27 Current and
Resistance
27-1 Moving Charges and Electric Currents
electric currents——that is,
charges in motion.
a. A loop of copper in
electrostatic equilibrium.
b. This movement of charges
is a current i.
27-2 Electric Current
The current I through the conductor has the
Same value at planes aa`,bb`,cc`.
An electric current I in a conductor is defined by:
We can fine the charge that passes through the
plane in a time interval extending from 0 to t by
Integration:
The SI unit for current is the coulomb per
Second,also called the ampere(A):
1 ampere=1 A=1 coulomb per second=1C/s
The Direction of Currents
A current arrow is drawn in the direction in
which positive charge carriers would move,
Even if the actual charge carriers are
negative and move in the opposite direction.
CHECKPOINT 1
What are the magnitude and direction of the
current I in the lower right-hand wire?
Sample Problem 27-1
Step one:
charge electrons molecules
i= per per per
electron molecule second
or
Step two:
molecules molecules moles mass volume
per = per per unit per unit per
second mole mass volume second
Step three:
Step four:
Step five:
27-3 Current Density
Drift Speed
Sample Problem 27-2
Step one:
Step two:
(b)
Step one:
Step two:
Sample Problem 27-3
Step one:
atoms atoms moles mass
n= per unit = per per unit per unit
volume mole mass volume
Step two:
Step three:
Step four:
27-4 Resistance and Resistivity
1 ohm =1Ω＝1 volt per ampere = 1V/A
definition of ρ
definition of σ
Calculating Resistance from
Resistivity
Resistance is a property of an object.
Resistivity is a property of a material.
Variation with Temperature
Sample Problem 27-4
Step one:
Step two:
Step three:
27-5 Ohm’s Law
Ohm’s law is an assertion that the current
through a device is always directly proportional
to the potential different applied to the device.
A conducting device obeys Ohm’s law when
the resistance of the device is independent
of the magnitude and polarity of the applied
difference.
A conducting material obeys Ohm’s law when
the resistivity of the material is independent
of the magnitude and direction of the applied
electric field.
27-6 A Microscopic View of
Ohm’s Law
Sample Problem 27-5
Step one:
Step two:
27-7 Power in Electric Circuits
or
Sample Problem 27-6
Step one:
Step two:
27-8 Semiconductors
27-9 Superconductors
Superconductors are materials that lose all
electrical resistance at low temperatures.