Download Lab 6: Parallel Circuits - Differences between series and parallel

Lab 6: Parallel Circuits
- Differences between series and parallel circuits
- Adding devices to a parallel circuit
- Current in a parallel circuit
16) The diagram below shows the circuit diagram for a string of holiday light ornaments
(where the .......... represents more of the same pattern of bulbs):
What kind of circuit is this? What is the main disadvantage to stringing lights in this
manner? Draw a circuit diagram representing a better way to connect a similar
configuration of lights.
This is a series circuit. The main disadvantage is that if one light
burns out on the string, all the lights go out (and finding the burnt out
bulb is very difficult....) Wiring them in parallel as follows is a better
idea:
17) Below is a simple parallel circuit for a room with 3 100-watt light bulbs, connected to
a 120 volt source.
120 V
a) A circuit breaker is needed in case the current gets too high. Where a good place for
a circuit breaker, and why? Mark this spot with an “X” on the diagram.
b) What is the current through the circuit breaker if one switch is closed?
current = power / voltage = 100 watts / 120 volts = 0.83 amperes
c) What is the current through the circuit breaker if all 3 switches are closed?
In a parallel circuit, additional elements do not reduce the current
(they all use the full voltage), so the current near the source is
0.83 amperes ✕ 3 = 2.5 amperes
d) How does the total power used change between cases b) and c) ?
The voltage doesnʼt change, and the current triples, so the power
used also triples.
18) Draw a circuit diagram containing batteries, a switch, an LED and a capacitor, such
that when the switch is closed, the LED lights and capacitor charges, and that when
the switch is opened, the batteries are cut off, and the LED is powered by the
capacitor alone. The symbol for a capacitor is
.
When the switch below is closed, the LED and capacitor are
connected in parallel, and power goes to both. When the switch is
opened, the battery is removed from the circuit, and the current flows
only from the capacitor to the LED, until the capacitor is discharged.
Lab 7: More Complicated Circuits
- Circuits with both series and parallel elements
- Resistance
- Electrical safety
19) Given the following circuit:
A
B
C
D
Predict the relative brightnesses of light bulbs A, B, C and D, if all the bulbs are
identical, explaining your answer.
A is the brightest, as it receives the full current in the circuit. The
current then splits, making B the 2nd brightest, and C & D are tied for
least bright, because they are connected in series and the remaining
voltage must be shared among them.
20) Tyler and Stephanie were both informed that if you are trapped outside during an
electrical storm, that the safest place to seek shelter is in your car. Tyler says that
this is because the rubber tires act as an electrical insulator. Stephanie recalls the
data given in Problem 14), and said this canʼt be correct, there must be another
reason youʼre safe in a car during a thunderstorm.
a) Rubber insulating shoes are sold online for electrical workers, capable of insulating
against up to a few thousand volts. Given the data in Problem 14), is it reasonable to
think that only a slightly thicker tire can insulate against a lightning bolt? (Who is
correct then, Tyler or Stephanie?)
Stephanie is correct; if rubber shoes insulate only against a few
thousand volts, thereʼs no way that tires, which are only a bit thicker,
can insulate against a 100 million volt lightning bolt.
b) A lightning bolt is a discharge between the sky and ground. Thinking about the
materials a car is made from, what is the path of least resistance in the car, should a
lightning bolt strike it?
The carʼs frame is made of metal - metal has a much lower resistance
than your body and air in the car, so the electrical current of a
lightning bolt is more likely to travel through the frame of the car than
a person in the car.
c) Are you actually safer in a car than outside during a thunderstorm? If so, why?
Yes, you are safer in a car than standing outside, for the reasons
stated above, not because of the rubber tires, but because youʼre
sitting inside a cage of metal with low resistance. This sounds
counterintuitive, but it is true - the metal makes the lightning form a
“short circuit”, and the current avoids you. If your car gets struck by
lightning, it would likely receive damage, but you would probably be
relatively safe inside. ITʼS NOT BECAUSE OF THE RUBBER TIRES!!
21) An old 4800-watt electric clothes dryer is plugged into a 240 volt outlet. A frayed wire
inside causes the metal frame of the dryer to come into contact with the circuit. Does
this situation cause a potential danger? What precaution is taken in most major
appliances to lessen this danger?
Yes, this can be VERY dangerous, because the frame of the dryer
becomes part of the circuit, and if you touch it, YOU could possibly
become part of the circuit and receive a potentially lethal shock. For
this reason, appliances with frayed/worn wiring should either be
professionally repaired or discarded. In most appliances, there is a
3rd prong on the plug, the “ground” wire. This prong is normally
inactive, but is connected to the frame of the appliance, so that if
something DOES go wrong, it provides a path of low resistance for
the stray current, and the current avoids YOU.