song (shs546) – Homework 9 – weathers – (22202)
1
This
printout
should
have
10
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
001(part1of2)10.0points
An electric heater operating at full power
draws a current of 13
.
7 A from a 124 V circuit.
What is the resistance of the heater?
Correct answer: 9
.
05109 Ω.
Explanation:
Let :
I
= 13
.
7 A
and
V
= 124 V
.
From Ohm’s law,
R
=
V
I
=
124 V
13
.
7 A
=
9
.
05109 Ω
.
002(part2of2)10.0points
Assuming
constant
R
,
how
much
current
should the heater draw in order to dissipate
810 W?
Correct answer: 9
.
46002 A.
Explanation:
Let :
P
= 810 W
.
Since
P
=
I
2
R
,
I
1
=
radicalbigg
P
R
=
radicalbigg
810 W
9
.
05109 Ω
=
9
.
46002 A
.
003(part1of2)10.0points
Suppose that you want to install a heating
coil that will convert electric energy to heat
at a rate of 399 W for a current of 0
.
76 A.
Determine the resistance of the coil.
Correct answer: 690
.
789 Ω.
Explanation:
Let :
P
= 399 W
and
I
= 0
.
76 A
.
Power is
P
=
I
2
R .
The resistance of the coil is
R
=
P
I
2
=
399 W
(0
.
76 A)
2
=
690
.
789 Ω
.
004(part2of2)10.0points
The resistivity of the coil wire is 1
.
35
×
10
−
6
Ω
·
m, and its diameter is 0
.
461 mm.
Determine its length.
Correct answer: 85
.
409 m.
Explanation:
Let :
ρ
= 1
.
35
×
10
−
6
Ω
·
m
and
r
= 0
.
2305 mm = 0
.
0002305 m
.
We use the formula for resistance as a function
of resistivity
ρ
, length
ℓ
and crosssectional
area
A
:
R
=
ρ
ℓ
A
=
ρ
ℓ
π r
2
,
where
r
is the radius of the wire. Solving for
ℓ
,
ℓ
=
R
ρ
π r
2
=
(690
.
789 Ω)
π
(0
.
0002305 m)
2
1
.
35
×
10
−
6
Ω
·
m
=
85
.
409 m
.
005
10.0points
A 10
.
3 V battery delivers 114
.
8 mA when
connected to a 33
.
7 Ω load.
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 Spring '00
 Littler
 Power, Work, Heat, Resistor, SEPTA Regional Rail, Correct Answer, Electrical resistance, Ω

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