Resistance in Electrical Systems

1
Resistance in Electrical Systems

• 4.3

2
Conductors

• Conduct electric current well
• Metals, some liquids and plasmasbecause of their
  free electrons
• Superconductorsresistance disappears at very low
  temperatures

3
Insulators

• Do not conduct electric current well
• Wood, plastic, glass, rubber, and
  mineralsbecause of tightly bound atoms or
  molecules

4
Semiconductors

• Good conductors when small amounts of impurities
  are added
• Silicon, germanium, gallium, arsenic

5
Electrical Resistance

• Caused by collision in the wire
• All electrical devices resist charge flow
• R ?V/I ( Electrical Resistance change in
  potential difference voltage divided by current
  
• the Ohm(O) is the unit of electrical resistance
• 1 O 1 V/A ( one ohm one volt per ampere)

6
Ohms Law

• resistance is constant, independent of potential
  difference and current
• ?V IR (change in voltage current times
  resistance)
• is not obeyed by all materials and devices (but
  resistance always applies to all devices)

7
Resistivity

• measure of the capacity of a material to resist
  electric charge flow
• unit of resistivity Om ?(rho) is the
  symbol
• R ? L/A (Resistance resistivity times length
  divided by area)

8
Resistivity

• Depends on 3 things
• length of the wirelonger wire, greater
  resistance
• the radius of the wiresmaller the wire, greater
  resistance
• material of which the wire is composed (p. 205
  for resistivity chart)

9
Series Circuits

• One path for current
• 4 important rules for series circuits
• The current through each device and everywhere in
  the series circuit, is the same.
• The total resistance in a series circuit is the
  sum of the individual resistances.
• The voltage drop across each device is the
  product of the lamps resistance and the current.
  (?V IR)
• The sum of the voltage drops across the devices
  the total potential difference(voltage) of the
  power supply the product of the total circuit
  resistance and the current.

10
Parallel Circuits

• More than one path for current
• 4 important rules for parallel circuits
• The total current through the circuit the sum
  of the currents through all the devices.
• 1/Rtotal 1/R1 1/R2
• The current through each device is the ratio of
  the voltage drop across the device to the
  resistance of the lamp. (I ?V/R)
• The voltage drop across the lamps is the same,
  and equals the potential difference of the power
  supply.

11
Resistors

• electrical devices that have a specific
  resistance
• made of long wires, carbon, or semiconductors
• value is labeled on the outside of a resistor
• used to reduce current in a circuit