1. 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!
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2. Problem 1
Consider a ceiling lamp made from bulbs wired in parallel. The bulbs are rated 100, 75 and 60 watts respectively and operate at 120 volts.
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.
Answer B
Answer C

3. Answer 1A
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.
Problem 1
Problem 1
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. I = 0 amps.
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4. Answer 1B
Problem 1
Problem 1
60 w R1
100 w R3
120 V
75 w R2
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.
I1 = 0.5 amps I2 = amps I3 = amps
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5. Problem 2
Lamp amps
Monitor 2.2 amps
Speakers 1.5 amps
Scanner amps
Computer 4.0 amps
How much power is used to operate this computer workstation? Each device operates at 120 volts.
Video Camera amps
Printer amps
Answer

6. Back TOTAL CURRENT = 10.46 AMPS TOTAL POWER = 1255 WATTS (1.26 kW)
Answer 2
Lamp watts
Video Camera watts
Scanner watts
Computer 480 watts
Speakers 180 watts
Monitor 264 watts
Printer watts
TOTAL CURRENT = AMPS
TOTAL POWER = WATTS (1.26 kW)
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7. If Voltmeter Were a Car It Would Park In Parallel!
Consider the adjacent Series Circuit:
Draw an equivalent circuit and calculate the equivalent resistance.
Calculate the current reading on the ammeter.
Calculate the voltage drop across each Resistor.
R1=4.5 Ohms R2=7.5 Ohms R3=8 Ohms
Answer

8. DC SERIES CIRCUIT Back The equivalent resistance is Req = 20 Ohms.
Answer 3
DC SERIES CIRCUIT V A
Req
The equivalent resistance is Req = 20 Ohms.
The current in the circuit is I = 0.75 Ampere.
The Voltage Drops across:
VR1 = Volts
VR2 = Volts
VR3 = 6.0 Volts
15 V
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9. It costs how much? You've got to be kidding!
Problem 4
Scanner watts
Speakers 180 watts
Lamp 144 watts
Computer 480 watts
Monitor watts
Video Camera watts
Calculate the cost of operating this computer system for 1 month at $ 0.10 per kwh. (24 hours x 30 days.)
Printer watts
You've got to be kidding!
Answer

10. Answer 4
It costs how much?
$ per month
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11. Series Circuit Summary
Summary of DC Circuits
Parallel Circuits
The voltage is constant across circuit elements in parallel.
The current through circuit elements in parallel can change.
The sum of individual currents add to equal the total system current.
The sum of the individual element’s power add to equal the total system power.
Power = Energy / Time = I x V = V2/R = I2 x R
Series Circuit Summary

12. Summary of DC Circuits Series Circuits
The current circuit elements wired in series is constant.
The voltage can change across a circuit element wired in series.
The sum of voltage drops across individual circuit elements equals the voltage of the power supply.
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.