Aim To investigate the properties of a capacitor
Equipment
A selection of capacitors (e.g. 100 µF, 470 µF, 100 µF) and above, resistor (about 470 kΩ or so), 9 volt battery, 1 LED
Method
1 Charge a capacitor as shown, ensuring that the
positive terminal on the capacitor touches the positive battery terminal for a couple of seconds.
Connecting the wrong way is dangerous and could damage either object. Carry out step 2 as soon as possible.
[Practical activities
]
3.5
UNIT
Prac 1 Unit 3.52 Complete a row of the table for each resistor.
The resistor colour code
Note: Depending on experience and equipment, the
following circuits may be constructed using a variety of methods, e.g. card/sticky tape, soldering, breadboard, drawing pins/balsa wood, etc. The card/tape method is not recommended for more complex circuits such as those in Pracs 5 and 6. Resistor colours below are based on the four-band system.
Method
1 Draw a larger version of the table shown below.
Resistor Colours (first 3 bands) Value
A B C
2 Connect the capacitor to an LED and resistor as shown. 3 Repeat steps 1 and 2 for different capacitors, noting any
differences in your results.
9 V
+– 100F
Fig 3.5.18
Aim To determine the values of resistors using the colour code
Equipment
A selection of resistors mounted on small pieces of cardboard (as in Figure 3.5.17) and labelled alphabetically.
Fig 3.5.17 100μF + + – – 470 Ω resistor LED Fig 3.5.19 Questions
1 Predict the effect of using a larger capacitor in
step 2.
2 Explain why it was important not to wait too long after
step 1 before performing step 2.
3 a Predict the effect of placing another capacitor side
by side with the original one.
b You may wish to combine with another group to investigate this effect.
Electronics
Electronics
UNIT
UNIT
3.5
3.5
Prac 3 Unit 3.5A diode/resistor circuit
Aim To investigate a circuit of a capacitor, diode and resistor
Equipment
A light-emitting diode (LED), 330 Ω resistor (orange, orange, brown), 470 Ω resistor (yellow, violet, brown), 1 kΩ resistor (brown, black, red), 9 volt battery, battery snap, connecting wire (2 pieces, each 10 cm), a piece of card on which to lay out the circuit, sticky tape or Blu-Tack
Method
1 Assemble the circuit exactly as shown in Figure 3.5.20,
otherwise you could damage the components.
Blu-Tack or tape
9 V
Fig 3.5.20
2 Notice that one of the legs of the LED is longer than
the other. Try placing the LED in the circuit both ways around. Note whether the longer leg must be as close to or as far away as possible from the positive terminal of the battery.
3 Try different resistors in the circuit, both ways around.
Questions
1 When an LED lights up, the positive leg is the one that
goes closest to the battery. Identify which leg (long or short) is the positive one on an LED. Justify your answer.
2 Predict the effect of a larger resistance on the LED. 3 Discuss whether it matters which way around a resistor
goes.
4 Explain why a resistor is used in this prac. 5 Construct a circuit diagram for this prac.
Prac 4 Unit 3.5 Blu-Tack or tape 9 V E B C long leg short leg Fig 3.5.22
The transistor
Aim To investigate the properties of a transistor
Equipment
A transistor (type BC548), 1 light-emitting diode (LED), 470 Ω resistor (yellow, violet, brown), 1.5 kΩ resistor (brown, green, red), 9 volt battery, battery snap, connecting wire (2 pieces, each 10 cm), piece of card on which to lay out the circuit, sticky tape or Blu-Tack
Method
1 Connect and observe
the circuit shown in Figure 3.5.22. Note that the transistor terminals can be identified as shown here.
2 Add a 1.5 kΩ resistor and extra LED in series between the transistor base and the positive battery terminal, as shown in Figure 3.5.23. Blu-Tack or tape 9 V E B C long leg short leg Fig 3.5.23 BC548 collector base emitter Fig 3.5.21
>>
3 Explain why a transistor is sometimes referred to as an
electronic switch.
4 Construct a circuit diagram for step 2 of this prac.
Questions
1 Explain how you can tell when current is flowing
(or not flowing) in the circuits on page CD21.
2 Compare the size of the current going into the base
of the transistor with that going into the collector in the second circuit.
3 Lie detectors work by using circuits similar to but
more sensitive than the one in this prac. Explain how such a circuit could detect when a person supposedly tells a lie. 470Ω 9 V E B C long leg short leg + – BC548 transistor probe probe Fig 3.5.24 Prac 5 Unit 3.5
A moisture detector
Aim To construct a circuit that detects moisture
Equipment
A transistor (type BC548), 1 light-emitting diode (LED), 470 Ω resistor (yellow, violet, brown), 100 kΩ resistor (brown, black, yellow), 9 volt battery, battery snap, connecting wire (2 pieces, each 10 cm), piece of card on which to lay out the circuit, sticky tape or Blu-Tack
Method
1 Assemble the circuit as shown.
2 Touch the probes together to test the circuit. The LED
should glow brightly.
3 Keeping the probes apart by a few millimetres, touch
them to a dry object, then a damp one (e.g. lick a finger if it’s clean).
Questions
1 Discuss how this circuit could be used as:
a a water detector b a soil moisture content detector 2 Predict the resistance of:
a a dry finger b a wet finger
Prac 6 Unit 3.5
Flasher
Aim To construct a circuit containing flashing lights
Equipment
A circuit ‘breadboard’, 2 transistors (type BC548), 2 100 µF capacitors, 2 light-emitting diodes (LED), 2 470 Ω resistors (yellow, violet, brown), 2 10 kΩ resistors (brown, black, yellow), 9 volt battery, battery snap, connecting wire (8 pieces, each 10 cm), other capacitors (e.g. 470 µF, 330 µF, 220 µF)
Method
1 Use the circuit diagram in Figure 3.5.25 as a guide to
construct a flasher circuit.
2 Once the circuit is working, try exchanging one of the
capacitors with a different one and note the effect on the circuit. 9 volt battery 470 Ω 10 KΩ 10 KΩ 470 Ω LED LED 100 μF 100 μF BC548 BC548 + + + + – – – – Fig 3.5.25 Questions
1 Deduce what effect the size of a capacitor has on the circuit. 2 Explain your answer to Question 1 in terms of charge movement. 3 Predict what else you might change (besides a capacitor) to alter
the flashing rate. If you have permission, the time to do so and the equipment, design an experiment to test your prediction.
4 Construct a circuit diagram for this circuit.
Electronics
Chapter review
[Summary questions]
1 State the units used for the following measurements,
giving the full name and short version in each case.
a voltage b resistance c current
2 Distinguish between a series circuit and a parallel
circuit.
3 Match the following terms to their definitions. Term load voltage current conducting path resistance switch Definition
Uses up electrical energy
The ability of a substance to reduce the flow of current
Wires for the electricity to flow through
The flow of charge, usually electrons Turns the current on and off
The energy available to push current through a circuit
4 Distinguish between AC and DC.
5 Copy the following and modify any incorrect statements
to make them true.
a A magnetic field is produced by a coil or coils of wire,
not by a straight wire.
b Electricity can cause magnetism and magnetism can
cause electricity.
c Electromagnets can be turned on and off. d A relay is an electromagnetic switch.
e A generator produces current when a magnet sits
inside or near its coils.
f More energy is lost in power transmission lines when
the voltage is higher.
6 List two types of wave that are possible in a slinky. 7 Explain why not all power is transmitted at 240 volts. 8 List five modern electronic devices.
9 List four categories of electromagnetic waves and state
a use for each type.
10 Outline how early telegraphs used electromagnetism. 11 State what each of the following people are famous for.
a Samuel Morse b Alexander Bell c Almon Strowger d William Shockley
[Thinking questions]
12 Construct diagrams of the following circuits: a a series circuit with two lights and a switch
b a circuit with three lights in parallel, and switches to
turn all lights off separately
c a circuit with three lights in parallel, and a single
switch to turn all lights off
13 Complete the following table comparing a water circuit
to an electrical circuit.
Electrical circuit Water pump circuit
switch battery resistor voltage or energy current wire
14 a Construct a graph of Ohm’s law using the
experimental results listed below.
b Identify what the slope of the graph represents. c Calculate the slope of the graph.
15 Use Ohm’s law to complete the following table:
Voltage, V (volts) Current, I (amps)
0 0
2.5 2
5 4
7.5 6
10 8
Current Voltage Resistance
3 amps 15 V
10 amps 6 k Ω
[Interpreting questions]
24 The following wave was produced in 10 seconds.
Calculate the: a frequency b wavelength c amplitude
25 The following signal consists of two messages sent using
time division multiplexing in groups of four characters: MYHO THEP VERC RICE RAFT OFEG ISFU GSHA LLOF SGON EELS EUP!
Analyse the signal and record its two messages. 26 Propose how 0s and 1s could be used to send a digital
message originally written in words.
16 Correct the following statements by identifying the
correct word in brackets: Series circuits:
a The voltage is shared (unequally/equally) between each
resistor.
b The current is (the same/different) for each resistor. c If any component is removed, the circuit (will/will not)
work. Parallel circuits:
d The voltage is (the same/different) for each resistor. e The current (divides into/is the same in) each branch of
the circuit.
f If one branch of the circuit is broken the other
branches (will/will not) still work.
17 State which type of transformer is used close to homes,
and explain why.
18 Contrast the visible spectrum with the electromagnetic
spectrum.
19 State what happens to the wavelength of electromagnetic
waves as the frequency increases.
20 Contrast laser light with light from the Sun.
21 State three ways in which messages are sent within
today’s global communications network and outline an advantage of each method.
22 Predict what might happen if the same frequency was
used for two different calls in a mobile phone cell.
23 Explain why radios were so large and heavy before
transistors were invented.
Worksheet 3.5 Electricity and communications technology crossword
Worksheet 3.6 Sci-words
>>>
By the end of this chapter you should be able to: explain the terms ‘gene’, ‘chromosomes’ and ‘DNA’ identify how genetic information is passed to
offspring
explain how recessive and dominant genetic characteristics are inherited
predict the probability of a person being affected by a particular characteristic
recall that sex cells have half the chromosomes of body cells
explain two different ways in which cells reproduce
identify the likely inheritance of genes from a pedigree.
1 Blue-eyed parents can produce brown-eyed children. True or false?
2 Why are approximately the same number of boys and girls born?
3 Why is colour blindness rare in girls but common in boys?
4 Why can one ear of corn produce kernels of many different colours, as in the picture on the left? 5 What is a clone?
6 What is GM food?
7 Could a dinosaur fossil be used to ‘create’ a live dinosaur?
8 List techniques that forensic scientists use to prove guilt.
9 Jeans for Genes Day is held each August to raise money for gene research. Why is gene research important?