Download DC Circuits

DC Circuit – Practice Problems
Problem 1 – Parallel bulbs, ceiling lamps and you.
Problem 2 – The Current Issue of Powerful Computing?
Problem 3 – If a voltmeter were a car it would park like this?
Problem 4 – It costs how much? You’ve got to be kidding!
Summary - What did I learn? Here is what you should learn!
Click on this icon to return to the this slide.
Copyright - Adam Randall & Ann Roemer
P
r
o
b
l
e
m
1
Consider a ceiling lamp made from
3 bulbs wired in parallel. The bulbs
are rated 100, 75 and 60 watts
respectively and operate at 120 volts.
A. Clearly Sketch and label a circuit diagram
modeling the ceiling lamp.
The lamps are the only elements using energy in the circuit .
The circuit is protected by a 15 amp circuit breaker.
B. Calculate the current through each individual bulb.
C. Discuss relationships that exists between individual bulb
currents, total lamp current & the circuit breaker.
Copyright - Adam Randall & Ann
Roemer
A
n
s
w
e
r
1
A
P
r
o
b
l
e
m
1
P
r
o
b
l
e
m
1
The current through each individual bulb
depends on the power rating of the bulb. The
power rating on each bulb measures the rate
of which the bulb can transform electrical
energy into heat and light energy: the greater
the power, the greater the current.
The greatest current is through the 100 watt
bulb and least through the 60 watt bulb.
The sum of individual bulb currents add to equal the total lamp current.
If the lamp current exceeds the limit set by the circuit breaker the
circuit will open resulting in no current flow. I = 0 amps.
Copyright - Adam Randall & Ann Roemer
Answer
1B
P
r
o
b
l
e
m
1
P
r
o
b
l
e
m
1
60 w
R1
120 V
75 w
R2
100 w
R3
Power is the product of current and voltage therefore, current is
the ratio of power to voltage. In parallel each bulb has the same
voltage across it.
P  IV
I
P
V
P1
I1 
V
I1 = 0.5 amps
P2
I2 
V
I2 = 0.625 amps
P3
I3 
V
I3 = 0.833 amps
Copyright - Adam Randall & Ann Roemer
Problem 2
Monitor
2.2 amps
Lamp
1 .2 amps
Speakers
1.5 amps
How much power is used to operate
this computer workstation? Each
device operates at 120 volts.
Computer
4.0 amps
Video Camera
.375 amps
Scanner
.52 amps
Printer
.667
amps
Copyright - Adam Randall & Ann Roemer
Answer 2
Lamp
144 watts
Video Camera
45 watts
Scanner
62.4 watts
Computer
480 watts
Speakers
180 watts
Monitor
264 watts
Printer
80 watts
TOTAL CURRENT = 10.46 AMPS
TOTAL POWER = 1255 WATTS (1.26 kW)
Copyright - Adam Randall & Ann Roemer
If Voltmeter Were a Car It Would Park In Parallel!
V
Consider the adjacent Series Circuit:
R1
15 V
R2
A
R3
V
R1=4.5 Ohms
V
1) Draw an equivalent circuit and
calculate the equivalent
resistance.
2) Calculate the current reading
on the ammeter.
3) Calculate the voltage drop
across each Resistor.
R2=7.5 Ohms
R3=8 Ohms
Copyright - Adam Randall & Ann Roemer
Answer 3
15 V
Req
A
V
1) The equivalent resistance is
Req = 20 Ohms.
2) The current in the circuit is
I = 0.75 Ampere.
3) The Voltage Drops across:
VR1 = 3.375 Volts
VR2 = 5.625 Volts
VR3 = 6.0 Volts
Copyright - Adam Randall & Ann Roemer
P
r
o
b
l
e
m
4
Scanner
62.4 watts
Speakers
180 watts
Computer
480 watts
Monitor
264 watts
Lamp 144
watts
Calculate the cost of operating
Video Camera
48 watts
this computer system for 1 month
at $ 0.10 per kwh. (24 hours x 30 days.)
Printer
80 watts
Copyright - Adam Randall & Ann Roemer
Answer 4
$ 90.36
per
month
Copyright - Adam Randall & Ann Roemer
Summary of DC Circuits
Parallel Circuits
1) The voltage is constant across circuit elements in parallel.
2) The current through circuit elements in parallel can change.
3) The sum of individual currents add to equal the total system
current.
4) The sum of the individual element’s power add to equal the total
system power.
5) Power = Energy / Time = I x V = V2/R = I2 x R
Copyright - Adam Randall & Ann Roemer
Summary of DC Circuits
Series Circuits
1) The current circuit elements wired in series is constant.
2) The voltage can change across a circuit element wired in
series.
3) The sum of voltage drops across individual circuit elements
equals the voltage of the power supply.
4) The equivalent resistance of a circuit with more than one
circuit element wired in series is equal to the sum of the
individual circuit element’s resistance.
Copyright - Adam Randall & Ann Roemer