Exam 1 Review19-1,2 Electric Charge, Conductors, and Insulators • Measure of charge is Coulomb and its quantized • Electrons have negative charge = - 1.6 x 1019 C • Protons have positive charge = 1.6 x 1019 C • Create static electricity in insulators: – Giving materials extra electrons – Take them a away….19-1,2 Electric Charge, Conductors, and Insulators • Insulators – Electrons localized to atoms, cannot move freely within material • Conductors – Electron delocalized, can move freely within material • Semi-conductors – Somewhere in between… – Ability to conductor depends on environment • Temperature, light etc…..19-3 Coulombʼs Law • Determines force between 2 charges… • Force attractive for unlike charges, repulsive for like…19-3, 4, 5 Electric Field and Field Lines • Measures force per unit of charge on test charge q0 • Fields point in the direction of where free positive charge will move… +q -q019-6 Electric Shielding • For conductors with excess charge, charges move to surface • Leads to zero field at the center – Shielding…19-7 Electric Flux and Gaussʼs law • Gaussʼs Law – Flux through a surface enclosing charge q is….20-1,2,3 Electric Potential and Potential Energy • Electric Potential can be thought of as electric potential energy per unit of charge • If a charges gains or loses energy – Moves through a region with a potential difference…20-1,2,3 Electric Potential and Potential Energy • Change in electric potential energy is equal to the work done… Electric field distance moved • For point charges… +q -q020-4 Equipotential Surfaces • Surfaces of “equal” electric potential… • Run perpendicular to electric field lines20-5,6,7 Capacitors, Dielectrics, and Energy stored… • Capacitance of a system potential difference charge • For parallel plates… area separation• Dielectrics boost capacitance by decreasing potential difference… • If V becomes too big, dielectric breakdown I.e. it stops working… 20-5,6,7 Capacitors, Dielectrics, and Energy stored… • Potential energy in a capacitor given by:21-1 Electric Current • Flow of electric charge around closed path • Direction of current is direction of positive charge • EMF provides a potential difference, drives current • Electrics drift slowly, electric signal propagates quickly…21-2 Resistance and Ohm’s Law • Ohm’s relates voltage to current • R = resistance, measured in Ohms – High electrical -> small currents • Resistivity: Another way of characterizing resistance – Given material dimensions, we can figure out resistance…21-3 Energy and Power in Electric Circuits • Movement of charge across potential differences leads to change in potential energy • Power is change in energy divided by change in time - Can be expressed in current and voltage… • Usually dissipates in the form of heat, use Ohmʼs law to link to resistance…21-4 Resistors in Series and Parallel • Req is “equivalent” or total resistance of circuit21-5 Kirchhoff’s Rules • The junction rule: - Current entering the junction must equal the current leaving it. • Loop rule: - Sum of the potential differences around a loop must be zero21-6 Circuits Containing Capacitors • Ceq is “equivalent” or total capacitance of circuit21-7 RC Circuits • Circuits with a resistor and capacitor • Current flow and charge on capacitor change after circuit is switched on • Characteristic time22-1 The Magnetic Field • Magnetic poles generate magnetic field… • Like poles repel, opposites attract… • Arrows show direction of force on north pole • Earth’s north pole is actually a magnetic south pole!• Moving charge experiences force in a magnetic field • θ is angle between velocity and field directions • Force is perpendicular to both velocity and field directions • Right hand rule can be used to determine force direction 22-2 The Magnetic Force on Moving Charges θ22-3 The Motion of Charged Particles in a Magnetic Field • Since the magnetic force is always perpendicular to the direction of motion - Path of a charge is circular in uniform field22-4 and 22-5 Forces on Wires in Magnetic Field Torque on current loop in magnetic field A is area of loop…22-6 Electric Currents, Magnetic Fields22-7 Current Loops and Solenoid • Magnetic field inside a solenoid is uniform… L is length of solenoid N is number of turns22-8 Magnetism in Matter • Ferromagnetic – Turn field on, material becomes magnetic – Turn field off, material remains magnetic • Paramagnetic – Turn field on, material becomes magnetic – Turn field off, material stops being magnetic • Diamagnetic – Opposes magnetic fields… Field off… Field