Chapters 17 through 23 Midterm Review

Midterm Exam ~ 1 hr, in class 15 questions 6 calculation questions One from each chapter with Ch. 17 and 18 combine Require use of calculator 9 conceptual questions One from each chapter plus some extra Don’t need calculator, just need brain

Conceptual vs. Calculation This is a calculation question: “Given that a bulb is a 2 meters away, how long does it take light from the bulb to reach me?” This is a conceptual question: “If the electric field is pointing up and the magnetic field is pointing right, which direction is the EM wave moving?”

Not on Exam What NOT to Study! Also Potential to Kinetic Energy, and vice versa Electric or Magnetic Energy density Complicated 3-Branch circuits Torque Intensity and Polarization

Charge and Field Lines

Electric Flux and Gauss’s Law Electric flux measures electric field penetrating any surface Gauss’s law gives an easy way to calculate electric flux through a closed surface

Conductors vs. Insulators Conductors Charge is free to move around Interior is shielded In equilibrium, E in = 0 Most metals are conductors Insulators Charge stays where it is placed

Polarization and Induction Polarization Charges in material align with external electric field Object remains with no net charge Occurs in insulators Induction Charge moves about object Flow of charge followed by “a separation” induces a net charge on the object Occurs with conductors

Equipotential Surfaces Surfaces are perpendicular to electric field Moving between surfaces changes energy of system Moving along a surface requires no work

Summary

Magnetic Fields Moving charges create magnetic fields Field lines point from north to south poles No isolated poles have been discovered Field due to a wire can be calculated by Ampere’s Law and Right-hand Rule 1

Ampere’s Law and Right-hand Rule 1 Ampere’s law relates magnetic field along a closed path to current penetrating the enclosed surface For a current wire, direction is given by Right-hand Rule 1

Magnetic forces and Right-hand Rule 2 Magnetic fields exert a force on isolated charges F B = q v B sin θ And on current wires F wire = I L B ext sin θ Direction is given by Right-hand Rule 2

Magnetic Flux and Faraday’s Law Magnetic Flux is similar to electric flux, but for magnetic fields Φ B = B A cos θ Faraday’s Law relates change in flux to induced voltage Direction of induced current given by Lenz’s Law “The magnetic field produced by an induced current always opposes any changes in the magnetic flux”

Electromagnetic Radiation E and B field oscillate E and B are perpendicular to each other and to the direction of propagation E, B, and Propagation related by Right-hand Rule 2 Travels at the “speed of light” in vacuum, and at slower speeds in material Electromagnetic Spectrum Runs from Radio Waves (long wavelength) to Gamma Rays (short wavelength)

Current Involves flow of charge Indicates direction of flow of positive charge carriers Flows from high potential to low potential Microscopically, involves drifting charges I = - n e A v d

Batteries and Ohm’s Law Batteries supply a potential difference to push charge in circuit Know as emf or voltage Ohm’s law relates current in a component to voltage difference “across” component Ohm’s law is very general In capacitors and inductors, other effects must be accounted for

Kirchhoff’s Rules, Summary Kirchhoff’s Loop Rule The total change in the electric potential around any closed circuit path must be zero Kirchhoff’s Junction Rule The current entering a circuit junction must be equal to the current leaving the junction These are actually applications of fundamental laws of physics Loop Rule – conservation of energy Junction Rule – conservation of charge The rules apply to all types of circuits involving all types of circuit elements Section 19.4

Resistors and Power Resist the flow of charge Resistance can be calculated from material and geometric properties Only resistors dissipate power P = I V = I² R = V² / R Ideal capacitors and inductors store and release power without dissipation

Capacitors Store energy and charge Capacitance can be calculated from geometric and material properties For parallel-plate capacitors With the inclusion of a dielectric

Series vs. Parallel Current is same through different components in series For resistors in series, For capacitors in series, Voltage is same across different components in parallel For resistors in parallel, For capacitors in parallel,

Inductors Oppose changes in current (Self-) Inductance can be calculated from material and geometric properties For a long solenoid Inductors store energy in magnetic field PE ind = ½ L I 2

DC Circuits For RC circuit, τ = RC For RL circuit, τ = L / R

AC Circuits

LRC Circuits and Impedance In an LRC Circuit Energy performs simple harmonic motion between capacitor and inductor Resistor damps motion Voltage source drives motion Impedance characterizes circuit Impedance is a “sum” of component “resistances”

Resonance Resonance amplifies current in circuit Occurs when reactance of inductor matches reactance of capacitor Characteristic resonant frequency is

Transformers Used to “step-up” (increase) or “step-down” (decrease) voltage Power in must equal power out The relation between voltage in and voltage out is