Chapter 35 Electric Circuits Review Questions
1. Are all the electrons flowing in a circuit provided by the battery? (35.1)
No, the electrons in a circuit are in both the battery and the circuit itself.
2. Why must there be no gaps in an electric circuit for it to carry current? (35.1)
A gap interrupts flow like closing valve in a water pipe stops the flow.
3. Distinguish between a series circuit and a parallel circuit. (35.2)
Series: single path only; parallel: 2 or more paths.
4. If three lamps are connected in series to a 6-volt battery, how many volts are impressed across each lamp? (35.3)
By symmetry, if 3 identical lamps are connected in series across a 6-volt battery then each lamp has a 2 volt drop
across it since the Σ VLoop = 0 or Σ VGain = Σ VLoss .
5. If one of three lamps blows out when connected in series, what happens to the current in the other two? (35.3)
If a lamp blows out in a series circuit the current in the other two will stop since there is a break in the circuit—an
open circuit has an infinite (  ) resistance.
6. If three lamps are connected in parallel to a 6-volt battery, how many volts are impressed across each lamp? (35.4)
Since each bulb is directly connected to the power source, each lamp will have 6 volts impressed across it—however,
the current flowing from the battery will be split across the lamps.
7. If one of three lamps blows out when connected in parallel, what happens to the current in the other two? (35.4)
Current continues unchanged in other two—in parallel there is more than one choice for the current to continue to
flow uninterrupted!
8. a. In which case will there be more current in each of three lamps—if they are connected to the same battery in series
or in parallel?
If lamps are hooked up in parallel there will be more current than in series since the resistance will be less—3 lamps
in parallel will have 3 times less resistance than 1 lamp in series since there are 3x more pathways for the current to
flow (think of traffic on a 3-lane highway vs. a 1-lane highway).
b. In which case will there be more voltage across each lamp? (35.4)
In parallel, there will be 6 V across each lamp; whereas in series the voltage is divided equally across the identical
lamps at 2 V a piece.
9. What happens to the total circuit resistance when more devices are added to a series circuit? To a parallel circuit?
(35.6)
In a series circuits, as more devices are added the resistance increases; whereas in a parallel circuit the overall
resistance decreases since there are pathways for the current to flow.
10. What is the equivalent resistance of a pair of 8-ohm resistors in series? In parallel? (35.6)
In series, the equivalent resistance of 2 8-ohm resistance would be 16 ohms since there is twice the resistance to
push through, while in parallel the equivalent resistance would only be half as much, 4 ohms since there is twice the
pathway.
11. Why does the total circuit resistance decrease when more devices are added to a parallel circuit? (35.6)
The overall circuit resistance decreases as more devices are added in parallel because there ware more pathways
for the current to flow.
12. What does it mean when you say that lines in a home are overloaded? (35.7)
Circuits are overloaded when they are carrying more than a safe current; the wires get hot and eventually can start
fires!
13. What is the function of a fuse or circuit breaker in a circuit? (35.7)
The function of a fuse or circuit breaker is to halt dangerously high current by breaking the circuit.
14. Why will too many electrical devices operating at one time often blow a fuse or trip a circuit breaker? (35.7)
When a circuit draws excess current, either the fuse wire will burn out or the bimetallic strip in a circuit breaker will
pop open, causing the circuit to break and prevent the flow of charge from causing overheating in the house wires.
15. What is meant by a short circuit? (35.7)
A short circuit occurs when a path of much less resistance is created, allowing excessive amounts of current to
flow—like when the positive lead of a power supply is connected to the negative lead; this can be very dangerous!
Think and Explain
1. Sometimes you hear someone say that a particular appliance “uses up” electricity What is it that the appliance actually
“uses up,” and what becomes of it?
An appliance actually “uses up” the fuel that produces electricity, and eventually that energy is lost to heat.
2. Why are the wingspans of birds a consideration in determining the spacing between parallel wires in a power line?
The wings of a bird may touch and connect wires, causing a dangerous short circuit and potential difference across
the poor bird!
3. Why are household appliances almost never connected in series?
The current would decrease as new appliances connected since the resistance would go up in the circuit; and the
voltage across each appliance would decrease as more devices are added in series! Your lamps would get dimmer,
hair dryer blow more weakly, toaster toast less, etc.!
4. As more and more lamps are connected in series to a flashlight battery, what happens to the brightness of each lamp?
The current diminishes and lamps glow dimmer as lamps are added in series since the overall resistance goes up in
the circuit!
5. As more and more lamps are connected in parallel to a battery; and if the current does not produce heating inside the
battery; what happens to the brightness of each lamp?
The current stays the same of lamps connected in parallel since the voltage stays the same so the brightness of each
lamp is not affected!
6. In the circuit shown, how do the brightnesses of the identical bulbs compare?
Which light bulb draws the most current? What happens if bulb A is unscrewed?
If bulb C is unscrewed?
Light bulb C is the brightest while A and B are only 1/4th as bright since they only
have ½ the voltage in series and only ½ the current in the branch sine they have
twice the resistance in series. Light bulb C draws twice the current of A & B since
it has half the resistance; if light bulb A is unscrewed then B will go dark in series
while C stays lit; if C is unscrewed than A & B will stay lit in the other branch.
7. A number of light bulbs are to be connected to a battery. Which will provide more
overall brightness, connecting them in series or in parallel? Which will run the
battery down faster, the bulbs connected in series or the bulbs connected in
parallel?
Connecting light bulbs in parallel will produce more overall brightness since the voltage will not drop and the overall
current drawn from the battery will increase to match the number of devices added. However, the tradeoff is the
battery will run down faster with devices hooked up in parallel since more current is drawn (P = IV).
8. A three-way bulb uses two filaments to produce three levels of illumination (50 W, 100 W, and 150 W) using a 120-V
socket. When one of the filaments burns out, only one level, the 50 W or the 100 W is available. Are the filaments
connected in series or parallel?
The filaments are connected in parallel! When 50 W is used, the thinner filament is lit up (higher resistance so less
current is drawn) by allowing current to flow down that branch. When 100 W is used, the thicker filament is lit up
(less resistance, more current drawn) by allowing current to flow down that branch of the circuit. When a 150 W is
used, both filaments are lit!
9. How does the line current compare with the total currents of all devices connected in parallel?
The total line current is equal to the sum of the total currents of the devices connected in parallel so that charge is
conserved! Σ IIn = Σ I Out
10. A 60-W and a 100-W bulb are connected in series in a circuit. Which bulb has the greater current flowing in it?
Which has the greater current when they are connected in parallel?
Both lightbulbs will have the same current flowing through them when connected in series because there is only 1
pathway available. In parallel, the 100-W bulb will have more current flowing through it than the 60-W bulb because
it has a lower resistance (thicker filament) to allow more current to flow and thus dissipate more power!