Download 07Circuits

More complicated circuits
How are houses wired, so that we can turn things on and off independently?
Materials
For each group
2 batteries
Some wires
4 light bulbs
Light emitting diode
Buzzer
Motor
Multimeters
Discussion: We will begin class by discussing the questions from last time:
> The circuit for the lights in a house resembles the circuit drawn at right.
The bulbs are all in parallel. What are the advantages of wiring them this way,
instead of putting them in series?
> Terry claims there is another advantage: the additional light bulbs don’t cost anything to operate.
He argues that the light bulbs are independent of each other, and so there is no way for the power
company to know how many are turned on. Paula says there is a way for the power company to
tell when a light bulb is turned on. Please explain who is right.
> Susan made an extra bright flashlight. It has two batteries (like most flashlights) but three light
bulbs. It gives off three times as much light, but the batteries don't last very long.
Here is a diagram of Susan's flashlight. Explain how the concepts
voltage, current, power, and energy are involved in this device; by
the time you are done, you should have completed explained the
sentence, " It gives off three times as much light, but the batteries
don't last very long. "
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1. Build this circuit (for all of the circuits on this page, the “battery”
should be two flashlight batteries in series)
In this circuit which light bulbs have larger currents, and which have
smaller currents?
Which have about the same currents?
Explain how the currents in the various bulbs are related to each other.
Measure the voltage between the two sides of each light bulb.
each other?
How are the voltages related to
Measure the current through each light bulb. How are these related to each other?
2. Carla is putting an overhead fan and light in her bedroom. She wants to be able to turn the fan
and light on and off using switches on the wall. Please draw in the wires that will make this work.
Check: Explain your diagram to an instructor.
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3. Here is a diagram for a house. The power company is represented by a battery. Please draw in
lines representing wires that will let the various switches turn on the lights, the fan, and ring the
doorbell. The toaster has its own built-in switch that is not shown.
Real houses also have circuit breakers. Please leave them out of this drawing (they would make it
too complicated)
4. Build your diagram, using a pair of batteries for the power company, a motor for the fan, a
buzzer for the doorbell, and a red light-emitting diode for the toaster.
Demonstrate your working circuit to an instructor.
Definitions:
Materials that can be used as part of an electrical circuit are called electrical conductors. Some
materials do not transmit current at all; these are called insulators. There are also materials that are
poor conductors: they transmit some current but are not good conductors.
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The property of a material that determines the ability of a material to resist a current is called the
resistance. Good conductors have low resistance: it’s easy to drive a large current through these.
Poor conductors (also called insulators) have high resistance. It takes a high voltage to get even a
small current through these.
Find the resistors in your kit. There is a picture of them at right. When you apply
a voltage to one of these, a current results that is proportional to the voltage but
also determined by the resistance of the device. Keeping the voltage constant (3
V, for example), we can get very different currents, as indicated in the table
below.
The symbol for a resistor is
which were folded up to
resistance decreases the current.
Resistance
10 Ohm
100 Ohm
1000 Ohm
10000 Ohm
because the early resistors were long wires
save space. Notice from the table that increasing the
Current
at 3 V
0.3 Amp
0.03 Amp
0.003 Amp
0.0003 Amp
8. Put each resistor in series with a light bulb, a buzzer, or a light-emitting diode and two batteries,
as shown in the diagram. If the current that the resistor permits is larger than
the current the device needs, the device continues to work, and the resistor
doesn't have much effect on it. If the current that the resistor permits is much
smaller than the current the device needs, the device doesn't do anything. If the
current that the resistor permits is about the same as the current the device needs,
it will function but not quite as well (the light bulb is only half as bright, for
example. Record in the table below what happens with each combination of
resistor and device, to determine the current that each device needs.
Resistance
Current
at 3 V
10 Ohm
0.3 Amp
100 Ohm
0.03 Amp
1000 Ohm 0.003 Amp
10000 Ohm 0.0003 Amp
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What the light
bulb does
What the
buzzer does
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What the LED does
Each group should hand in one copy of this page at the end of class
Group:
Names of group members present:
9. If you plug in too many things, the circuit breaker will throw. What is it that makes the circuit
breaker unhappy?
Why do they put circuit breakers in the house’s wiring?
10. Some students are discussing the circuit shown at right, and comparing
it to the usual circuit that has just one battery.
Alex says, "This is the usual arrangement of two batteries in a flashlight."
Bill says, "The light bulb will be brighter than with one battery"
Cheryl says, "This is about the same as carrying around a spare battery to
put in when the first one goes dead."
Dan says, "Nothing will happen to the battery on the left at all."
Eva says, "This isn't a good idea, because it will use up the two batteries just as fast as the usual
circuit with one battery."
Flossie says, "This is a very bad idea. It will quickly ruin the batteries."
Do you agree with any of them, or are they all wrong?
Homework assignment: read the two pages on Electrical Safety.
There might be a question about electrical safety on the test.
If you don’t know this material you might get killed.
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Electrical Safety
The batteries we are using in our activities are low voltage, and are quite safe -- assuming, of
course, that people can resist the temptation to swallow them, or throw them at each other!
However, household electricity can be dangerous, and so it should be emphasized that these battery
experiments are not to be repeated at home using the wall-socket kind; and a discussion of
electrical safety should accompany the study of electrical circuits.
Electricity poses two sorts of hazards: fire hazard and health hazard.
Fire hazard
Electrical energy can start a fire if it gets out of control. This is equally true of gas, oil, gasoline,
and even firewood; electricity is actually safer than the alternate sources of energy, because it can
be turned off quickly and easily, and because it has to stay in wires (or at least in conductors).
The electrical fire hazard can be greatly decreased by always using equipment that has been
designed for wall-socket electricity (that is, 115 Volts), and not using equipment that has a bad plug
or a wire with broken insulation. Circuit breakers and fuses help prevent fires of electrical origin,
by turning off a circuit when too much power is going into it.
Health hazard
The nervous system is based on electrical signals. An electrical current in the body will cause a
very painful shock (all out of proportion to the amount of energy involved) because it confuses the
human signaling system. A current passing through the chest is capable of causing the heart to stop
beating, because the nervous system itself uses electrical signals to control the heart.
It only takes a current of 1/100 Ampere to cause a shock -- far less than the current required to
operate a light bulb (about 1 Ampere). Fortunately, people are not good conductors, so that voltages
up to 10 Volts can be handled with complete safety. Beyond this level, electricity should be
handled respectfully. As we all know, wall-socket voltage (115 Volts) can cause serious and even
fatal shocks. The dividing line between safety and danger depends on factors such as how dry your
hands are, because wetness makes skin a better conductor. It is just the size of the current that is
dangerous, and not whether it is ac or dc, from batteries or from the power plant. If you were to
continue piling washers and damp paper until you had 200 washers of each kind, you would have a
90 V battery which could give you a very nasty shock. You should consider any voltage greater
than 20 volts to be hazardous.
It is the current through the chest, and not the voltage, that is dangerous. The first rule of electrical
safety is to avoid being part of a circuit. Birds can perch on power lines because they are not part
of a circuit: their feet are in contact with very high voltage, but there is nowhere for a current to go,
through the bird. Using wall-socket equipment in the bathtub or in a flooded basement or while
standing on a metal ladder on damp ground are all bad ideas, because they make it more likely that
you will become part of a circuit.
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We use several strategies to make electrical power safer.
The first is to make sure that all the conducting objects that you are
likely to touch are at the same voltage. We say the objects are
grounded because the traditional way to establish this condition is
to connect everything to a pipe driven into the ground. Having the
lamp base and the stove frame and the water pipes all at the same
voltage means that you will not get a shock if you happen to touch two of them at once.
Whenever you plug something in, one of the slots in the wall socket is supposed to be connected to
ground. The reason why the plug only goes in one way is so that the device will know which wire
is the ground wire.
Devices that have three-wire plugs are even better, because
now the ground wire is separate from the wires that carry
the current. The outside case of the container for the
device is connected to the third wire, so that any current
that tries to escape from the device (by way of a loose wire,
for example) gets intercepted.
Fuses and circuit breakers lower the chance of a fire originating in an electrical device. They are
detecting the amount of current that is flowing. When the current becomes large it means a lot of
electrical power is going somewhere. If the current becomes too large, the circuit breaker decides
there must be a short circuit, and disconnects the line. Unfortunately, the current needed to give a
lethal shock is too small for a circuit breaker to notice.
The most hazardous place (electrically speaking) used to be the bathroom, because there were good
conductors (pipes and water) and electrical devices (shavers, hairdryers, heaters) in close
combination. This has changed somewhat by the requirement that all bathroom circuitry be
protected by a "ground fault interrupter” (GFI). This is a device that compares the amount of
current going into a device to the amount that returns from it. If these differ by as little as 0.005
Amperes, the circuit is broken or interrupted -- in 1/1000 second! Of course you should still be
careful when using appliances that operate on household power, especially in situations where there
is water and pipes around.
Outside power lines (overhead and buried) are at even higher voltages than in the home. The
distribution lines out in the street are already at 7200 V. Things that we usually consider to be
insulators (such as wood) are sufficiently good conductors to deliver a lethal shock, when high
voltages are involved. Let's stay away from these power lines when trimming trees or working with
ladders -- even wooden ladders.
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