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Physics 2080 Chapter 21 Notes: Electric Current

by: Amanda Biddlecome

Physics 2080 Chapter 21 Notes: Electric Current Physics 2080

Marketplace > Clemson University > Physics 2 > Physics 2080 > Physics 2080 Chapter 21 Notes Electric Current
Amanda Biddlecome
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About this Document

These notes cover everything from Chapter 21 which is all about electric currents, Ohm's Law, resistance, resistivity, Kirchhoff's Rule, resistors in parallel, and resistors in series.
General Physics 2
Dr. Pope
Class Notes
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This 5 page Class Notes was uploaded by Amanda Biddlecome on Thursday February 18, 2016. The Class Notes belongs to Physics 2080 at Clemson University taught by Dr. Pope in Fall 2016. Since its upload, it has received 24 views. For similar materials see General Physics 2 in Physics 2 at Clemson University.


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Date Created: 02/18/16
Physics  2080   Chapter  21:  Electric  Current   February  16,  2016   Amanda  Biddlecome     1)  Electric  Current     I=Q/t=ΔQ/Δt   *I=current   *Q=charge   *t=time       -­‐battery  pumps  charge  around  the  current     -­‐current       *unit=ampere  (A):  1A=1C/s       *give  current  the  same  direction  as  the  flow  of  positive  charge       *have  to  have  a  closed  path  to  propagate  current       -­‐potential  difference  of  a  batter       *emf  (ε)=electromotive  force;  similar  to  voltage;  across  the  battery;  is         not  a  force;  electric  potential     -­‐amount  of  work  to  move  charge     W=ΔQε   *W=work   *Q=charge   *ε=emf       -­‐batteries  always  have  a  positive  and  a  negative  terminal       *the  positive  terminal  is  longer  than  the  negative  terminal       *current  flows  from  positive  to  negative  if  it’s  being  used     -­‐flow  of  current  and  flow  of  electrons  are  opposite     2)  Electric  Current-­‐Drift     -­‐motion  of  charges  in  wire  is  slow       *electrons  bounce  around  and  don’t  get  anywhere       *electric  signal  moves  very  quickly  despite  this     -­‐electrons  have  drunkard’s  walk       *move  but  move  with  no  purpose  and  no  net  movement     -­‐with  a  battery,  we  have  ΔV  that  gives  electric  charge  and  the  movement  of     the  electric  field  gives  the  current     -­‐current  is  the  motion  of  charged  particles       *mobile  charge  carriers  are  those  that  are  free  to  move  in  the  sample     Q=q(nAΔx)         *carriers  move  slowly  (drift  speed)     ΔQ=q(nAv )   dt   (ΔQ/Δt)=I=qnAv   d       *all  electrons  move  at  the  same  time  (slowly)  but  the  machine  comes         on  quickly  because  carriers  are  all  throughout  the  wire     -­‐have  current  because  of  free  charge  carriers  that  conduct  electricity     3)  Resistance  and  Ohm’s  Law     -­‐Ohm’s  Law=the  relationship  between  current  and  voltage       *resistance  depends  on  what  type  of  material  you  have;  all  wires  have         resistance       *empirical  law=we  observe  it  happening  but  we  don’t  know  why     ΔV=IR   *Ohm’s  Law   *under  normal  circumstances   *R=constant  resistance=ΔV/I       -­‐as  voltage  changes,  have  constant  resistance     -­‐non-­‐Ohmic  resistor  doesn’t  obey  Ohm’s  Law       *transistors,  lightbulbs       *resistance  isn’t  constant     -­‐units  of  resistance=V/A=Ω=(ohm)=[R]       *[1/R]=1mho     -­‐resistance  depends  on  the  material,  the  area,  and  the  length       *due  to  collisions  between  carrier  electrons       *as  ΔV  increases,  I  increases     -­‐Ohmic  materials=Ohm’s  Law  holds       *when  you  plot  V  versus  I,  the  slope  is  the  resistance     -­‐sometimes  Ohm’s  Law  holds  and  sometimes  it  doesn’t     -­‐we  will  use  Ohm’s  Law  a  lot     4)  Resistivity     -­‐resistivity  (ρ)       *good  conductors  have  a  low  resistivity       *poor  conductors  have  a  high  resistivity       *it’s  different  for  different  materials       ρ=R(A/l)   *ρ=resistivity   *R=resistance   *A=cross  sectional  area   *l=length         *semi  conductor  resistivity  changes  with  temperature     -­‐changes  with  temperature       *most  materials  resistivity  increases  with  an  increase  of  temperature         because  the  molecules  are  moving  faster  and  colliding  more  often     ρ=ρ [1+0(T-­‐T )]   0 *ρ =r0sistivity  at  the  reference  temperature     *a=temperature  coefficient  of  resistivity  (constant)     R=R [1+a(0-­‐T )]   0 *R=resistance     5)  Power  in  Electric  Circuits     -­‐charges  lose  potential  energy  as  they  go  through  resistors       P=IΔV   *P=power  (watts)   *I=current   *V=potential  difference     P=IΔV=I R=(ΔV /R)   2 *all  equivalent   *derived  by  using  Ohm’s  Law       -­‐power  companies  bill  you  by  kilowatts/hour  of  energy  that  you  use       *1kWh=(10 W)(3600s)=(1000J/s)(3600s)=3.6X10 J   6     *choose  wires  with  proper  power  rating  to  be  safe  and  efficient     6)  Resistors  in  Series     -­‐current  leaves  the  battery  from  the  positive  terminal  and  travels  through  the     circuit       *only  one  possible  path  that  you  can  follow  in  series     -­‐every  single  electron  travels  through  all  resistors       *current  through  each  element  has  to  be  the  same     ΔV=IR +IR 1 2 *R 1 and  R =re2istance  1  and  2     ΔV=IR   eq *R =ReqR   1 2 *don’t  know  what  pieces  make  up  this  part  so  we  put  it  under  a  big  hat  (R )   eq       *R (ieq  series)  is  greater  than  any  individual  R       7)  Resistors  in  Parallel     -­‐can’t  make  closed  loop  with  one  path     -­‐have  potential  difference  across  the  battery       *potential  difference  across  each  resistor  is  the  same     -­‐charge  is  conserved       *the  number  of  charge  (the  current)  that  goes  into  a  junction  has  to         equal  the  number  of  charge  (the  current)  that  comes  out  of  the           junction       *current  takes  least  resistant  path,  so  more  current  flows  through  the         resistor  with  the  lowest  resistance     ΔV=IR   eq I1=ΔV/R 1   I2=ΔV/R   2 (1/R )=(1/R )+(1/R )+(1/R )+…   eq 1 2 3 *R (peqallel)  is  smaller  than  the  smallest  R  in  the  group       -­‐power  in  houses  run  in  parallel     -­‐to  find  R ,eq  you  have  to  find  the  reciprocal  of  1/R   eq   8)  Kirchhoff’s  Rule     -­‐sometimes  batteries  can’t  be  in  parallel  or  in  series     -­‐Junction  Rule:  conservation  of  charge       *current  going  in=current  going  out     I in  out I 1I +I 2  or 3  I -­‐I 1­‐I 20 3 *these  are  equivalent       -­‐Loop  Rule:  sum  of  potential  differences  across  all  elements  in  closed  loop     has  to  be  0       *conservation  of  energy     -­‐choose  the  current  through  each  branch  (guess  doesn’t  matter  because  if  it’s     negative  you  know  it  just  goes  the  opposite  direction)     -­‐resistors  always  dissipate  energy     -­‐junction=current  can  go  multiple  ways     -­‐Junction  Rule  gives  one  equation,  Loop  Rule  provides  the  rest  to  find  the     individual  currents     -­‐find  three  equations  to  solve  for  the  currents       *analyze  them       *use  Junction  Rule+Loop  Rule+Loop  Rule  to  find  the  three  equations     9)  Adding  Capacitors     -­‐capacitors  in  parallel:  equivalent  capacitance=C +C   1 2   -­‐capacitors  in  series:  equivalent  capacitance=(1/C )+(1/C )+…   1 2   -­‐opposite  of  resistors        


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