Physics

In

General Science

Electronics,

Electricity & Magnetism

Produced by the Education Sub-Committee of

Australian Institute of Physics (Victorian Branch)

Australian Institute of Physics (Vic Branch) Education Committee - Nov 2008

1

Index

Electronics

Activity 1:What do Electronic Components look like?2

Activity 2:Resistance with Multimeters3

Activity 3:What do Electronic Components do in a Circuit?9

Activity 4:How Does a Transistor Work?11

Activity 5:Building a Delay Circuit13

Activity 6:A Light Sensitive Indicator15

Explanations of Transistor Operation16

Electronics Terminology18

Electricity and Magnetism

Activity 7:Building a DC Motor19

Teacher Notes: 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.

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.

Activity 2

Resistance 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 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.

2.2Making 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. 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?

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 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.

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 / Black / Brown / Orange / Green / Grey
Value / 0 / 1 / 2 / 4 / 6 / 7 / 9
  1. 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
  1. 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)
  1. Did your measured values of resistance agree with the calculated values?

If the values are different what could be the reasons for these differences?

  1. 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 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.

Circuit Diagram

  1. Construct the circuit as shown in the circuit diagram. Connect to the 9 Volt battery.
  1. Join leads P and Q. If the circuit is connected correctly, the Light Emitting Diode (LED) should light up.
  1. Write a few lines describing how this circuit works.

Begin with “Current flows from the positive terminal of the battery, then ______

______

______

  1. 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.

Resistors

i) ______

______

ii)______

______

Diode

iii)______

______

iv)______

______

LED

v)______

______

vi)______

______

LDR

vii)______

______

viii)______

______

  1. 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.

Australian Institute of Physics (Vic Branch) Education Committee - Nov 2008

1

Activity 4

How does a Transistor Work?

Components:

One transistorOne 1kohm resistor (Brown, Black, Red)

Two LEDsOne 390 ohm resistor (Orange, White, Brown)

BatteryOne lead wire with clips attached

Three 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.

  1. 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.

  1. 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.

  1. 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.

______

  1. 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

Australian Institute of Physics (Vic Branch) Education Committee - Nov 2008

1

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 transistorOne 1kohm resistor (Brown, Black, Red)

One LEDOne 390 ohm resistor (Orange, White, Brown)

BatteryOne 100k ohm resistor (Brown, Black, Orange)

Three connecting wiresOne 100 F capacitor

One lead wire with clips attachedOne 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.

  1. 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.

______

______

______

______

______

______

  1. Predict what will happen when a 10 F capacitor replaces the 100 F capacitor. Test your ideas on the real circuit.

______

______

______

  1. 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 transistorOne 1kohm resistor (Brown, Black, Red)

One LEDOne 390 ohm resistor (Orange, White, Brown)

BatteryOne 1.2k ohm resistor (Brown, Red, Red)

Three connecting wiresOne Light Dependent Resistor

  1. 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.