Transcript living with the lab - Louisiana Tech University

living with the lab
Currents Through Parallel Resistors
470W
220W
5V
© 2012 David Hall
Gnd
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living with the lab
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living with the lab
Select Resistors
Find the 220W and the 470W resistors from your parts kit.
color
digit
black
0
brown
1
red
2
first
digit
second
digit
gold = ±5%
tolerance
silver = ±20%
number
of zeros
orange
3
yellow
4
green
5
blue
6
violet
7
4 = yellow
7 = violet
Add 1 zero to 47 to make 470, so 1 = brown
gray
8
So, 470 = yellow, violet, brown
white
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Example: 470W resistor
Now, find the 220W resistor.
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living with the lab
build the parallel circuit below
measure the total current from the power source
cu rren t
m easurem ent
Vo= 5V
current flow
+
-
current flow
+
-
Compute the current after you make your measurement.
220W
use Ohm’s law:
𝑉 =𝐼∙𝑅
𝑜𝑟
𝐼=
𝑉
𝑅
𝑅𝑒𝑞 =
1
1
+
220Ω 470Ω
= 149.9Ω
𝑉
5𝑉
=
= 0.033𝐴
𝑅𝑒𝑞
149.9Ω
-
+
𝐼=
1
470W
compare
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living with the lab
modify the parallel circuit as show
measure the total current through the 220Ω resistor
cu rren t
m easurem ent
Vo= 5V
+
-
+
220W
current flow
Compute the current after you make your measurement.
𝑉 =𝐼∙𝑅
𝑜𝑟
-
+
use Ohm’s law:
5𝑉
𝑉
= 0.023𝐴
𝐼= =
𝑅 220Ω
compare
470W
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living with the lab
modify the parallel circuit as show
Vo= 5V
measure the current through the 470Ω resistor
+
-
cu rren t
m easurem ent
+
220W
+
470W
Compute the current after you make your measurement.
use Ohm’s law:
𝑉 =𝐼∙𝑅
𝑜𝑟
5𝑉
𝑉
= 0.011𝐴
𝐼= =
𝑅 470Ω
compare
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living with the lab
check for conservation of charge
~
~
Vo= 5V
more current goes through smaller resistor
I = 22.7 mA
-
+
I = 10.7 mA
I = 33 mA
+
-
220W
Kirchoff’s Current Law (KCL)
The algebraic sum of currents entering a node is zero.
Current In = Current Out
+
470W
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