Electrical Resistance and Resistivity Syllabus Statements 5.1.6-5.1.7, 5.2.3 We are in chapter 17 of...
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Transcript of Electrical Resistance and Resistivity Syllabus Statements 5.1.6-5.1.7, 5.2.3 We are in chapter 17 of...
Electrical Resistance and
ResistivitySyllabus Statements 5.1.6-5.1.7, 5.2.3
We are in chapter 17 of your textbook!
What IS Resistance?On what will resistance depend?
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!
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 = 4dB
b) dA = 2dB
c) dA = dB
d) dA = 1/2dB
e) dA = 1/4dB
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 resistivity
lR
A
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) dA = 4dB
b) dA = 2dB
c) dA = dB
d) dA = 1/2dB
e) dA = 1/4dB
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 ITwo 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 = 4dB
b) dA = 2dB
c) dA = dB
d) dA = 1/2dB
e) dA = 1/4dB
lR
A
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 4
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 4
lR
A
Resistivities for Common Conductive Materials (table 17.1)
Practice Problem!
What is the resistance of a piece of copper wire that is 10.0 m long and 1.2 mm in diameter?
2r
L
A
LR
m
mmmr
mL
81070.1
0006.06.0
0.10
150.0
)0006.0(
)0.10)(1070.1(2
8
R
R
Temperature effect on R Would you expect a higher temperature to cause
an INCREASE or a DECREASE in the resistance of a wire? Why?
Temperature effect on R Temperature increases causes
resistance to increase:
R0 = resistance at some reference temperature
Rf = resistance at some temperature t (in °C) above the reference temperature
a = temperature coefficient for material
0 (1 )fR R t
Effective Resistance: Series
Resistors in Series: Example Given the following information, what is the
effective resistance of this series of resistors:
323
k 50.1
425
3
2
1
R
R
R
2250 2248
)323()0015()425(
eff
ieff
R
RR
Effective Resistance: Parallel
The effective resistance will be smaller than the lowest resistance in the parallel combination
Resistors in Parallel: Example Determine the effective resistance of this parallel configuration of
resistors:
175
210
150
3
2
1
R
R
R
58 3.5818
1050
1050
18
1050
6
1050
5
1050
71
175
1
210
1
150
111
eff
eff
ieff
R
R
RR
Daily Wrap-up!
Covered today:
Resistivity
Resistors in Series and Parallel
Due Tomorrow:
Lab: Resistors and Resistance