Science Olympiad Division B

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Science OlympiadDivision B

• Shock Value

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 Use a single D battery, a single bare wire and
a light bulb. Find four different ways to light
the light bulb using only a battery, one wire and
the bulb.
Quite often one thinks that they must run
electricity from the battery to the bulb. They
try from either end of the battery. The bulb
wont light!
If the wire is HOT the bulb wont light.
Disconnect the wire from the battery and try
something else.
So, what will work?
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 Use a single D battery, a single bare wire and
a light bulb. Find four different ways to light
the light bulb using only a battery, one wire and
the bulb.
There are two positions of the bulb at each end
of the battery. The wire must go from a different
part of the bulb to the other end of the battery.
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Obviously, there are two important parts of the
battery. The positive terminal and the negative
terminal. These terminals are at opposite ends of
the battery.
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If you look closely at the bulb, there is a
filament that glows when it is properly wired.
Many filaments are coiled.
You will also notice that one end of the filament
connects to the tip of the bulb and the other to
the threads.
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In order to light the bulb with two wires and the
bulb away from the battery, one wire must go from
one terminal of the battery () to the tip of the
bulb.
The other wire must go from the other terminal of
the battery (-) to the threads of the bulb.
It does not matter which wire goes to the tip and
which goes to the thread as long as one end of
the battery is connected to one part of the bulb
and the other terminal is connected to the other
part of the bulb.
Now we can say that we have a complete circuit.
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How would one define a complete circuit, which we
will call just a circuit?
A pathway for charge to flow from one terminal of
the battery, through the light bulb and back to
the other terminal of the battery.
A battery is an example of a SOURCE. A source
is a device that converts some form of energy
into electrical energy.
A battery converts chemical energy into
electrical energy (EE).
A generator converts mechanical energy into EE.
A solar (photovoltaic) cell converts light
(radiant) energy into EE.
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A light bulb is an example of a LOAD. A load is
a device that converts electrical energy into
some other form of energy.
A light bulb converts EE into thermal (heat)
energy and light (radiant) energy. Incandescent
bulbs need a lot of heat to have the filament
glow and give off a lot of light.
Other types of bulbs, CFLs, LEDs, etc. are more
effective in converting EE into light with little
or no heat. Which do you think require less
electricity?
A motor converts EE into mechanical energy.
A heater converts EE into thermal energy (heat).
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SCHEMATIC SYMBOLS
Battery
Ammeter
A
Light Bulb
Voltmeter
V
Resistor
Wire
Switch (open)
Junction (wires connected)
Switch (closed)
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Photo of set-up. My sketches are terrible.
Schematic of the same circuit.
What does a switch do? How does it do it?
A closed switch completes the circuit or turns
the bulb on and an open switch breaks the circuit
or turns the bulb off.
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Using an Ammeter
An ammeter measures the current flowing through
the circuit.
Current is the flow of charge through the wires.
In physics the unit for charge is a coulomb. The
symbol for current is an I and the unit is an
ampere which is abbreviated amp or A. An amp is a
coulomb per second.
An ammeter is wired in series in a circuit. That
means that you have to break the circuit to
insert the ammeter.
I would recommend an analog ammeter. When my
students used the ammeter on the multimeter, they
usually made a mistake and blew the fuse. It is
easy to blow the fuse and not too easy to replace
it.
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Using an Ammeter
A series circuit means that there is only one
pathway for the current to flow. If you look at
the circuit, the current must pass through the
ammeter and then the bulb and then the switch.
This leads to an interesting question. What is
the direction of the current?
e-
We know now that electrons move in a metal wire.
So electrons flow out of the negative terminal of
the battery, go through the ammeter, the light
bulb, and the switch and return to the positive
terminal of the battery.
But, years ago we did not know about electrons!
So what then?
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Benjamin Franklin is credited with the idea of
positive and negative charge. Positive meant an
abundance of electrical charge. So it made sense
to him that positive charge flowed from an
abundance of charge () to and absence of charge
(-).
If you look in a physics book it will say that
current is the flow of positive charge from the
positive terminal of the battery, through the
switch, the light bulb, and the ammeter and
return to the negative terminal of the battery.

Many electronics books use the flow of electrons
for current. Make sure that you look to see which
way current is defined. Also, you may want to
clarify this with the event supervisor.
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For the rest of this presentation I will use
electron flow for current.
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Using an Ammeter
Does it matter where we put an ammeter in a
circuit? What happens to the current as it passes
through the light bulb?

• Lets put the ammeter on the opposite side of the
  circuit. What will be different about the
  current?
• The current will be less.
• The current will be the same.
• The current will be greater.
• Make your prediction.

Now wire it and see for yourself.
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Hopefully, by now it is obvious that the current
into the bulb is the same as the current out of
the bulb.
So, it does not matter where you put the ammeter
or the switch in a series circuit. The current
everywhere is the same!
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If the values for the current are not the same,
make sure that you have good connections in the
circuit. To convince you I put two ammeters in
this circuit one before the bulb and one after
the bulb.
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Multi-scale ammeter.
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500
Here you have to look at where the wires are
connected. There is always one wire connected to
the black (-) terminal.
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If the other wire is connected to the 50
terminal, you read the 50 mA scale.
If the other wire is connected to the 500
terminal, you read the 500 mA scale.
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50
500
If the other wire is connected to the 5
terminal, you read the 5 A scale.
Only one scale can be used at a time!
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Voltage or Potential Difference
So, the current does not change when it passes
through a light bulb, then what does cause the
bulb to light? To answer this there is another
property that we need to measure.
A voltmeter measures the voltage (potential) gain
at the battery (source) or the voltage
(potential) drop or loss at the light bulb
(load). Look back at sources and loads.
A voltmeter is wired in parallel to the circuit.
You do not need to break the circuit. All you
have to do is touch the red () probe of the
voltmeter to the positive terminal of the battery
and the black (-) probe of the voltmeter to the
negative terminal of the battery. If done
properly the reading should be positive. If you
get a negative value you either connected the
probes backwards on the voltmeter or to the
battery.
A parallel circuit has multiple paths. You are
actually diverting a very tiny current through
the voltmeter in order to get a reading. You will
observe series and parallel circuits for light
bulbs later.
The symbol for voltage is a capital V and the
unit is a volt, V.
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Voltage or Potential Difference
Orienting a voltmeter is a little more
challenging than an ammeter.
Touch the red () probe of the voltmeter to the
positive terminal of the battery.
Touch the black (-) probe of the voltmeter to the
negative terminal of the battery.
If done properly the reading should be positive.
If you get a negative value you either connected
the probes backwards on the voltmeter or to the
battery.
WHY?
If you move the probes together around the
circuit to the light bulb you should have the
probes connected in the following orientation and
the reading should be negative.
The value should be close to the reading at the
battery, except it is negative.
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Remember that electrons flow around the circuit
from the negative terminal of the battery through
the loads along the way and back to the positive
terminal of the battery. The size of the current
never changes.
One way to illustrate this is to say that the
electrons pick up energy at the battery (positive
voltage or voltage gain) and gives up its energy
at the light bulb (negative voltage or voltage
loss).
You may find that the voltage loss at the light
bulb is a little bit less than the voltage gain
at the battery. This is due to losses in the
ammeter, switch, wires poor connections or a
combination of both.
Keep in mind that energy is conserved. So as one
goes around the circuit in a complete loop, the
voltage gains must be equal to the voltage losses.
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Two Bulbs in Series
You will probably find that the sum of the
voltage losses at the bulbs, V1 V2, is close in
value to the voltage gain at the battery, VS.
Remember there can be other losses.
You will notice that the bulbs are very dim or
maybe out, but there is still current registered
on the ammeter.
If you are lucky the bulbs are identical and they
are both the same brightness. Chances are they
are not and one is dim and the other is dimmer or
out.
Note In any complete loop around the circuit
from the negative terminal of the battery,
through the loads, to the positive terminal of
the battery, and back to the negative terminal,
that the sum of the voltage gains equal the sum
of the voltage losses.
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Two Bulbs in Series
What happens when you unscrew Bulb 1?
Bulb 2 goes out. The ammeter indicates that the
current, I ,is zero. The circuit is broken!
What happens when you unscrew Bulb 2?
Bulb 1 goes out. The ammeter indicates that the
current, I ,is zero. The circuit is broken!
Some of the older Christmas lights were wired in
series and when one burned out they all went out.
This drove many a person crazy trying to find out
which one blew out.
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Two Bulbs in Series
4. "Christmas Lights" are often wired in series
just like the circuit you just tested. a) What
happens when one of the lights burns out? Why?
As we just saw, as one bulb goes out (the
filament breaks, which is just like unscrewing a
bulb) the circuit is broken and there is no
pathway for the current and all the bulbs go out.
The challenge then is to figure out which one
went out and replace it.
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Two Bulbs in Series
4. "Christmas Lights" are often wired in series
just like the circuit you just tested. b) We can
make a string of lights in series just like the
"Christmas Lights." If you were given light bulbs
that are designed to operate with a voltage of 14
volts, how many would you have to wire in series
before it is connected to a 120-volt outlet?
Remember that the sum of the voltage gains equals
the sum of the voltage losses. If the bulb is
designed for 14 volts then 14x 120 or x 8.57.
I will need nine bulbs. I had a bunch of 14 volt
bulbs and wired them and it does work.
CAUTION! I would not recommend trying this unless
you really know what you are doing. And
DEFINITELY, DO NOT LET A STUDENT DO THIS!
A better suggestion If you have an old 80 or 100
bulb string. Cut off a socket and strip the wires
and then wire it to a 1.5 volt AA, C or D battery
and watch it light up.
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Two Bulbs in Series
4. "Christmas Lights" are often wired in series
just like the circuit you just tested. c) If you
have a one hundred bulb string set of lights that
are wired in series, what is the voltage drop
across each bulb?
SVGAIN SVLoss 120 V V1 V2 V100 120 V
100V V 1.20 volts
SVGAIN SVLoss The voltage drop across each bulb
is 1.2 volts
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Two Bulbs in Series
So, what can we do to make both bulbs bright
again?
If we now put two batteries in series, connecting
the negative terminal of the second with the
positive terminal of the first, what happens?
The voltage gain is doubled, the current
increases, and the bulbs return to their former
brightness when one bulb was wired to one battery.
AGAIN Note In any complete loop around the
circuit from the negative terminal of the
battery, through the loads, to the positive
terminal of the battery, and back to the negative
terminal, that the sum of the voltage gains equal
the sum of the voltage losses. There is still
only one path for current to travel through the
circuit.
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Two Bulbs in Parallel
What is different when you wire two bulbs in
parallel?
e-
You might want to try this with one battery first.
Both bulbs are bright. V1 V2 VS
Trace the path taken by the current.
The current leaves the battery and it all goes
through the ammeter illustrated.
The current comes to a junction and splits, some
going through Bulb 1 and the other going through
Bulb 2.
The current rejoins at the next junction goes
through the switch and returns to the battery.
Note that there are two pathways between the
junctions, Parallel circuit.
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Two Bulbs in Parallel
What happens when you unscrew bulb 1?
Bulb 1 goes out. Bulb 2 stays lit about the
same. The current drops (roughly in half). VS
V2 and stays about the same as before bulb 1 was
unscrewed.
What happens when you screw Bulb 1 back in and
unscrew Bulb 2?
Bulb 2 goes out. Bulb 1 stays lit about the
same. The current drops (roughly in half). VS
V1 and stays about the same as before bulb 1 was
unscrewed..
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How is a home wired?
 Using what you observed in series and parallel
circuits, do you think that your house in wired
in series or parallel? Explain your answer. Lets
look at the options.
If your house were wired in series, what would
happen if one light bulb blew out or was turned
off?
All the other lights and appliances would go out!
Thats no good!
If your house were wired in parallel, what would
happen if one light bulb blew out or was turned
off?
All the other lights and appliances would stay
on! Thats good!
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Resistance
When you place a meter long piece of copper wire
in the circuit, there should be no significant
change in the current.
The voltage drop across the copper wire, VCW,
will be very small. At most VCW 0.02 V
The voltage drop across the light bulb, VB, will
be close to VS.
Copper wire is a good conductor so it is said to
have little resistance.
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Resistance
When you replace the copper wire with a meter
long piece of nichrome wire, what is different?
It works best if you have 28 or 30 gage nichrome
wire. You may be able to get a piece from your
physics department in the high school.
The current in the ammeter goes down quite a
bit. VNW becomes sizable usually more than
VB. The bulb is barely lit or may not glow at
all. VS VNW VB.
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Resistance
Nicrome wire is said to have a lot of resistance.
What is resistance?
Resistance is the property of the wire the
inhibits the flow of charge.
Resistance (R) is the ratio of the voltage drop
across the device to the current passing through
the device.
R V/I and its unit is called an ohm,
abbreviated O. Named after Georg Simon Ohm.
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Resistance

• As you shorten the nichrome wire you will notice
  that
• The current increases.
• VNW decreases.
• VB increases.
• The bulb finally starts to glow or gets brighter.

What qualities of a given wire affects its
resistance?
Length, the longer the wire the greater the
resistance.
Thickness or diameter, the thicker the wire the
resistance is less.
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Resistance
Nichrome wire will glow and get hot with a great
enough voltage and current. Nichrome wire is used
to give off heat in a toaster, hair dryer or
space heater.
This is fun to do. If you go to Radio Shack and
get a cheap rheostat and replace the nichrome
wire. As you turn the knob the same thing will
happen when you changed the length of the
nichrome wire.
A rheostat acts as a dimmer switch or a volume
control by varying the resistance.
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Resistance
The symbol for a rheostat is
The schematic of the circuit is
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Short Circuit
When you do this section, only close the switch
long enough to take the required readings. You
are KILLING the battery!
What happens when you place the wire between the
terminals of Bulb 1?
the current, I, increases. Bulb 1 goes out. Bulb
2 gets brighter.
VS stays the same or increases slightly. The wire
gave a low resistance path around Bulb 1.
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Short Circuit
When you do this section, only close the switch
long enough to take the required readings. You
are KILLING the battery!
What happens when you place the wire between the
terminals shown?
VS goes to zero or close to it. The wire gave a
low resistance path from one terminal of the
battery to the other, mostly bypassing the bulbs.
the current, I, increases a lot. Bulbs 1 2 go
out.
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Short Circuit
When you do this section, only close the switch
long enough to take the required readings. You
are KILLING the battery!
What happens when you place the wire between the
terminals shown?
VS goes to zero or close to it. The wire gave a
low resistance path from one terminal of the
battery to the other, mostly bypassing the bulbs.
the current, I, increases a lot. Bulbs 1 2 go
out.
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THE END!