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ap-physics-b-review-electromagnetism

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ap-physics-b-review-electromagnetism

  1. 1. AP Phys B Test Review Electrostatics, Circuits, and Magnetism 4/29/2008
  2. 2. Overview  Electrostatics  Electric Potential  Dielectrics and Capacitance  Electric Current  DC Circuits  Magnetism
  3. 3. Electrostatics  Charge is carried by subatomic particles (protons, electrons) • 99% of all charged effects caused by electron transfer  Charging by Conduction • Physical contact  Charging by Induction • No physical contact
  4. 4. Coulomb’s Law  This law determines the force of attraction or repulsion between 2 charged objects • ε0 is a constant – permittivity of free space • Positive force = repulsive, negative force = attractive • Remember: force is a vector! F Q Q r q = 1 4 0 1 2 2π ε
  5. 5. Electric field lines  A visual representation of an electric field. • More lines = stringer force • Point away from positive, toward negative.
  6. 6. Electric Fields and conductors  The electric field inside any conductor is zero  The electric field is always perpendicular to the surface of a conductor
  7. 7. Gauss’ Law  Electric Flux: The amount of an electric field passing through an area  Gauss’ Law: The total electric flux passing through a closed surface is proportional to the charged enclosed in that surface. Φ = Q e n c l o s e d ε 0 Φ = E A c o s θ
  8. 8. Electric Potential Energy  Electric Potential energy can be determined using mechanics  Electric potential is defined as the electric potential energy per unit charge ∆ U q E d= − V U q W q = = − ∆ ∆U q V= −
  9. 9. Equipotential lines or surfaces  An equipotential surface is a surface over which all points have the same potential. • An equipotential surface must be perpendicular to the electric field!
  10. 10. Potential due to a point charge V Q r = 1 4 0π ε • Remember: potential is a scalar!
  11. 11. Capacitance  A capacitor is a device that stores electric charge.  The capacitance of an object is defined as:  Capacitance is measured in farads. C Q V =
  12. 12. Parallel plate capacitors and dielectrics  For a parallel plate capacitor (two conducting plates with a vacuum between the plates)  Often, an insulator known as a dielectric is placed between the plates to enhance capacitance • Dielectric constant: measures the strength of the dielectric C A d = ε 0
  13. 13. Capacitors and energy  A charged capacitor stores an amount of electric energy given by • This energy can be thought of as stored in the electric field between the plates. U Q V= 1 2 2
  14. 14. Electric Current  Electric current is defined as the amount of charge that flows past a given point in a second
  15. 15. Ohm’s Law  Ohm’s Law related the resistance of an object to the decrease in electric potential across a point and the current flowing through that point. R V I =
  16. 16. Electric Resistance  Electric resistance is the innate ability of a material to inhibit the passage of electrons. • Measured in ohms. • Given by the resistivity as well as the geometry of the object. R L A = ρ
  17. 17. Circuits – emf and terminal voltage  A device that transforms one type of energy into electrical energy is a “source of electromotive force” • emf: the potential difference between the terminals of a battery when there is no current flowing to an external source. • A battery has some internal resistance • The real voltage of a battery is then V E I r= −
  18. 18. Resistors in series  Voltage and resistance are additive  Current is constant everywhere in a series circuit R Re q i i = ∑ V Vt o t a l i i = ∑ I I It o t a l = = =1 2 . . .
  19. 19. Resistors in parallel  Current additive  Voltage is constant everywhere in a series circuit  More resistors = smaller equivalent resistance 1 1 R Re q ii = ∑I It o t a l i i = ∑ V V Vt o t a l = = =1 2 . . .
  20. 20. Complex Circuits
  21. 21. Kirchhoff’s rules  Junction rule: At any junction point, the total current into the junction has to be equal to the total current out of the junction.  Loop rule: The sum of changes in potential around and closed loop is zero.
  22. 22. Kirchhoff’s Rules
  23. 23. Magnetism  Every magnet has two poles: north and south  Magnetic field & magnetic field lines: analogous to electric field • Direction: points north to south  Electric current (moving charge) produces a magnetic field!
  24. 24. Force due to magnetic fields  The force on a charged particle moving through a magnetic field  The force in a current carrying wire immersed in a magnetic field F q v B= s i n θ F I L B= s i n θ
  25. 25. Right hand rule
  26. 26. Ampere’s Law  A moving charge (current) creates a magnetic field. • For a long wire, ∆l = 2πr • Two wires can attract or repel due to this effect. • A solenoid is a long coil of wire. B l Ii i e n c l o s e d∆∑ = µ 0
  27. 27. Faraday’s Law  A changing magnetic field induced an emf. • A current produced by an induced emf moves in a direction such that its magnetic field opposes the original change in flux (Lenz’s Law) • A coil rotating in a magnetic field is a good example of this. E N t = − ∆ Φ ∆

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