What IS Resistance On what will resistance depend Electrical Resistance A measure of how easily charge flows through a material A Resistor is a material of significant resistance that has been placed in an electric circuit in order to control current or voltage ID: 557521
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Slide1
Electrical Resistance and Resistivity
Section 5.2Slide2
What IS Resistance?
On what will resistance depend?Slide3
Electrical Resistance
A measure of how easily charge flows through a material
A
Resistor
is a material of significant resistance that has been placed in an electric circuit in order to control current or voltageSlide4
On what will a wire’s resistance depend?
There are 4 primary factors when determining a wire’s resistance:
Material composition
Length of the wire
Cross-sectional Area of the wire
Temperature
HOW?
Turn and talk!Slide5
Question 17.3a
Wires I
Two wires,
A
and
B
, are made of the
same metal
and have
equal length
, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare?
a)
d
A
= 4
d
B
b)
d
A
= 2
d
B
c)
d
A
=
d
B
d)
d
A
= 1/2
d
B
e)
d
A
= 1/4
d
BSlide6
Resistivity (
r
) of a wire
A physical property of the wire that depends on the material of which the wire is constructed; determines how easily current can flow through the material
When the temperature is CONSTANT:
Units =
W
·m
Conductivity (
s
)
is the inverse of resistivitySlide7
Question 17.3a
Wires
I—try again
Two wires,
A
and
B
, are made of the
same metal
and have equal length, but the resistance of wire A is four times the resistance of wire
B
. How do their diameters compare?
a)
d
A
= 4
d
B
b)
d
A
= 2
d
B
c)
d
A
=
d
B
d)
d
A
= 1/2
d
B
e)
d
A
= 1/4
d
BSlide8
The resistance of wire A is greater because its
area is less
than wire B.
Since
area
is related to
radius (or diameter) squared, the diameter of A must be two times less than the diameter of B.
Question 17.3a
Wires I
Two wires,
A
and
B
, are made of the
same metal
and have
equal length
, but the resistance of wire
A
is four times the resistance of wire B. How do their diameters compare?
a)
dA = 4dBb) dA = 2dBc) dA = dBd) dA = 1/2dBe) dA = 1/4dBSlide9
Question 17.3b
Wires II
A wire of resistance
R
is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance?
a)
it decreases
by a factor of 4
b)
it decreases
by a factor of 2c) it stays the samed) it increases
by a factor of 2
e) it increases
by a factor of 4Slide10
Keeping the volume (= area x length) constant means that if the length is
doubled
,
the area is
halved
.
Since
,
this increases the resistance by a factor of
4. Question 17.3b Wires II A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance?
a)
it decreases
by a factor of 4
b)
it decreases
by a factor of 2
c)
it stays the same
d) it increases
by a factor of 2
e) it increases by a factor of 4Slide11
Resistivities
for Common Conductive Materials (table 17.1)Slide12
Practice Problem!
What is the resistance of a piece of copper wire that is 10.0 m long and 1.2 mm in diameter (
r
=1.70 x 10
-8
W·m
)?Slide13
Temperature effect on R
Would you expect a higher temperature to cause an INCREASE or a DECREASE in the resistance of a wire? Why?Slide14
Temperature effect on R
Temperature increases causes resistance to increase:
R
0
= resistance at some reference temperature
Rf
= resistance at some temperature
t
(in °C) above the reference temperaturea = temperature coefficient for materialSlide15
Resistors
Items placed in an electrical circuit in order to limit the current that can flow.
Made of a (relatively) conductive carbon-composition core, with wires embedded on either end.
In the picture, you can see a cutaway showing the insides of some resistors.
Which do you think has the highest resistance? Why?Slide16
Measuring Resistance
Units = Ohm (
W
)
Multimeter
: A tool designed to measure multiple electrical quantities, such as resistance, AC and DC current, AC and DC voltage, and capacitance.Color Code:
Each band represents a value in the overall resistance
Each Color represents a different numberSlide17
Resistor Color Code
COLOR
VALUE
Black
0
Brown
1
Red
2
Orange
3
Yellow
4Green5Blue6Violet
7
Grey
8
White
9Slide18
Effective Resistance: SeriesSlide19
Resistors in Series: Example
Given the following information, what is the effective resistance of this series of resistors:Slide20
Effective Resistance: Parallel
The effective resistance will be smaller than the lowest resistance in the parallel combinationSlide21
Resistors in Parallel: Example
Determine the effective resistance of this parallel configuration of resistors:Slide22
Daily Wrap-up!
Covered today:
Resistivity
Resistors in Series and Parallel
Due Tomorrow:
Lab: Resistors and Resistance