# APCH17HW: DIRECT CURERENT

Urban and Civil

Nov 15, 2013 (4 years and 4 months ago)

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APCH17HW: DIRECT CURERENT

Multiple Choice: No calculators or equation sheet

1.

The five resistors shown below have the lengths and cross

sectional areas indicated and are
with the same resistivity. Which has the greatest resistance?

2.

An immersion heater of resistance R converts elec
trical energy into thermal energy that is
transferred to the liquid in which the heater is immersed. If the current in the heater is I,
t
he thermal energy trans
ferred to the liquid in time t is

(A) IRt (B) I
2
Rt (C) IR
2
t (D) IRt
2

(E) IR/t

3.

Which of the following will cause the electrical resistance of certain materials known as
superconductors to suddenly
decrease to essentially zero?

(A) Increasing the voltage applied to the material beyond a certain threshold voltage

(B) Increasing the pressure applied to the material beyond a certain threshold pressure

(C) Cooling the material below a certain threshold t
emperature

(D) Stretching the material to a wire of sufficiently small diameter

(E) Placing the material in a sufficiently large magnetic field

4.

Kirchhoff’s loop rule for circuit analysis is an expression of which of the following?

(A) Conservation of cha
rge (B) Conservation of energy (C) Ampere's law

(D) Faraday's law (E) Ohm's law

5.

How much current flows through a 4 ohm resistor that is dissipating 36 watts of power?

(A) 2.25 A

(B) 3.0 A

(C) 4.24 A

(D) 9.0 A

(E) 144 A

6.

The
five resistors shown below have the lengths and cross

sectional areas indicated and are
made of material with the same resistivity. Which resistor has the least resistance?

7.

When there is a steady current in the circuit

above
, the amount of charge passing a point
per unit of time is

(A) the same everywhere in the circuit

(B) greater in the 1

resistor than in the 2

resistor

(C) greater in the 2

resistor than in the 3

resistor

(D) greater at point X than at point Y

(E) greater in the 1

resistor than in the 3

resistor

8.

certain coffeepot draws 4.0 A of current when it is operated on 120 V household lines. If
electrical energy costs 10 cents per kilowatt

hour, how much does it cost to operate the
coffeepot for 2 hours?

(A) 2.4 cents (B) 4.8 cen
ts (C) 8.0 cents (D) 9.6 cents (E) 16 cents

9.

A wire of length
L

r

has a resistance
R
. What is the resistance of a second wire
the same material that has a length
L/2

r/2
?

(A)

4R
(B)
2R
(C)
R

(D)

R/2
(E)

R/4

10.

The operating efficiency of a 0.5 A, 120 V electric motor that lifts a 9 kg mass against gravity
at an average velocity of 0.5 m/s is most nearly

(A) 7% (B) 13% (C) 25% (D) 53% (E) 75 %

11.

When lighted, a 100

watt light bulb operating on a 110

volt household circuit has a
resistance closest to

(A) 10

2

(B)

10

1

(C) 1

(D) 10

(E) 100

12.

The power dissipated in a wire carrying a constant electric current I may be written as a
function of the length
l

of the wire, the diameter d of the wire, and the resistivity

of the
material in the wire. In this expression, the power dissipated is directly proportional to
which of the following?

(A)
l

only (B) d only (C)
l

and

only (D) d and

only (E)
l
, d, and

13.

A wire of resistance

R

dissipates power
P

when a current

I

passes through it. The wire is
replaced by another wire with resistance
3R
. The power dissipated by the new wire when
the same current passes through it is

(A) P/9

(B) P/3 (C) P (D) 3P (E) 6P

14.

A narrow beam of protons produces a current of 1.6 × 10

3

A. There are 10
9

protons in each
meter along the beam. Of the following, which is the best estimate of the average speed of
the prot
ons in the beam?

(A) 10

15

m/s (B) 10

12

m/s (C) 10

7

m/s (D) 10
7

m/s (E) 10
12

m/s

15.

A hair dryer is rated as 1200 W, 120 V. Its effective internal resistance is

(A) 0
.1 Ω (B) 10
Ω (C) 12 Ω (D) 120 Ω (E) 1440 Ω

16.

Two conducting cylindrical wires are made out of the same material. Wire X has twice the
length and twice the diameter of wire Y. What is the ratio
R
x
/R
y

of their resistances?

(A)
1/4 (B) ½ (C) 1 (D) 2 (E) 4

17.

You are given three 1.0

resistors. Which of the following equivalent resistances
CANNOT
be produced using all three resistors?

(A) 1/3

(B) 2/3

(C) 1.0

(D) 1.5

(E) 3.0

18.

A
variable resistor is connected across a constant voltage source. Which of the following
graphs represents the power P dissipated by the resistor as a function of its resistance R?

19.

Wire I and wire II are made of the same material. Wire II has twice the diameter and twice
the length of wire I. If wire I has resistance
R
, wire II has resistance

(A)
R
/8 (B)
R
/4 (C)
R
/2 (D)
R
(E) 2
R

20.

A heating coil is rated 1200 watts

and 120 volts. What is the maximum value of the current
under these conditions?

(A) 10.0 A (B) 12.0 A (C) 14.1 A (D) 0.100 A (E) 0.141 A

21.

What is the resistance of a 60 watt light bulb designed to operate at 120 volts?

(A)

0.5

(B) 2


(C) 60


(D) 240


(E) 7200


22.

An electric heater draws 13 amperes of current when connected to 120 volts. If the price of
electricity is \$0.10/kWh, what would be the approximate cost of running the heater for 8
hours?

(A) \$0.19

(B) \$0.29

(C) \$0.75

(D) \$1.25

(E)

\$1.55

23.

A cylindrical resistor has length
L

r
. This piece of material is then drawn so that it
is a cylinder with new length 2
L
. What happens to the resistance of this material because of
this process?

(A) the resistance is quartered.

(B) the resistance is halved.

(C) the resistance is unchanged.

(D) the resistance is doubled.

24.

A
cylindrical graphite resistor has length L and cross

sectional area A. It is to be placed into
a circuit, but it first must be cut in half so that the new length is ½ L. What is the ratio of the
new resistivity to the old resistivity of the cylindrical r
esistor?

(A) 4

(B) 2

(C) 1

(D) ½

(E) ¼

25.

A current through the thin filament wire of a light bulb causes the filament to become white
hot, while the larger wires connected to the light bulb remain much cooler. This happens
because

(A) the

larger connecting wires have more resistance than the filament.

(B) the thin filament has more resistance than the larger connecting wires.

(C) the filament wire is not insulated.

(D) the current in the filament is greater than that through the conn
ecting wires.

(E) the current in the filament is less than that through the connecting wires.

26.

A household iron used to press clothes is marked “120 volt, 600 watt.” In normal use, the
current in it is

(A) 0.2 A (B) 2 A (C) 4 A (D) 5 A

(E) 7.2 A

27.

Approximately how much would it cost to keep a 100
W
light bulb lit continuously for 1
year at a rate of \$0.10 per
kW

hr?

(A)
\$1
(B)
\$10
(C)
\$100
(D)
\$1000
(E)
\$100000

28.

In terms of the seven fundamental SI units in the MKS system, the Ohm is written as

(A)

(B)

(C)

(D)

(E)

29.

A
junior Thomas Edison wants to make a brighter light bulb. He decides to modify the
filament. How should the filament of a light bulb be modified in order to make the light bulb
produce more light at a given voltage?

(A) Increase the resistivity only.

(B) I
ncrease the diameter only.

(C) Decrease the diameter only.

(D) Decrease the diameter and increase the resistivity.

(E) Increase the length only

30.

Which of the following graphs would best represent the resistance versus temperature
relationship for a superconductor?

(A)

(B)

(C)

(D)

(E)

31.

Two cables can be used to wire a circuit. Cable
A
has a lower resistivity, a larger diameter,
and a different length than cable
B
. Which cable should be used to minimize heat loss if the
same current is maintained in either cable?

(A) Cable
A

(B) Cable
B

(C) The heat loss is the same for both.

(D) It cannot be determined without knowing the length of each cable.

(E) It cannot be determined without knowing the materials contained in each cable

32.

In a 30
-
minute interval, one kilowatt
-
hour of electrical energy is dissipated in a resistance
of 20 ohms by a current of

(A)
10 amp. (B)
20 amp. (C) 14.1 amp. (D) 36 amp. (E) 18 amp.

33.

A fixed voltage is applied across the length of a tungsten wire. An increase in the power
dissipated by the wire would result if which of the following could be increased?

(A) The
resistivity of the tungsten

(B) The cross
-
sectional area of the wire

(C) The length of the wire

(D) The temperature of the wire

(E) The temperature of the wire’s surroundings

34.

The Current in an electron beam in a cathode
-
ray tube is 7.0

10

How much charge hits
the screen in 5.0 s?

a.2.8

10

C

b.
3.5

10

C

c
.

5.6

10

C

d.

5.3

10

C

35.

T
he amount of charge that moves through the filament of a
light bulb

in 2.00 s is 2.67 C.

What is the current in the
light bulb
?

a.

5.34 amps

b.

1.33
amps

c.

0.835
amps

d.

0.417
amps

36.

When you flip a switch to turn on a light, the delay time before the light turns on is
d
etermined by

a.

the number of electron collisions per second in the wire.

b.

the drift speed of the electrons in the wire.

c.

the

speed of the electric field moving in the wire.

d.

the resistance of the wire.

37.

The power ratings on
light bulbs

are measures of the

a.

rate that they give off heat and light.

b.

voltage they require.

c.

density of the charge carriers.

d.

amount o
f negative charge passing through them.

38.

If a 75 W light

bulb operates at a voltage of 120 V, what is the current in the bulb?

a.
0.62
amps

b.

1.6
amps

c.
1.95

10

amps

d.

9.0

10

amps

39.

If the current through a 5.00

10

W heater is 4.00 A, what is the potential difference

across the ends of the heating element?

a.

2.00

10

V

b.
1.25

10

V

c.
2.50

10

V

d.

8.00

10

V

40.

If a 325 W heater has a current of 6.0 A, what is the resistance of the
heating element?

a.

88

b.

54

c.

9.0

d.

4.5

41.

Which process will double the power dissipated by a resistor?

a.

doubling the current while doubling the resistance

b.

doubling the current and making the resistance half as big

c.

doubling the current and doubling the potential difference

d.

doubling the current while making the potential difference half as big

42.

How much does it cost to operate a 695 W heater for exactly 30.0 min if electrical energy

costs \$0.060 per kW

h
?

a.

\$0.02

b. \$0.18

c.

\$0.36

d. \$0.90

43.

A microwave draws 5.0 A when it is connected to a 120 V outlet. If electrical energy costs

\$0.090 per kW

h, what is the cost of running the microwave for exactly 6 h?

a.

\$2.70

b. \$1.60

c.

\$0.72

d. \$0.32

A

1.

R =

L/A. Greatest resistance is the longest, narrowest resistor.

2.

W = Pt = I
2
Rt

3.

Resistance varies directly with temperature. Superconductors have a resistance
that quickly goes to zero once the
temperature lowers beyond a certain
threshold.

4.

The loop rule involves the potential and energy supplied by the battery and it’s
use around a circuit loop.

5.

P = I
2
R

6.

R =

L/A. Least resistance is the widest, shortest resistor

7.

The upper branch, with twic
e the resistance of the lower branch, will have ½ the
current of the lower branch.

8.

Power = IV = 480 W = 0.48 kW. Energy = Pt = (0.48 kW)(2 hours) = 0.96 kW
-
h

9.

R =

L/A. If L ÷ 2, R ÷ 2 and is r ÷ 2 then A ÷ 4 and R × 4 making the net effect R ÷ 2
× 4

10.

The motor uses P = IV = 60 W of power but only delivers P = Fv = mgv = 45 W of
power. The efficiency is “what you get” ÷ “what you are paying for” = 45/60

11.

P = V
2
/R

12.

P = I
2
R and R =

L/A giving P

L/d
2

13.

P = I
2
R

14.

Dimensional analysis:
1.6 × 10

3

A = 1.6 × 10

3

C/s ÷ 1.6 × 10

19

C/proton = 10
16

protons/sec ÷ 10
9

protons/meter = 10
7

m/s

15.

P = V
2
/R

16.

R =

L/A

L/d
2

where d is the diameter.
R
x
/R
y

= L
x
/d
x
2

÷ L
y
/d
y
2

=

(2L
y
)d
y
2
/[L
y
(2d
y
)
2
] = ½

17.

Using all three in series = 3

, all three in parallel = 1/3

. One in parallel with
two in series = 2/3

, one in series with two in parallel = 3/2

18.

P = V2 / R and if V is constant P

1/R

19.

R =

L/A

L/d
2

where d is the diameter. R
II
/R
I

= LII/d
II
2

÷ LI/d
I
2

= (2L
I
)d
I
2/[LI(2d
I
)
2
]
= ½

20.

P = IV

21.

P = V
2
/R

22.

P = IV = 1.56 kW. Energy = Pt = 1.56 kW × 8 h = 12.48 kW
-
h

23.

Since the volume of material drawn into a new shape in unchanged, when the
length is doubled, the area is halved. R =

L/A

24.

Resistivity is dependent on the material. Not
to be confused with resistance

25.

In series circuits, larger resistors develop more power

26.

P = IV

27.

1 year = 365 days × 24 hours/day = 8760 hours. W (energy) = Pt = 0.1 kW × 8760
hours = 867 kW
-
h × \$0.10 per kW
-
h = \$ 86.7

28.

R = V/I where V = W/Q and Q = It
giving R = W/I
2
t and W = joules = kg m
2
/s
2

29.

For more light at a given voltage, more current is required, which requires less
resistance. R =

L/A

30.

Superconductors have a property where the resistance goes to zero below a
certain threshold temperature.

31.

R
=

L/A

32.

1 kW
-
h = 1000 W × 60 min = 60,000 W
-
min = I
2
Rt = I
2
(20

)(30 min)

33.

P = V
2
/R and R =

L/A giving P = V
2
A/

L

34.

Q = I

t

35.

I =

Q /

t

36.

I = P /

V

37.

V = P / I

38.

R = P / I
2

Given

P

= 695 W

t

= 30.0 min

Energy
cost

= \$0.060 per kW

h

Solution

Given

I

= 5.0 A

V

= 120 V

Energy cost

= \$0.090 per kW

h

t

= 6 h

Solution