? the same amount of kinetic energy as it loses in potential energy

  29. Energy and Charge Movements • A positive charge gains electrical potential energy when it is moved in a direction opposite the electric field • If a charge is released in the electric field, it experiences a force and accelerates, gaining kinetic energy • As it gains kinetic energy, it loses an equal amount of electrical potential energy • A negative charge loses electrical potential energy when it moves in the direction opposite the electric field

  30. Potentials in Practice • Rank the points from largest potential (V) to smallest.

  31. Electrical Potential Energy of Two Charges • V1 is the electric potential due to q1 at point P • The work required to bring q2 from infinity to P without acceleration is q2V1 • This work is equal to the potential energy of the two particle system

  32. Problem Solving with Electric Potential (Point Charges) • Draw a diagram of all charges • Note the point of interest • Calculate the distance from each charge to the point of interest • Use the basic equation V = keq/r • Include the sign • The potential is positive if the charge is positive and negative if the charge is negative

  33. Problem Solving with Electric Potential, cont • Use the superposition principle when you have multiple charges • Take the algebraic sum • Remember that potential is a scalar quantity • So no components to worry about

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