Physics 202
Thursday, March 4, 1999
Announcements:
Lecture notes:
Resistors in Series
•
In this case, the current going through R
1
is the same as that going through R
2
.
•
Hence,
ξ
 iR
1
 iR
2
= 0
o
ξ =
i (R
1
+ R
2
)
o
Therefore. R
eq
= R
1
+ R
2
•
If there are 3 resistors in series, then R
eq
= R
1
+ R
2
+ R
3
.
•
If there are n resistors in series, then R
eq
= R
1
+ R
2
+ R
3
+...R
n
.
Resistors in parallel
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There is an equal emf (potential) across R
1
and R
2
.
•
ξ
= i
1
R
1
= i
2
R
2
(Equation 1), but i = i
1
+ 1
2
(Equation 2)
o
From equation 1: i
1
=
ξ
/ R
1
and i
2
=
ξ
/ R
2
o
From equation 2: i = i
1
+ 1
2
=
(ξ
/ R
1
) + (
ξ
/ R
2
) =
ξ [(1
/ R
1
) +
(1
/ R
2
)]
•
For equivalent circuit,
ξ
= i
1
R
eq
or i =
ξ
/ R
eq
.
o
Therefore,
ξ
/ R
eq
=
ξ [(1
/ R
1
) +
(1
/ R
2
)]
o
or similarly, (1/R
eq
) =
€[(1
/ R
1
) +
(1
/ R
2
)]
•
If there are more than 2 resistors in parallel, then (1/R
eq
) =
€[(1
/ R
1
) +
(1
/ R
2
) +
(1/R
3
) +.
... (1/R
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 Spring '07
 MAHLON,GREGORYDA
 Current, Magnetism, The Circuit, Electrical resistance, Series and parallel circuits

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