Transcript Document

Resistance in Electrical
Systems
4.3
Conductors
► Conduct
electric current well
► Metals, some liquids and plasmas—because
of their free electrons
► Superconductors—resistance disappears at
very low temperatures
Insulators
► Do
not conduct electric current well
► Wood, plastic, glass, rubber, and minerals—
because of tightly bound atoms or
molecules
Semiconductors
► Good
conductors when small amounts of
impurities are added
► Silicon, germanium, gallium, arsenic
Electrical Resistance
► Caused
by collision in the wire
► All electrical devices resist charge flow
► R = ΔV/I ( Electrical Resistance =
change in potential difference [voltage]
divided by current
► the Ohm(Ω) is the unit of electrical
resistance
► 1 Ω = 1 V/A ( one ohm = one volt per
ampere)
Ohm’s Law
► resistance
is constant, independent of
potential difference and current
► ΔV = IR (change in voltage = current times
resistance)
► is not obeyed by all materials and devices
(but resistance always applies to all devices)
Resistivity
► measure
of the capacity of a material to
resist electric charge flow
► unit of resistivity: Ω·m
ρ(rho) is the
symbol
► R = ρ L/A (Resistance = resistivity times
length divided by area)
Resistivity
► Depends
on 3 things:
 length of the wire—longer wire, greater
resistance
 the radius of the wire—smaller the wire, greater
resistance
 material of which the wire is composed (p. 205
for resistivity chart)
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 lamp’s resistance and the current. (ΔV = 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.
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 = ΔV/R)
 The voltage drop across the lamps is the same, and
equals the potential difference of the power supply.
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