Download Project 1: Basic Testing Circuit

Physics
In
General Science
Electronics,
Electricity & Magnetism
Produced by
Vicphysics Teachers' Network Inc.
http://www.vicphysics.org/years9-10.html
Vicphysics Teachers' Network Inc. - July 2015
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Index
Electronics
Activity 1:
What do Electronic Components look like?
Activity 2:
Resistance with Multimeters
Activity 3:
What do Electronic Components do in a Circuit?
Activity 4:
How Does a Transistor Work?
Activity 5:
Building a Delay Circuit
Activity 6:
A Light Sensitive Indicator
Explanations of Transistor Operation
Electronics Terminology
2
3
9
11
13
15
16
18
Electricity and Magnetism
Activity 7:
Building a DC Motor
Teacher Notes:
19
21
Extracts from VELS, Level 6
Learning Focus
“… They investigate how energy may be responsible for the changes observed in …
physical processes and applications. Examples include: electromagnetism, the
operation of electronic systems, … photonics …”
“… They also explore the ways in which science concepts, language and perspectives
can be misunderstood and misrepresented. This involves students applying their
conceptual understandings to the consideration of issues significant to themselves as
individuals and to the broader society in which we live; for example …, electronic
gadgets, …”
Standards
Science knowledge and understanding
“… Students explain change in terms of energy in a range of … physical contexts. …”
Science at work
“… They use …, equipment, electronic components and instruments responsibly and
safely. …”
Safety
“… As students progress through their schooling they develop skills in the safe use of
scientific apparatus, including heating and electrical equipment …”
This material has been produced by the following members of the Australian Institute
of Physics (Victorian Branch) Education Sub-Committee: Keith Burrows, Helen Lye
and Dan O’Keeffe.
Vicphysics Teachers' Network Inc. - July 2015
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Activity 1
What do electronic components looks like?
What are their circuit symbols?
Take the components from your box one at a time and place each component on the word adjacent to
its picture.
Component
Picture
Symbol
Battery
Resistor
Light Emitting Diode
(LED)
Diode
Transistor
Capacitor
Light Dependent Resistor
(LDR)
Connecting components
The components have been placed in clear plastic holders and connected to springs. The plastic has
the symbol of the component on it.
You can use the short wires to join components together.
Bend the spring back and place the end of the wire in the spring. To place a second wire in the same
spring bend the spring in the opposite direction.
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Activity 2
Resistance with multimeters
2.1 Using a Multimeter
Probes
Turn to select:
DC volts,
amps, ohms,
AC volts
COM
black
10A
red
V mA
red







Setting up
Put the black probe into the black com socket and the red probe into the red V socket.
Use the dial to select the function you need.  measures resistance, V… measures voltage or
potential difference in a direct current circuit.
Select the range you need. You would usually start with the highest range and move to a lower
range if you need more sensitivity.
Measuring resistance: 1 on the display means that you need a higher range. For example, if you
select the 200 scale and connect the probes to a 500 resistance, the display will show 1. If you
then move the dial to the 2000 scale, the display will read 500.
Multimeters are robust in normal usage but care needs to be taken when using them to measure
electric current. The Vsocket has a maximum current of about 200 milliamps. This is only 0.2
amps so it is easy to blow the fuse if the A… scale is mistakenly used when measuring volts or
ohms.
Measuring current. Connect the red probe to the red 10A DC socket and select the A… range 10.
The values on the display screen will be in ampere (written A or amps). A break in the circuit must
be made and the multimeter connected in the gap in the circuit.
Reading the scales
 On the resistance scale, k means times one thousand. That is the 20k scale reads up to 20000
, and the 2000k scale reads up to 2000000  A reading of 25.1 could be 25.1  or
25.1kdepending on the scale selected. 25.1k can also be written as 25100  or 25100
ohms. 
 On the voltage scale m means milli or divided by 1000. A reading of 10.5 on the display when
the 20 volt scale is selected indicates a value of 10.5 volts. When the 200m scale is selected the
display would mean a reading of 10.5 mV(millivolts) or 0.0105 volts.
 On the electric current scale (A…) the symbol m means milli or divided by 1000, while the
symbol stands for micro and means divided by one million. A display reading of 151.6 when
the 200m scale is selected indicates 151.6 milliamps. This can be written 151.6mA or
0.1516A.
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2.2 Making Resistors
Resistors are made of a material with a high resistance to electrical current. They are used to control
the size of the electrical current in a circuit.
You can make resistors using the carbon in ordinary grey lead pencils.
Materials
You will need a selection of pencils including 2B, 4B and 6B pencils, paper, ruler and a multimeter
Method
1.
Rule a 3 cm pencil line on this paper using the 2B pencil. Go over the line two or three times.
2.
3.
4.
5.
6.
Connect the black probe to the black com socket on the multimeter.
Connect the red probe to the red V socket.
Turn the multimeter dial to the section and select the highest range (probably 2000k)
Touch the metal tips of the probes together. The display should read 0.0. Separate the probes.
The display should read 1.
Place the tips of both probes on your pencil line about 1 cm apart and press down firmly. The
reading on the display screen of the multimeter is the resistance of 1 cm of this pencil line.
Pencil line
Probe
Multimeter
7.
Move the probes so that they are further apart on the pencil line and read the resistance in the
new position. What happens to the resistance as the length of the line increases?
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8.
Use the multimeter to investigate the resistance of pencil lines
You could use the data table on the next page to record your measurements or you could draw up your
own data table.
 Decide what length of line you will use if you want to compare the resistance of lines made by
different pencils.
 Decide how you will draw your lines so that you can compare the resistance of different lines.
 Does the thickness of the line on the paper change the resistance?
 Does the width of the line change the resistance?
 Do you need to make two or three measurements of the resistance of each line and find an
average?
Make notes here about the method you have chosen.
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9. Data Table
You can draw the lines you are investigating in the space in the table
Pencil type
Pencil line
Length
Resistance
Questions
10.
What conclusions can you make about the type of pencil and the electrical resistance of lines
drawn with this pencil?
11.
What conclusions can you make about the width or thickness of the lines and the measured
resistance?
12.
How could the way you drew the lines have affected your results and conclusions?
13.
You could investigate the resistance of the leads used in ‘Pacer’ pencils. You could use the
leads themselves as well as the lines you make with them.
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14. 2.3 Resistor Colour Code
The resistor colour code can be used to work out the value of many resistors. In this exercise you will
use the colour code to find the resistance of small four band resistors.
Materials
 Card or board with five resistors attached. Resistor colour code chart.
 Multimeter.
Method
1. Use the resistor colour code chart to complete this table
Colour
Value
Black
0
Brown
1
Orange
Green
2
4
Grey
6
7
2. Label this diagram of a 12000 resistor by putting in the numbers for each band. The band
colours are brown, red, orange.
Tolerance:
silver or gold
Band 1:
first value
of the
resistance
3.
Band 2:
second
value of the
resistance
Band 3:
number of
zeros
What would be the code for the following resistors? Complete the table.
Resistance
Colour code
Drawing
25
200
33000
68k
Red, red, grey
Brown, black, black
1M
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9
4.



Collect a set of resistors.
Record the colour code in the table
Calculate the resistance of each one using the resistor colour code and enter the value in the table.
Use the multimeter to measure the resistance of each one and complete the table.
Resistor colour code
Calculated value (ohms)
Measured value (ohms)
5. Did your measured values of resistance agree with the calculated values?
If the values are different what could be the reasons for these differences?
6. Collect some circuit boards that have been part of electrical or electronic devices. Look for
resistors on these boards and calculate and measure the resistance of some of them. What
differences do you see when you compare them with the resistances that you used in the previous
activity?
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Activity 3
What do Electronic Components do in a Circuit?
Basic Testing Circuit
Circuit Diagram
The circuit diagram gives a simple description of the layout of the circuit, using component symbols
and connections. Your first circuit, shown below, is used to test the operation of other electronics
components. When the probes are connected, the LED lights up.
Red
Black
Circuit Diagram
a.
Construct the circuit as shown in the circuit diagram. Connect to the 9 Volt battery.
b.
Join leads P and Q. If the circuit is connected correctly, the Light Emitting Diode (LED) should
light up.
c.
Write a few lines describing how this circuit works.
Begin with “Current flows from the positive terminal of the battery, then ________________
___________________________________________________________________________
___________________________________________________________________________
d.
Place (one at a time) each of the components shown overleaf across points P and Q. Describe
how the brightness of the LED changes compared to the original brightness in ‘b.’ above.
Write logical statements.
e.
The transistor shown has 3 connections. Connect the P and Q leads to all possible pairs, that is
P to B and Q to E, then P to E and Q to B, and so for the each other possible pairing: B & C, C
& B, C & E, E & C.
Note that when the LED is bright, there is little resistance in the circuit. When the LED is dim, the
resistance of the circuit is high.
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Resistors
i) ________________
Resistors
(i)
(ii)
1 k
P
Q
1 k
1 k
P
Q
__________________
ii)
Diode
(iii)
__________________
(iv)
P
_____________
P
Q
Q
Diode
iii) ______________
Cathode (K)
Anode (A)
___________________
Light Emitting Diode LED
iv)
(v)
(vi)
P
P
Q
Q
Comment on when this LED lights.
when
(viii)
Q
Exposed to light
(ix)
P
Q
Covered from light
NPN transistor
E
B (Base), C (Collector), E (Emitter)
f.
LED
v)
______________
vi)
C
B
___________________
___________________
Light Dependent Resistors LDR
(vii)P
______________
______________
___________________
LDR
vii) ______________
___________________
viii) ______________
___________________
Find out which two cases cause current to flow in the circuit. Make a note of this, with
diagrams, in your log book. Remember that the current flows from the positive terminal to the
negative.
Connected to P
C B E
Connected
C X
to
B
X
Q
E
X
Y: LED glows, N: LED does not glow
g.
Complete the following statements:
* The LED becomes (dimmer/brighter) as the resistance in the circuit increases.
* Diodes must be placed in the circuit so that their [anode (A) /cathode (K)] is nearest the
positive terminal of the battery.
* The resistance of an LDR is large when it is (exposed to/covered from) a light source.
* In a transistor, there (is / is not) a connection between the collector and the emitter.
* If the base of a NPN transistor is joined to the positive side of a circuit, a current
(will / will not) flow from the base to emitter.
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Activity 4
How does a Transistor Work?
The Positive Rail
Lead wire
390 
+
9 Volt
D.C. Power
LED 2
-
LED 1
C
1 k
B
Flat side of E
transistor
faces left
The Negative Rail
Components:
One transistor
Two LEDs
Battery
Three connecting wires
a.
b.
c.
One 1kohm resistor (Brown, Black, Red)
One 390 ohm resistor (Orange, White, Brown)
One lead wire with clips attached
Connect the circuit to the battery and switch on.
Briefly touch the wire called the “lead wire” to the positive rail and note that both LED’s glowing.
(Ask for help if the LED’s do not glow) Disconnect the yellow lead from the positive rail.
Briefly touch a short piece of wire from C to E. Now remove it. What do you observe?
_______________________________________________________________________________
The wire enables the current to bypass the transistor. If LED 2 goes on, this tells you that the
transistor has no internal connection from C to E. There is no way for the current to get from C to E.
d.
Now touch the yellow lead to the positive rail. What do you observe now?
_______________________________________________________________________________
Current is flowing along two paths from the positive rail to the negative rail.
On the circuit diagram above draw the two circuit paths starting at the positive terminal of the battery
and finishing at the negative terminal.
Describe each of these paths.
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
The current flowing into the base and out the emitter acts as a switch to allow current to flow from the
collector to the emitter. This ‘Internal Switch’ then, causes LED 2 to light up.
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“When no base current (current flowing into the base), there is no collector current. When there is a
base current, the collector current flows”.
The next question then is “How much base current is needed to cause the collector/emitter to turn
on?” The next task answers this question.
e.
Connect the following resistors between the end of the yellow lead and the positive rail. Observe the
brightness of each LED. LED 1 indicates how much current is flowing into the base. LED 2 indicates
how much current is flowing into the collector and out the emitter. Describe your observations of
LED 1 and LED 2 for each resistor.
(i)
a 1 K resistor (Brown Black Red)
_______________________________________________________________________________
(ii)
a 10 K resistor (Brown Black Orange)
_______________________________________________________________________________
(iii) a 100 K resistor (Brown Black Yellow)
_______________________________________________________________________________
(iv)
a Wet finger from the positive rail to the yellow lead.
_______________________________________________________________________________
f.
Complete the statement: “It can be seen that a (tiny/large) current flowing into the base of a transistor
is necessary to cause the collector to internally join to the emitter”.
g.
Summarise the important points on how a transistor works as follows:
In a transistor, there is no connection between the collector and emitter unless _______________
_______________________________________________________________________________
Quite a tiny amount of base current is needed to ._______________________________________
_______________________________________________________________________________
Don’t disconnect your circuit,
you can use it for the next project
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Activity 5: Building a Delay Circuit
This circuit introduces you to a new component – a capacitor. Capacitors are used in circuits involving
time. e.g.
Photographic exposure timers.
Delay in the turning off of lights.
Flashing warning lights.
The Positive Rail
390
100 k 
+
9 Volt
D.C. Power
LED 2
C
X
Lead
Wire
1 k
B
Flat side of E
transistor
faces left
100 F
The Negative Rail
Components:
One transistor
One LED
Battery
Three connecting wires
One lead wire with clips attached
a.
b.
One 1kohm resistor (Brown, Black, Red)
One 390 ohm resistor (Orange, White, Brown)
One 100k ohm resistor (Brown, Black, Orange)
One 100 F capacitor
One 10 F capacitor
Connect the battery and touch the lead wire to point X. The LED should not be glowing.
Now lift the lead wire and watch the LED. Repeat the process a few times. How long a delay is
there until the LED glows fully?
_____________________________________________________________________________
A capacitor is like two half-full buckets of water. When a battery is connected across it, the battery
‘pumps water’ from one bucket to the other. When one bucket becomes full, and the other empty, we say
that the capacitor is “fully charged”. Before the pumping began, the capacitor was said to be
“discharged”.
The time taken to charge depends on the size of the buckets. This is shown by the size of the capacitor,
measured in micro Farads. (F) The greater capacity of the capacitor, the longer it will take to fully
charge. When discharging, any resistance in the wires connected to the capacitor will slow down the time
taken.
In our actual circuit, the capacitor remains discharged as long as the yellow lead touches point X. Once
the lead is lifted, the battery pumps charge from the negative side of the capacitor to the positive side via
the 100 k resistor. Only when the capacitor is fully charged does current flow at X into the 1 k
resistor.
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c.
Describe in your own words the explanation of how the circuit works. First draw the circuit paths
for the two cases on the diagram on the previous page. i) lead wire joined to X and ii) lead wire
lifted from X.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
d.
Predict what will happen when a 10 F capacitor replaces the 100 F capacitor. Test your ideas
on the real circuit.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
e.
Predict what will happen when the 100k ohm resistor is replaced. Test your ideas on the real
circuit.
____________________________________________________________________________
____________________________________________________________________________
Don’t disconnect your circuit,
you can use it for the next project
Vicphysics Teachers' Network Inc. - July 2015
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Activity 6: A Light Sensitive Indicator
Positive Rail
LDR
+
9 Volts
390 
1k 
C
Flat at
Bottom
B
E
1.2k 
Negative Rail
Components:
One transistor
One LED
Battery
Three connecting wires
One 1kohm resistor (Brown, Black, Red)
One 390 ohm resistor (Orange, White, Brown)
One 1.2k ohm resistor (Brown, Red, Red)
One Light Dependent Resistor
a.
Connect the battery and cover the Light Dependent Resistor (LDR). Describe what happens.
b.
________________________________________________________________________________
Expose the LDR to sunlight. Describe what happens this time.
c
d.
________________________________________________________________________________
Refer to your notes about the LDR from the earlier activity to describe the operation of an LDR
“An LDR becomes a (high/low) resistance resistor when exposed to bright light and has a
(high/low) resistance in the dark.”
Describe how the circuit works under two headings:
(i) LDR exposed to Bright Light
“The current flows from the positive battery terminal to the junction of the 390 resistor and
LDR. Some current goes through the LDR. If the LDR is exposed to bright light, then a
(large/small) current goes through the LDR and into the base of the transistor and …”
_______________________________________________________________________
_______________________________________________________________________
(ii)
_______________________________________________________________________
LDR in the Dark
Begin with the description as in (i) above.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
e.
There are many uses for circuits like this one, that respond to differing light conditions. Describe a
couple of applications.
_________________________________________________________________________
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Explanations of Transistor Operation
You will now realise that the current into the base of a transistor controls the current flowing into
the collector and out the emitter. This is why you must always start your explanation by looking
to see if there is a base current or not before discussing if, for example, an LED connected to the
collector will glow or not.
So there are two paths in a transistor, as shown in the example below. They are described as IB
(Base current) and IC (Collector current).
Positive Rail
High Energy Level
(IB + IC)
X
IC
C
+
IB
B
E
(IB + IC)
(IB + IC)
Negative Rail
Low Energy Level
The current from the battery breaks up at point ‘X’ into the base current, IB, and the collector
current, IC. These two separate currents rejoin again at the emitter, E. So the current through the
battery is equal to IB + IC. Current can only flow from the positive rail to the negative rail.
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The transistor as a current-operated switch
One of the several uses for a transistor is as a special switch, in which a small current is used to turn a
large current on or off. A transistor is an electronic component with three terminals, called the collector
(c), base (b) and emitter (e). The circuit symbol for a transistor is shown in figure 3. It is usually
connected into a circuit something like the one shown in figure 4.
collector
Y
X
base
emitter
Figure 3
Figure 4
To understand how the transistor circuit works, let us first think about what would happen in a very
similar circuit without the transistor, shown in figure 5. There are two parallel paths through this circuit.
Most of the current will take the path through resistor Y, since X has much greater resistance. If we made
the resistance of X even larger, say 20,000, then almost no current would flow through X, but the
current through Y would stay pretty much the same.
X
2,000
Y
100
Figure 5
However, with the transistor in the circuit (figure 4) things change. The resistance of the transistor itself
is small compared to the other resistors (X & Y) in the circuit, and makes very little difference in that
regard. But, if you make the resistance of X very large, so that the current through that path in the circuit
almost stops, then no current will flow through Y either. A small current through X (the base current) is
necessary for the larger current to flow through Y (the collector current). In other words, the small base
current ‘switches on’ the larger collector current.
Suppose we place the light globe next to resistor Y in the circuit in diagram 4, and the LDR in place of
resistor X. In the dark the LDR has a very high resistance, and almost no current flows through the base
of the transistor. This means that no current can flow through the indicator light, and so the light is off.
In bright light, the LDR’s resistance is low, and a small current flows through the transistor base. This
allows a larger current to flow through the indicator light, making it turn on. So using the transistor in the
circuit makes it possible to have a fairly large current through the light globe, even though the current
through the LDR is much smaller.
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Electronics Terminology
Amplifier
An electronic circuit that increases the amplitude of the input signal.
Alternating Current(AC) The current type in which electricity moves back and forth rapidly.
Australian household electricity alternates at 50 Hz (Hertz-times per
second). Measured in Amps.
Battery
Often the terms battery and cell are confused. A battery is actually a group
of cells connected together. In electric circuits, the red lead is positive (+),
the black lead is negative (-). In the circuit diagram the longer line is the
positive.
Capacitor
Components that store electric charge. The larger the capacitor, the more
charge that can be stored. Capacitance is measured in Farads, but unit such
as microfarads and picofarads are commonly used.
Cell
A container with substances that react to produce a chemical reaction.
Chemical energy is converted into electrical energy.
Conductor
A substance that conducts electricity, with a very low resistance.
Current
The movement of electrons along a wire or other conductor. By convention,
current always flows from the positive, through the circuit, and back to the
negative terminal of the battery. It is likened to water flowing through pipes.
The unit of current is the Amp (A).
Diode
Components which allow current to flow in one direction only. In resistance
terms, they have a very high resistance when connected in one way, and a
very low resistance in the other. They must be placed carefully in circuits,
facing the correct way.
Direct Current (DC)
The current type in which electricity flows in one direction only. Produced
by batteries. Used by electronic circuits. Measured in Amps.
LED
Light Emitting Diode. When connected correctly, they glow brightly. Used
in many electronic displays.
LDR
Light Dependent Resistor. In darkness, they have a very high resistance
(millions of Ohms). In light they have a low resistance. (hundreds of Ohms)
Microphone
A device able to convert sound energy into electrical energy.
Resistance
The ability of a component to oppose the flow of current. The higher the
value, the less current allowed to flow. Measured in Ohms (
to friction caused by water as it flows along a pipe.
Semiconductor
Neither a conductor nor insulator. Often it has the property to be able to
change its resistance by various means.
Switch
A device for connecting and disconnecting power.
Transistor
A semiconductor with three connections: base, collector emitter. Its
collector-emitter resistance changes when a small current flows from the
base to the emitter. There are two types: NPN and PNP.
Speaker
A device for converting electrical current into sound energy.
Voltage
Described as the ‘pressure’ exerted by the power supply on a circuit. The
higher the voltage, the more current that will flow through a circuit.
Measured in Volts (V).
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Activity 7: Making a DC Electric Motor
Principle of a DC Motor
An electric motor works because of the magnetic force on current carrying wires. In most motors, the
magnetic field is created by either permanent magnets or electromagnets which are held stationary,
and hence called the stator. The current carrying wires are formed into a coil which is made to rotate by
this magnetic force. This coil is normally wound on an iron core and is called the armature.
How it works
Here is a picture of our simple motor. The magnetic field is supplied by the disc magnet shown on top of
the battery. Our “armature” is a simple coil of copper wire but has no iron core. You will realise that
another important part of an electric motor is the commutator. This is the part that carries the current to
the rotating coil and switches direction every half turn. The commutator in this motor is very simple; it
consists of the wire extending from the coil, sitting on the bent paper clips that both hold it up and
connect it to the battery. The current is not reversed every half turn, it is simply switched off for half of
each turn.
Equipment Needed
One D size 1.5 volt battery
One magnet
One thick rubber band
One test tube
Two 50mm paper clips
90 cm of enamelled 22 gauge copper wire
One piece of sand paper or steel wool
One pencil or biro
Steps in making a DC Motor
Making the Copper Coil
1.
2.
Wind about 5 or 6 turns of insulated copper wire (22 gauge or thereabouts) around a piece of
dowel (or a test tube) about 2 cm in diameter.
Leave about 4 cm of wire at either end. Tie the ends around the loop as shown and straighten the
bits that stick out so that the coil is balanced and turns freely. You may find it best to loop the wire
around itself where it goes around the coil so that the wire sticking out is directly in line with the
centre of the coil.
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Bending the Paper Clips
3.
Straighten out two paper clips.
4.
To make the loop place the middle of each straightened paper clip across the pen and wrap the
wire around the pen. The ends of the paper clips do not need to be even.
5.
Attach the bent paper clips to the battery by a rubber band as shown. Ensure the band is tight and
the paper clips are in contact with the metal ends of the battery.
Assembling the components
6.
The next bit needs to be done carefully. Arrange the coil in the loops in the paper clips and see how it
swings. Bend the wires, if necessary, until it is well balanced and rotates fairly freely.
7.
Now take the coil out and sand off the insulation from half the circumference of
the wire along the section that was resting on the support when the coil is in
the vertical position as in the diagram on the previous page. Do this for each
end. Ensure that the bared halves are on the same side of the wire as you look at
it. Sand the wire right up to the coil.
8.
Now put the coil back in the support.
9.
Place the magnet on the battery and see if it runs! If it doesn't, give it a little flick. If it still doesn't, try
switching the coil around or taking it out and re-balancing it. Experiment with the position of the coil
relative to the magnet. Also, check to see that the area you sanded is quite free of insulation and that
the supports are in contact with that area. Also check that the paper clips are in contact with the
terminals of the battery.
It is not necessary to put the magnet on the battery as shown. It may be easier to simply place the magnet on
the table and hold the battery and coil above the magnet. A disc magnet is shown but almost any magnet will
do if you experiment with the position.
NOTE: The resistance of the coil is fairly low and so a fair bit of current will flow. For this reason don’t leave
your motor going for too long at one time. You could try finding ways to reduce the current flow!
Activities
1.
Which way does the coil turn?
_______________________________________________________________________
2.
Turn the magnet over and try again. Which way does the coil turn now?
_______________________________________________________________________
3.
Now take the coil out of the supports and put it back in the opposite way. Which way does the coil
turn now?
_______________________________________________________________________
4.
Nudge the coil across the magnet from one end to the other. How does the spin rate change?
_______________________________________________________________________
5.
Other things to try:
Put the magnet on its edge
Use a shorter piece of copper
Try a bar magnet
Clean the enamel off all way round the ends of the wire
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Teacher Notes
Using a Multimeter
1.
2.
3.
4.
The probes can be used by touching the tips to the part of a circuit or component to be tested or
jumper leads can be used to connect the probes to a component.
Care must be taken to connect each probe to the correct socket on the multimeter. For
measurement of voltage and resistance the black probe is connected to the black com socket on
the front of the multimeter and the red probe to the red Vsocket. The appropriate section of the
multimeter is then selected using the dial.
Multimeters can be used as ammeters, AC or DC voltmeters or ohm meters and have other
functions such as continuity test and transistor test.
A basic digital multimeter costs $10-$12.
Making resistors
1.
2.
3.
4.
5.
6.
7.
8.
Pencil leads contain a mixture of graphite and clay. The softer pencils (B, 2B and so on) have
more graphite. Graphite is a form of the element carbon that conducts electricity although carbon
is a non-metal element. Connecting a pencil sharpened at both ends in a circuit will allow an
electric current to flow in the circuit. Making pencil lines on paper forms a thin layer of graphite
with a measurable resistance.
Any pencils can be used but generally anything harder than HB has too little graphite in it to give
a measurable resistance.
The question in method 7. Generally the resistance increases as the length of the line between the
multimeter probes increases.
The pencil lines can be drawn in the data table and the resistance of different lengths of each line
recorded.
Question 10. Students should be encouraged to make inferences and draw conclusions consistent
with their data. In general softer pencils with a greater proportion of graphite would have lower
resistance and shorter lines have lower resistance.
Question 11. Generally wider lines would be expected to have lower resistance.
Question 12. You may want to ask students to discuss differences between the data and /or
conclusions of different groups in the class.
13 is offered as an extension.
Resistor colour code
1.
2.
3.
4.
Resistor types:
Fixed resistors have a particular value of resistance which is taken as constant. They may be
constructed from wire (wire wound), carbon or metal oxide.
Variable resistors can supply a changing resistance. Potentiometers have a resistance that
depends on the length of the wire selected, light dependent resistors (LDRs) have a resistance that
changes under different light levels, thermistors have a resistance that changes with temperature.
Resistor use. Resistors are used to limit electric currents in circuits in ways that help the circuit to
function as intended.
Small carbon resistors covered with a protective coating and connected to a wire at each end have
a colour code on the outside of each resistor. Various forms of this colour code are available in
text books, from electronics suppliers and on the internet. The activities aim to familiarise students
with the use of the resistor colour code. There is no need for them to memorise it.
The colour code chart. In this exercise the resistor colour code chart will be used for four band
resistors with a gold or silver tolerance band at one end and three coloured bands at the other end.
The chart may also be used for more accurate 5 band resistors and may have up to 7 colours for
tolerance levels.
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5.
6.
7.
8.
9.
Remembering the code If you want students to remember the colour code as well as being able
to use it when given a chart, you could ask them to make up a sentence with the colours in order.
There are several around eg:
Barney Bull Runs Over Your Garden, Blue Violets Gone West.
Big Brown Rabbits Often Yield Great Big Vocal Groans When Gingerly Slapped.
Bad Beer Rots Our Young Guts But Vodka Goes Well.
Method 3. You may find drawing the coloured bands on the diagram in the table helps some
students or helps all focus on the task.
Method 4. Loose resistors can be used but mounting them in some way makes measurements
easier and keeps them from being lost. The measured values should be close to the calculated
values but do not have to be exactly the same.
Method 5. Students could consider the 5% or 10% tolerance and the possibility of damage to the
resistor.
Method 6 can be used as application or extension. Circuit boards from old clock radios, tape
players, phones etc and toys and novelties will often have identifiable resistors, transistors,
capacitors and diodes. Some boards are helpfully numbered R6, D2, C12 where R is resistor, D is
diode, C is capacitor and so on. Increasingly ICs (integrated circuits or chips) with only a few
other components carry out all the functions.
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Activity 3: What do Electronic Components do in a Circuit?
Basic Testing Circuit
Comments of the basic circuit:
 What does the 390 ohm resistor do?
It limits the size of the current through the Light Emitting Diode (LED),and so protects it.
 If the LED does not light up, it is probably connected in the wrong way round.
Answers:
c.
Current flows from the positive terminal of the battery, then through the 390 ohm resistor. It then
passes through the LED, lighting it up, and then on to the negative terminal of the battery.
d.
(i)
When the 1 kilohm resistor is placed in the circuit, the brightness of the LED reduces.
(ii)
With two 1 kilohm resistors the brightness is even less.
(iii)
The LED does not go on.
(iv)
The LED does light up as bright as if there was nothing between P and Q.
(v)
Neither LED lights up.
(vi)
Both LEDs light up as bright as if there was nothing between P and Q.
(vii) The LED lights up, but not very brightly
(viii) The LED is very dim.
f.
Connected to P
C B E
Connected
C X N N
to
B Y X Y
Q
E N N X
Y: LED glows, N: LED does not glow
g.
Conclusions
* The LED becomes dimmer as the resistance in the circuit increases.
* Diodes must be placed in the circuit so that their anode (A) is nearest the positive terminal
of the battery.
* The resistance of an LDR is large when it is covered from a light source.
* In a transistor, there is not a connection between the collector and the emitter.
* If the base of a NPN transistor is joined to the positive side of a circuit, a current will flow
from the base to emitter.
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Activity 4: How does a Transistor Work?
a.
b.
c.
d.
You will need a few short wires to connect components together.
If both LEDs don’t glow then one or more of the components is around the wrong way. Check the
symbol on the plastic sheet to see if it is in the same direction as the circuit diagram
LED 2 should go on, but not LED 1. There is no way for the current to go from the collector to
the emitter, the base is in the way. It is too high a barrier for the current “to get over”.
Both LEDs are on. Current is travelling by two paths around the circuit. The description of the
two paths: The current leaves the positive terminal of the battery, goes along the yellow lead to
LED 1, then through the 1 kilohm resistor to the base of the transistor. The current then travels
out of the emitter and back to the negative terminal of the battery. The other path is: The current
leaves the positive terminal of the battery, goes along past the yellow lead to 390 ohm resistor and
through the LED 2. The current then enters the collector of the transistor, passing through the
base to the emitter and back to the negative terminal of the battery.
Note: Some students will draw the first path leaving the transistor through the collector and
heading back to the positive terminal of the battery. For such students it is useful to use the
analogy of current, like water, is running down hill from the positive terminal to the negative
terminal, so such a path suggests the current is going back up hill, which like water, the current
does not do.
e.
As you increase the resistance connected to the base the current gets less. The LED 1 will get
dimmer, but LED 2 will still be quite bright. Even with a wet finger LED 2 will light up, even
though LED 1 is not on.
In fact you can try other possibilities:
 Hold one wire in your left hand and the other in your right hand
 Hold one wire in your hand and the other wire in someone else’s hand, then let the free hands
touch
f.
g.
It can be seen that a tiny current flowing into the base of a transistor is necessary to cause the
collector to internally join to the emitter.
Conclusions:
“In a transistor, there is no connection between the collector and emitter unless there is a current
entering the base.
“Quite a tiny amount of base current is needed to open the switch of a transistor”.
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Activity 5: Building a Delay Circuit
a.
b.
c.
d.
e.
You will need a few short wires to complete the circuit.
The LED should take several seconds to come on. It’s brightness should noticeably increase over
about one second.
i)
With the lead wire joined to X, the current flows from the positive terminal of the battery
through the 100 kohm resistor, bypasses the capacitor and goes to the negative terminal.
ii)
With the lead wire lifted from X, the current flows from the positive terminal of the battery
through the 100 kohm resistor, and begins to charge the capacitor. As the capacitor starts
to fill some current goes through the 1kohm resistor to the base of the transistor. This
current increases as the capacitor fills. Eventually the base current is large enough to
turn on the transistor and allow current to flow from the collector to the emitter when the
LED turns on.
With a 10 F capacitor, the LED comes on more quickly. The smaller capacitor takes a shorter
time to fill.
The resistor limits the size of the current. A larger resistance will produce a smaller current which
will take longer to fill the capacitor.
Activity 6: A Light Sensitive Indicator
a.
b.
c.
d.
e.
The LED will not go on. The resistance of the LDR is high when covered.
The LED goes on.
“An LDR becomes a low resistance resistor when exposed to bright light and has a high
resistance in the dark.”
i)
“The current flows from the positive battery terminal to the junction of the 390 resistor
and LDR. Some current goes through the LDR. If the LDR is exposed to bright light, then
a large current goes through the LDR and into the base of the transistor and turns on the
transistor, allowing current to flow through the LED to the Collector and the Emitter and
then back to the battery.”
ii)
The current flows from the positive battery terminal to the junction of the 390 resistor
and LDR. If the LDR is covered, then a small current goes through the LDR. This
current is too small to open up the transistor and it continues on back to the battery
through the 1.2 k ohm resistor.
Controlling street lights, turning on appliances when daylight appears. If the LDR and the 1.2
kohm resistor are interchanged the reverse response occurs.
Electronics References
Introductory Electronics, Wood and Sardi
Engaging Science Radio & Communications, Dangerfield, Curriculum Corporation
Heinemann Outcomes Science Four, Parsons.
Science Now Book Four, Stannard and Williamson
Dick Smith Electronics, www.dse.com.au
Vicphysics Teachers' Network Inc. - July 2015
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Kit List
1. Resistor Colour Code
 Six resistors
 Multimeter
2. What does this component do?
 9V battery and clip
 390 ohm resistor
 LED
Duct, non metallic PVC
 Two 1k ohm resistors, diode, LDR, LED, transistor
($35 for 4m)
 Two wires
Electrical & data supplies
How does a transistor work?
8 Rose Street, Doncaster
 9V battery and clip
(03) 9848 9122
 390 ohm resistor
 LED
 One 1k ohm, 10k ohm, 100k ohm resistors
 Three wires
 One wire with clips
3. Delay Circuit
 9V battery and clip
 390 ohm resistor
 LED
 One 1k ohm, 100k ohm resistors
 10 F, 100 F, 1000 F capacitors
 Three wires
4. Light Controller
 9V battery and clip
 390 ohm resistor
 LED
Conical compression springs(phosphor bronze)
 One 1k ohm, 1.2k ohm resistors
($0.65 each based on an order of 300)
 LDR
Bell Springs Pty Ltd.
P.O Box 344 Reservoir (03) 9464 6611
Overall
 9V battery and battery holder (S6100 $2.28 pack of 5)
 BC548 transistor (Z1308 $0.25 each)
 390 ohm resistor (R1064)
 2 LEDs (Z4084, $14.50 pack of 100)
 One 1k ohm (R1074 $0.04 each), 1.2k ohm (R1076), 10k ohm (R1098), 100k ohm (R1124) resistors
 Six resistors (Mixed values)
 LDR (Z4801 $2.50 each)
 10 F (R4315 $0.25 each), 100 F (R4360, $0.32 each), 1000 F (R4440 $0.70 each) capacitors
 Three wires
 One wire with alligator clips
 Multimeter
 Component holders (conduit with banana sockets (red P1730 and black P1732 $1.40 each or cheaper
(P1720 and P1726 $0.50 each) or thick plastic sheet with springs
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