Transcript Science Olympiad Division B

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 won’t light!
If the wire is
the bulb
won’t 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.
-
+
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, CFL’s, LED’s, 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
Light Bulb
Voltmeter
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?
Switch (closed)
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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Switch (open)
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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?
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.
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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?
Let’s put the ammeter on the
opposite side of the circuit. What will
be different about the current?
a)The current will be less.
b)The current will be the same.
c)The current will be greater.
Make your prediction.
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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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5
Multi-scale ammeter.
50
If the other wire is connected to the “50”
terminal, you read the 50 mA scale.
5
500
50
-
If the other wire is connected to the “500”
terminal, you read the 500 mA scale.
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.
I would not recommend trying this unless you really know what you are
doing. And
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?
ΣVGAIN = ΣVLoss
120 V = V1 + V2 + … + V100
120 V = 100V
V = 1.20 volts
ΣVGAIN = ΣVLoss
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?
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
going through
.
and splits, some going through
and the other
The current rejoins at the next
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. Let’s 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! That’s 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! That’s 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 Ω.
Named after Georg Simon Ohm.
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Resistance
As you shorten the nichrome wire you
will notice that:
1)The current increases.
2)VNW decreases.
3)VB increases.
4)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?
the current, I, increases
a lot.
Bulbs #1 & #2 go out.
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.
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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?
the current, I, increases
a lot.
Bulbs #1 & #2 go out.
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.
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THE END!
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