Physics Final

• Direct-current (dc) circuits


  Circuits in which the direction of the current does not change with time
• Direct-current circuits

  • Flashlights
  • Automobile wiring systems
• Alternating current (ac)


  Current that oscillates back and forth
• Resistors in series


  Connected one after the other so the current is the same in all of them
• Resistors in parallel


  Current through each resistor need not be the same, but the potential difference between the terminals of each resistor must be the same, and equal to Vab
• Junction


  A point where three or more conductors meet
• Loop


  Any closed conducting path
• Loop rule


  A statement that the electrostatic force is conservative
• Galvanometer


  Measures the current that passes through it
• Ammeter


  Measures the current passing through it
• Voltmeter


  Measures the potential difference between two points
• Ohmmeter


  Consists of a meter, a resistor, and a source (often a flashlight battery) connected in series
• Digital multimeter


  Can measure voltage, current, or resistance over a wide range
• Potentiometer


  An instrument that can be used to measure the emf of a source without drawing any current from the source
• Magnetism


  Interaction of moving electric charges
• The earth itself is a magnet
• The earth's north geographic pole is close to a magnetic south pole, which is why the north pole of a compass needle points north
• The earth's magnetic axis is not quite parallel to its geographic axis (the axis of rotation), so a compass reading deviates somewhat from geographic north
• Magnetic declination or magnetic variation

  The deviation of the compass reading from geographic north, which varies with location
• The magnetic field is not horizontal at most points on the earth's surface; its angle up or down is called magnetic inclination
• There is no experimental evidence for magnetic monopoles
• A moving charge (or current) creates a magnetic field in the surrounding space
• The magnetic field exerts a force on any other moving charge (or current) that is present in the field
• Magnetic field


  A vector field - a vector quantity associated with each point in space
• The magnitude of the magnetic force on a moving particle is proportional to the component of the particle's velocity perpendicular to the field. If the particle is at rest, or moving parallel to the field, it experiences zero magnetic force
• Cathode-ray tube


  • Electron beam shoots out a narrow beam of electrons
  • If there is no force to deflect the beam, it strikes the center of the screen
  • The magnetic force deflects the beam, and creates an image on the screen
• Magnetic field lines


  Lines that represent the magnetic field, with the line through any point tangent to the magnetic field vector at that point
• Magnetic field lines never intersect
• When a charged particle moves in a magnetic field, it is acted on by the magnetic force, which is always perpendicular to the velocity, so it cannot change the speed of the particle
• If the particle has velocity components parallel to and perpendicular to the field, its path is a helix, but the speed and kinetic energy of the particle remains constant
• Velocity selector


  Uses perpendicular electric and magnetic fields to select particles of a specific speed from a beam
• The Hall emf reveals whether the charge carriers in a current-carrying conductor are negative or positive
• A moving charge generates a magnetic field that depends on the velocity of the charge, and the distance from the charge
• The total magnetic field of several moving charges is the vector sum of each field
• The field lines around a long, straight, current-carrying conductor are circles, with directions determined by the right-hand rule
• Computer cables or cables for audio-video equipment create little or no magnetic field because the opposing currents in the closely spaced wires cancel each other's magnetic fields
• The magnetic field of the lower wire exerts an attractive force on the upper wire. If the wires had currents in opposite directions, they would repel each other
• MRI (magnetic resonance imaging) requires a magnetic field of about 1.5 T
• Ampere's law


  Relates electric current to the line integral around a closed path
• If the integral around the closed path is zero, it does not necessarily mean that the magnetic field is everywhere along the path, only that the total current through an area bounded by the path is zero