Transcript Slide 1

Today’s agenda:
Electric Current.
You must know the definition of current, and be able to use it in solving problems.
Current Density.
You must understand the difference between current and current density, and be able to
use current density in solving problems.
Ohm’s Law and Resistance.
You must be able to use Ohm’s Law and electrical resistance in solving circuit problems.
Resistivity.
You must understand the relationship between resistance and resistivity, and be able to
use calculate resistivity and associated quantities.
Temperature Dependence of Resistivity.
You must be able to use the temperature coefficient of resistivity to solve problems
involving changing temperatures.
Resistance
The resistance of a material is a measure of how easily a
charge flows through it.
Resistance: how much “push” is needed to
get a given current to flow.
V
R
I
1V
.
The unit of resistance is the ohm: 1  
1A
Resistances of kilohms and megohms are common:
1 k  103 , 1 M=106.
Every circuit component has resistance.
This is the symbol we use for a “resistor:”
All wires have resistance. Obviously, for efficiency in carrying a
current, we want a wire having a low resistance. In idealized
problems, we will consider wire resistance to be zero.
Lamps, batteries, and other devices in circuits have resistance.
Resistors are often intentionally used in
circuits. The picture shows a strip of five
resistors (you tear off the paper and
solder the resistors into circuits).
The little bands of color on the resistors have meaning. Here
are a couple of handy web links:
1. http://www.dannyg.com/examples/res2/resistor.htm
2. http://www.digikey.com/en/resources/conversioncalculators/conversion-calculator-resistor-color-code-4-band
Ohm’s Law
In some materials, the resistance is constant over a wide range
of voltages.
For such materials, we write V  IR, and call the equation
“Ohm’s Law.”
In fact, Ohm’s Law is not a “Law” in the same sense as
Newton’s Laws…
… and in advanced Physics classes you will write something
other than V=IR when you write Ohm’s Law.
Newton’s Laws demand; Ohm’s Law suggests.
I
Materials that follow Ohm’s Law are called
“ohmic” materials, and have linear I vs. V
graphs.
Materials that do not follow Ohm’s Law are
called “nonohmic” materials, and have
curved I vs. V graphs.
slope=1/R
V
I
V
I
Materials that follow Ohm’s Law are called
“ohmic” materials, and have linear I vs. V
graphs.
Materials that do not follow Ohm’s Law are
called “nonohmic” materials, and have
curved I vs. V graphs.
slope=1/R
V
I
V