ELECTRIC
CURRENT
Mr. Charge’s Journey
PROBLEM #1
The amount of charge that passes
through the filament of a certain
light bulb in 2 seconds is 1.67 C.
A) What is the current in the light
bulb?
B) What is the number of electrons
that pass through the filament in
one second?
A)
Q 1.67
I 
 0.835 A
t
2
B) In 2 s 1.67 C of charge passes
In 1 s : 0.835C of charge will pass
1e
 0.835C 
-19
1.6  10 C
18
 5.22  10 electrons
PROBLEM #2
1 x 1020 electrons flow through a
conductor in 3 s.
A) What is the charge flowing
through the conductor?
B) What is the current in the
circuit?
A) # e  1  10 20 e
1.6  10
Q  1  10 e 
1e
Q  16C
20
19
C
t  3s
Q
16
C
B) I 

 5.33 A
t
3s
PROBLEM #3
How many electrons pass a
point in 5 s if a constant
current of 8 A is maintained in
a conductor?
I  8A
Q?
t  5s
Q
I
t
Q  I t  8  5
1e
20
#  40C 

2.5

10
e
19
1.6  10
PROBLEM #4
The amount of charge that passes
through the filament of a certain
light bulb in 2 seconds is 12C.
A) What is the current in the light
bulb?
B) What is the number of electrons
that pass through the filament in
one second?
Q 12
A) I 

 6A
t
2
B) Current of 6 A means 6 C of charge
will pass through in 1 second.
 6(6.25  10 )  3.75  10 electrons
18
19
PROBLEM #5
How many electrons pass a
point in 10 s if a constant
current of 20 A is maintained in
a conductor?
I  20A
Q?
t  10s
Q
I
t
Q  I  t  20  10
18
21
#  200 (6.25  10 )  1.25  10 e
PROBLEM #6
How many amperes of current
flow in a wire through which
1018 electrons pass per
second?
I?
# electrons  1  10
18
Q?
t  1s
Q  1  10 (1.6  10
18
19
Q .16C
I 
 .16 A
t
1s
)  .16C
PROBLEM #7
The headlights of a typical
car are powered by a 12 V
battery. What is the resistance
of the headlights if they draw
3 A of current when turned
on?
I  3A
V  12V
R?
V 12
R

 4
I
3
PROBLEM #8
Find the resistance of a
portable lantern that uses
a 24 V power supply and
draws a current of 0.80 A.
I  .80 A
V  24V
R?
V
24
R

 30
I
.80
PROBLEM #9
The current in a resistor of is
0.50 A when connected
across a voltage of 120V.
What is its resistance?
I  0.50 A
V  120V
R?
V 120
R 
 240
I 0.5
PROBLEM #10
An automobile headlight with a
resistance of 30 ohms is placed
across a 12 V battery. What is
the current through the circuit?
I ?
V  12V
R  30
V 12
I 
 0 .4 A
R 30
PROBLEM #11
A motor with an operating
resistance of 32 ohms is
connected to a voltage
source. The current in the
circuit is 3.8 A. What is the
voltage of the voltage source?
I  3 .8 A
V ?
R  32
V  IR  3.8(32)  121.6V
PROBLEM #12
A transistor radio uses 0.0002 A
of current, when a 3 V battery
operates it. What is the
resistance of the radio circuit?
I  0.0002 A
V  3V
R?
V
3
R 
 15000
I 0.0002
PROBLEM #13
What is the resistance of an
electric frying pan that draws
12 amperes of current when
connected to a 120 V circuit?
I  12 A
V  120V
R?
V 120
R 
 10
I
12
PROBLEM #14
How much current is drawn by
a lamp that has a resistance of
100 ohms when a voltage of 50
volts is impressed across it?
I ?
V  50V
R  100
V
50
I 
 0.5 A
R 100
PROBLEM #15
The plate on a certain steam
iron states that the iron carries
a current of 6.4 A when
connected to a 120 V source.
What is the resistance of the
steam iron?
I  6 .4 A
V  120V
R?
V 120
R 
 18.75
I 6 .4
PROBLEM #16
The resistance of the hot plate
is 48 ohms. How much current
does the plate carry when
connected to a 120 V source?
I ?
V  120V
R  48
V 120
I 
 2 .5 A
R 48
EXAMPLE 1
A heater has a resistance of 10 .
It operates on 120 V.
A) What is the current through the
resistance?
B) What thermal energy is
supplied to the heater in 10 s?
V 120
I 
 12 A
R 10
E  Pt
 VIt
 120(12)(10)
 14400J
EXAMPLE 2
A current of 6 A flows
through a resistance of
300  for one hour. What
is the power loss? How
much heat is generated
in joules?
V  IR  6(300)  1800V
P  VI
 1800(6)
 10800 W
E  Pt
 10800 W (3600s)
 3.8  10 J
7
PROBLEM #17
A 15 ohm electric heater operates on
a 120 V outlet.
A) What is the current through the
heater?
B) How much energy does the heater
liberate in 30 seconds?
C) How much thermal energy does
the heater liberate in this time?
V 120
I 
 8A
R 15
E  Pt
 VIt
 120(8)(30)
 28800J
PROBLEM #18
A 30-ohm resistor is connected
across a 60 V battery.
A) What is the current in the
circuit?
B) How much energy does the
resistor use in 5 minutes?
V 60
I 
 2A
R 30
E  Pt
 VIt
 60(2)(5)(60)
 36000J
PROBLEM #19
A soldering iron draws
0.75 A at 120 V. How
much energy does the
resistor use in 15 minutes?
E  Pt
 VIt
 120(.75)(15)(60)
 81000J
PROBLEM #20
An electric lamp has an 80
ohm filament connected to a
110 V direct current line. What
is the current through the
filament? What is the power
loss in watts?
V 110
I 
 1.375 A
R 80
P  VI
 110(1.375)
 151.25 W
PROBLEM #21
How much does it cost to
burn 100 W light bulb for 24
hours if the electrical
energy costs $0.08 per
kilowatt-hour?
P  100 W  0.1kW
t  24h
E  Pt  0.1kW(24h)  2.4kWh
$.08
Cost  2.4kWh(
)  $0.192
kWh
PROBLEM #22
A modern color TV set draws 2 A
when operated on 120 V. A) How
much power does the set use? B) If
the set is operated for an average
of 7 hours/day, what energy in kWh
does it consume per month (30
days) C) At $0.11 per kWh, what is
the cost of operating the set per
month?
A)
P  VI
 120V (2 A)  240 W  .240kW
B)
E  Pt
 .240kW(7)(30h)  50.4kWh
$.11
)  $5.544
C) Cost  50.4kWh(
kWh
PROBLEM #23
An electric space heater draws 20 A
from a 120V source. It is operated on
the average of 5 hours each day.
A) How much power does the heater
use?
B) How much energy in kWh does it
consume in 30 days?
C) At $.11 per kWh, what does it cost
to operate it for 30 days?
A)
P  VI  120(20)
 2400 W  2.4kW
B)
E  Pt
 VIt
 2.4kW(5)(30h)  360kWh
C) Cost  360(.11kWh)
PROBLEM #24
A digital clock has an operating
resistance of 12,000 ohms and is
plugged into a 115 V outlet. Assume the
clock obeys ohm’s law.
A) How much current does it draw?
B) How much power does it use?
C) If the owner of the clock pays $0.09
per kWh, what does it cost to operate
the clock for 30 days?
A)
V
115
I 
 .0096 A
R 12000
P  VI
 115(.0096)  1.1W  .0011kW
B)
E  Pt
 .0011kW(24)(30h)  .792kWh
$.09
C) Cost  .792kWh(
)  $.07
kWh
PROBLEM #25
An electric heater is
rated 500 W.
A) How much energy is
delivered to the heater in
half an hour?
P  500 W  .5kW
E  Pt
 .5kW(.5h)  .25kWh
or
60 sec
E  500 W (30 min)(
)
1 min
 900000 Ws  900000J
PROBLEM #26
Each coil in a resistance box is
capable of dissipating heat at
rate of 4 W. What is the
maximum current that should
be allowed across the coil to
avoid overheating if the coil
has of 2 ohms?
P  4W
R  2
P  VI  ( IR)( I ) P  I R
2
I
P
4

 1.41A
R
2
PROBLEM #27
How much does it cost to
run a 2 W clock for one
year (365.25 days) if it costs
3.53 cents per kWh?
P  2 W  .002kW
E  Pt
 .002kW(24h)(365.25)
 17.532kWh
$.0353
Cost  17.532kWh(
)
kWh
 $.62
PROBLEM #28
A small electric furnace that expends 2
kW of power is connected across a
potential difference of 120 V.
A) What is the current in the circuit?
B) What is the resistance of the furnace?
C) What is the cost of operating the
furnace for 24 hours at 7 cents per kWh?
A)
P  2kW
V  120V
P 2000W
I 
 16.7 A
V
120V
V 120V
R 
 7.18
I 16.7 A
B)
E  Pt
 2kW(24hr )  48kWhr
$.07
)  $3.36
C) Cost  48kWh(
kWh