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Physics

In

General ScienceElectronics,

Electricity & Magnetism

Produced by the Education Sub-Committee of

Australian Institute of Physics (Victorian Branch)

http://www.vicphysics.org/index.php?id=38

Australian Institute of Physics (Vic Branch) Education Committee - Nov 20081

Index

ElectronicsActivity 1: What do Electronic Components look like? 2Activity 2: Resistance with Multimeters 3Activity 3: What do Electronic Components do in a Circuit? 9Activity 4: How Does a Transistor Work? 11Activity 5: Building a Delay Circuit 13Activity 6: A Light Sensitive Indicator 15Explanations of Transistor Operation 16Electronics Terminology 18

Electricity and MagnetismActivity 7: Building a DC Motor 19

Teacher Notes: 21

Extracts from VELS, Level 6Learning 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, …”

StandardsScience 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.


Activity 1What 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 componentsThe 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.


Activity 2Resistance with multimeters

2.1 Using a Multimeter

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 Vsocket 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 25.1kdepending 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.


COMblack

V mAred

10Ared

Turn to select: DC volts, amps, ohms, AC volts

Probes

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.

MaterialsYou will need a selection of pencils including 2B, 4B and 6B pencils, paper, ruler and a multimeter

Method1. Rule a 3 cm pencil line on this paper using the 2B pencil. Go over the line two or three times.

2. Connect the black probe to the black com socket on the multimeter.3. Connect the red probe to the red V socket.4. Turn the multimeter dial to the section and select the highest range (probably 2000k)5. Touch the metal tips of the probes together. The display should read 0.0. Separate the probes.

The display should read 1.6. 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.

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?


Pencil line

Multimeter

Probe

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.


9. Data TableYou can draw the lines you are investigating in the space in the table

Pencil type Pencil line Length Resistance

Questions10. 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.


14. 2.3 Resistor Colour CodeThe 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.

Method1. Use the resistor colour code chart to complete this table

Colour Black Brown Orange Green GreyValue 0 1 2 4 6 7 9

2. Label this diagram of a 12000 resistor by putting in the numbers for each band. The band colours are brown, red, orange.

3. 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


Tolerance:silver or gold

Band 1: first value of the resistance

Band 2: second value of the resistance

Band 3: number of zeros

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?


Activity 3What do Electronic Components do in a Circuit?

Basic Testing CircuitCircuit DiagramThe 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.

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.


Resistorsi) ________________

__________________

ii) _____________

__________________

Diodeiii) _____________

_

___________________

iv) _____________

_

___________________

LEDv) _____________

_

___________________

vi) _____________

_

___________________

LDRvii) _____________

_

___________________

viii) ______________

___________________

f. 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 PC B E

Connected C Xto B XQ 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.


Activity 4How does a Transistor Work?

Components:One transistor One 1kohm resistor (Brown, Black, Red)Two LEDs One 390 ohm resistor (Orange, White, Brown)Battery One lead wire with clips attachedThree connecting wires

a. Connect the circuit to the battery and switch on.b. 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.c. 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.


“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


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.

Components:One transistor One 1kohm resistor (Brown, Black, Red)One LED One 390 ohm resistor (Orange, White, Brown)Battery One 100k ohm resistor (Brown, Black, Orange)Three connecting wires One 100 F capacitorOne lead wire with clips attached One 10 F capacitor

a. Connect the battery and touch the lead wire to point X. The LED should not be glowing.b. 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.


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


Activity 6: A Light Sensitive Indicator

Components:One transistor One 1kohm resistor (Brown, Black, Red)One LED One 390 ohm resistor (Orange, White, Brown)Battery One 1.2k ohm resistor (Brown, Red, Red)Three connecting wires 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 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.”

d. 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.

_________________________________________________________________________


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).

+

-B

C

E

Positive RailHigh Energy Level

Negative RailLow Energy Level

(IB + IC)

(IB + IC)X

IB

IC

(IB + IC)

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.


X2,000

Y100

Figure 5

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.

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.

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.


Figure 4

YX

Figure 3

collector

emitter

base

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 (). It is likened 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).


Activity 7: Making a DC Electric MotorPrinciple of a DC MotorAn 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 worksHere 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 NeededOne D size 1.5 volt battery Two 50mm paper clipsOne magnet 90 cm of enamelled 22 gauge copper wireOne thick rubber band One piece of sand paper or steel woolOne test tube One pencil or biro

Steps in making a DC MotorMaking the Copper Coil1. 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.2. 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.


Bending the Paper Clips3. 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 components6. 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!

Activities1. 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 Try a bar magnetUse a shorter piece of copper Clean the enamel off all way round the ends of the wire


Teacher Notes

Using a Multimeter1. 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.2. 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 Vsocket. The appropriate section of the multimeter is then selected using the dial.

3. Multimeters can be used as ammeters, AC or DC voltmeters or ohm meters and have other functions such as continuity test and transistor test.

4. A basic digital multimeter costs $10-$12.

Making resistors1. 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.

2. Any pencils can be used but generally anything harder than HB has too little graphite in it to give a measurable resistance.

3. The question in method 7. Generally the resistance increases as the length of the line between the multimeter probes increases.

4. The pencil lines can be drawn in the data table and the resistance of different lengths of each line recorded.

5. 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.

6. Question 11. Generally wider lines would be expected to have lower resistance.7. Question 12. You may want to ask students to discuss differences between the data and /or

conclusions of different groups in the class.8. 13 is offered as an extension.

Resistor colour code1. 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.

2. Resistor use. Resistors are used to limit electric currents in circuits in ways that help the circuit to function as intended.

3. 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.

4. 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.


5. 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.

6. 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.

7. 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.

8. Method 5. Students could consider the 5% or 10% tolerance and the possibility of damage to the resistor.

9. 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.


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 PC B E

Connected C X N Nto B Y X YQ E N N X

Y: LED glows, N: LED does not glowg. 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.


Activity 4: How does a Transistor Work?

a. You will need a few short wires to connect components together.b. 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 diagramc. 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”.d. 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. 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.

g. 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”.


Activity 5: Building a Delay Circuit

a. You will need a few short wires to complete the circuit.b. The LED should take several seconds to come on. It’s brightness should noticeably increase over

about one second.c. 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.

d. With a 10 F capacitor, the LED comes on more quickly. The smaller capacitor takes a shorter time to fill.

e. 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. The LED will not go on. The resistance of the LDR is high when covered.b. The LED goes on.c. “An LDR becomes a low resistance resistor when exposed to bright light and has a high

resistance in the dark.”d. 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.

e. 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 ReferencesIntroductory Electronics, Wood and SardiEngaging Science Radio & Communications, Dangerfield, Curriculum CorporationHeinemann Outcomes Science Four, Parsons.Science Now Book Four, Stannard and WilliamsonDick Smith Electronics, www.dse.com.au


Kit List1. Resistor Colour Code Six resistors Multimeter2. 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 suppliesHow 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 clips3. Delay Circuit 9V battery and clip 390 ohm resistor LED One 1k ohm, 100k ohm resistors 10 F, 100 F, 1000 F capacitors Three wires4. 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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