Physics Chapter 6 Circuits

Physics Chapter 6 Circuits

1. 6.1Current

   • conductivity
     • inverse of resistance
       • Siemens (S) sometimes S/m
       • measure of permissiveness to current
     • metallic vs electrolytic(solution)
   • AKA flow of positive charge
   • Current
     • I=Q/t
       • I= Ampere= C/s
     • potential differences: Voltage: are made by cells.
       • electromotive force (emf): is a potential difference J/C: made when no charge is moving between two terminals of a cell and they have different values
       • J/s
   • circuit laws
     • Kirchhoff's laws
       • Junction Rule: sum of currents at a point equal sum of current away
       • Loop Rule
         • sum of voltagesources= voltage (potential) drop

2. 6.2 Resistance

   • Properties of Resistors
     • R= pL/A
       • p=resistivity, L= length of resistor, A= Cross sectional area
     • resistivity= ohm-meter(ohm*m)
     • more area= more resistance= more conduction pathways
     • Temperature increases resistance
   • Conductors vs insulators
     • Resistors= materals which offer resistance between the two extremes
   • OHM's Law and power
     • Ohm's law
       • V= IR
       • internal resistance would lower voltage
         • V=Ecell - i(r)int
           • Ecell= emf
     • Measuring power
       • Work/time
         • rate of energy dissipation can be found by power
           • P=IV = I^2R= V^2/R
     • secondary batteries can be recharged
       • normally discharching is from (+) to (-)
         • Galvanic (voltaic)
         • When recharging it acts as a electrolytic cell
   • Resistors Series & Parallel
     • Series
       • Voltages and resistors become additive
         • Voltage will change for each resistor
     • parallel
       • voltages are equal in each path
         • voltage will stay constant in each resistor
       • Although electrons prefer the path of least resistance
         • resistance is inverse addition of the parallel resistors
           • 1/Rp= 1/R1 + 1/R2...

3. 6.3 Capacitance and capacitors

   • can hold charge at a voltage
   • properties of capacitors
     • C=Q/V
       • voltage is applied across the places and charge is applied
       • SI= farad (F)
     • plates depend on area
       • C= Permittivity of free space (Eo) (A/d)
         • d= separation of plates
         • Eo= 8.85 *10^-12F/m
         • this creates Uniform electric Field
           • E=V/d
             • always (+) direction
     • potential energy stored= U = 1/2 CV^2
     • if voltage source removed then capacitor will discharge in opposite direction
   • dielectric materials
     • AKA insulation
     • when a material is added capacitance will increase by the constant value
       • C'= kC
     • isolated calacitors
       • charged capacitor that is disconnected form any circuit
         • when material is placed voltage drop will create a capacitance increase
       • dielectrics do not change charge
     • in circuit capacitors
       • When material placed in circuit with voltage source
         • capacitance will increase as voltage stays constant
   • Capacitors in series and parallel
     • series
       • similar to parallel resistors
         • as capacitors line they share voltage drop
           • 1/Cs= 1/C1 + 1/C2...
     • parallel
       • capacitors are additive

4. 6.4 Meters

   • Ammeters
     • measure current at some point in a circuit
       • need to be measured in series
         • low resistance parallel shunt helps reading
     • requires a very low resistance to not disrupt reading
   • Voltmeters
     • requires an active circuit
     • measure voltage drop
     • wired in parallel to minimize impact
       • better if it has alot of resistance
   • Ohmmeters
     • do not require an active circuit
     • contain own battery and act as an ammeter
       • ohm law is used since voltage and current is created through meter
         • better if zero resistance
   • measure circuit quantities