Lecture 16

Readings

http://umdberg.pbworks.com/w/page/73085087/Quantifying%20electric%20current%20%282013%29

http://umdberg.pbworks.com/w/page/51327369/Resistive%20electric%20flow%3A%20Ohm%27s%20law

Electric Current

• Coulombs/sec = Amperes, or Amp, or A
• $\mathrm{\Delta }q$$\Delta q$ = number of charges moving through area, specifically cross-sectional area of wire

• If 3 $C$$C$ passes through imaginary plane every sec, then current = 3 Amp

• Imagine it's positive charge moving ( even though it's usually negative electrons moving )

  • So positive current is defined as the direction of positive charge movement
  • Even though it's really negative electrons moving the opposite direction
  • For example, if negative charges are moving to the left, then positive current is to the right

• Electrons are pushed along a wire by an electric field inside the wire

  • not static
  • current is flowing

• They also feel a resistance to movement, kind of like viscosity

• Long derivation, result is $\mathrm{\Delta }V=I\ast R$$\Delta V = I*R$ , where:

  • $\mathrm{\Delta }V$$\Delta V$ is voltage difference between 2 places on the wire

  • $I$$I$ is the current

  • $R$$R$ is something called "resistance"

    • $R$$R$ has units of "ohms" $=\frac{Volts}{Amps}$$= \frac{Volts}{Amps}$
    • Symbol $\mathrm{\Omega }$$\Omega$

• Resistance depends on geometry of wire and wire material:

• $q$$q$ = charge on moving carrier object (electron)

• $n$$n$ = density o fcharge carriers

• $b$$b$ = "viscosity"

  • Depends on material

• $L$$L$ = length of wire

  • Depends on material

• $A$$A$ = cross-sectional area of wire

• Important to remember:

  • $R\propto \frac{L}{A}$$R \propto \frac{L}{A}$

    • $R$$R$ increases if $L$$L$ increases
    • $R$$R$ increase if $A$$A$ decreases