Presentation on theme: "Electrostatics, Circuits, and Magnetism 4/29/2008"— Presentation transcript:
1Electrostatics, Circuits, and Magnetism 4/29/2008 AP Phys B Test ReviewElectrostatics, Circuits, and Magnetism4/29/2008
2Overview Electrostatics Electric Potential Dielectrics and Capacitance Electric CurrentDC CircuitsMagnetism
3ElectrostaticsCharge is carried by subatomic particles (protons, electrons)99% of all charged effects caused by electron transferCharging by ConductionPhysical contactCharging by InductionNo physical contact
4Coulomb’s LawThis law determines the force of attraction or repulsion between 2 charged objectse0 is a constant – permittivity of free spacePositive force = repulsive, negative force = attractiveRemember: force is a vector!
5Electric field lines A visual representation of an electric field. More lines = stringer forcePoint away from positive, toward negative.
6Electric Fields and conductors The electric field inside any conductor is zeroThe electric field is always perpendicular to the surface of a conductor
7Gauss’ LawElectric Flux: The amount of an electric field passing through an areaGauss’ Law: The total electric flux passing through a closed surface is proportional to the charged enclosed in that surface.
8Electric Potential Energy Electric Potential energy can be determined using mechanicsElectric potential is defined as the electric potential energy per unit charge
9Equipotential 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!
10Potential due to a point charge Remember: potential is a scalar!
11Capacitance A capacitor is a device that stores electric charge. The capacitance of an object is defined as:Capacitance is measured in farads.
12Parallel 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 capacitanceDielectric constant: measures the strength of the dielectric
13Capacitors and energyA charged capacitor stores an amount of electric energy given byThis energy can be thought of as stored in the electric field between the plates.
14Electric CurrentElectric current is defined as the amount of charge that flows past a given point in a second
15Ohm’s LawOhm’s Law related the resistance of an object to the decrease in electric potential across a point and the current flowing through that point.
16Electric ResistanceElectric 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.
17Circuits – 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 resistanceThe real voltage of a battery is then
18Resistors in series Voltage and resistance are additive Current is constant everywhere in a series circuit
19Resistors in parallel Current additive Voltage is constant everywhere in a series circuitMore resistors = smaller equivalent resistance
21Kirchhoff’s rulesJunction 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.
23Magnetism Every magnet has two poles: north and south Magnetic field & magnetic field lines: analogous to electric fieldDirection: points north to southElectric current (moving charge) produces a magnetic field!
24Force due to magnetic fields The force on a charged particle moving through a magnetic fieldThe force in a current carrying wire immersed in a magnetic field
26Ampere’s Law A moving charge (current) creates a magnetic field. For a long wire, Dl = 2prTwo wires can attract or repel due to this effect.A solenoid is a long coil of wire.
27Faraday’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.