Topic 1.3 – Circuit Concepts.
Learning Objectives:
At the end of this topic you will be able to;
？ recognise standard symbols for components included within the
module;
？ apply the current at a junction rule;
？ apply the voltage divider rule;
？ explain how voltage at a point can be indicated relative to a 0V
reference;
？ appreciate that resistance is the opposition to current flow and that
is measured in ohms;
？ understand the relationship between current, voltage and resistance
in qualitative terms;
VR？？ select and use ;
I
？ recognise that analogue signals are continuously varying and digital
signals are two state;
？ state the power is dissipated when current flows through resistance
and is measured in watts;
？ understand the relationship between current, voltage and power in
qualitative terms;
？ perform calculations involving ; P？V！I
？ recognise and use the following multiple and sub-multiple indicators: p,
n, µ, m, k, and M.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Circuit symbols
Electronic circuits consist of components (parts) such as lamps, resistors and transistors connected to an electrical supply, e.g. a battery. The connections are wire or strips of a good electrical conductor such as copper. The connections and components must make a complete path, i.e. a circuit.
Circuits are represented by diagrams in which each part is shown by a symbol. Some examples that you should recognise are shown below are given below.
Using the circuit symbols above, draw a circuit diagram for a circuit containing a battery, a resistor, two lamps, a switch and an ammeter. 2
Topic 1.3 – Circuit Concepts.
Electric Current
What is it
An atom consists of a tiny core or nucleus with a positive (+) electric charge, surrounded by electrons which have an equal negative (-) charge. (see below)
Electrons (-)
Nucleus (+)
In a conductor, some electrons are loosely attached to their atoms. When the conductor is part of a circuit connected to a battery, the battery forces these electrons to move through the conductor from its negative (-) terminal towards its positive (+) terminal. An electric current is said to be flowing through the conductor when these electrons flow in one direction at a given time.
The ampere and ammeters
Current is measured in units called amperes (shortened to amps or A). The
current flowing in a circuit can be measured with an ammeter. The current
through a large torch bulb is about 0.5A and through a car headlamp 3A to 4A.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Individual ammeters are hardly ever used for circuit measurements as it is too expensive to have a large range of ammeters capable of reading different currents. Today we use an instrument called a multimeter which is capable of measuring lots of different currents and many other things besides all in one package. The multimeter will become a very important instrument for you. There are two main types of multimeter available. The first type is called an analogue meter and is shown below.
One terminal is marked ‘+‘ (or coloured red) and this is the one the conventional current must enter, that is, it must lead to the ‘+‘ terminal of the battery. Otherwise the pointer on the ammeter is deflected in the wrong direction and the ammeter may be permanently damaged.
The analogue meter has a moving needle which moves across multiple scales and it is up to the user to interpret the correct reading. To the inexperienced user this instrument is very hard to use and errors are frequently made. Good quality analogue meters are very expensive (over ?100.00) although cheaper versions are available they are not always very accurate.
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Topic 1.3 – Circuit Concepts.
More common these days are the digital multimeter as shown below:
The digital meter is much easier to use than an analogue meter because it gives a direct reading on its display of the quantity that it is measuring. The digital meter also has a red terminal which should be connected to the positive part of the circuit, however, unlike the analogue meter if a mistake is made all that will happen is that the display will shown a ‘-’ sign in the display
to indicate that current is flowing the wrong way. To rectify the problem simple reverse the connections in the circuit. No damage will be done to the
multimeter.
Throughout this course you should use a digital meter if at all possible during some of your practical work. During computer modelling sessions you will use a computerised version of the digital multimeter.
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GCSE Electronics.
Unit E1 : Discovering Electronics
In either case the symbol used for an ammeter is shown below.
Note : The symbol shown is for an ammeter because of the letter ‘A’ in
the circle. There is no separate symbol for a multimeter, because
as it’s name suggests the multimeter can be set up as a number of
different meters, one of which is an ammeter.
Two smaller units of current used in electronics are the milliampere (mA)
and the microampere (µA); (pronounced mu A).
11mA？Aor1000mA？1A1000
111，A？mA？Aor10001000000
1000，A？1mAor1000000，A？1A
To be able to convert between these units accurately and reliably is very important for calculations needed later. The following diagram will hopefully help.
x1000x1000
uAAmA
/1000/1000
Examples :
1. Convert the following currents into mA.
(1) 3A = 3 x 1000 = 3000 mA
(2) 1.5A = 1.5 x 1000 = 1500 mA
(3) 0.65A = 0.65 x 1000 = 650 mA
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Topic 1.3 – Circuit Concepts.
2. Convert the following currents into µA.
(1) 2A = 2 x 1000 = 2000 mA = 2000 x 1000 = 2,000,000 µA
(2) 1.8mA = 1.8 x 1000 = 1800 µA
(3) 0.32A = 0.32 x 1000 = 320 mA = 320 x 1000 = 320,000 µA
3. Convert the following currents into A.
(1) 1,500,000 µA = 1,500,000 ? 1000 = 1500 mA = 1500 ? 1000 = 1.5 A
(2) 1.3mA = 1.3 ? 1000 = 0.0013 A
(3) 65,000 µA = 65000 ? 1000 = 65 mA = 65 ? 1000 = 0.065 A
There are other multipliers that we will come across in this course, which are for very small quantities, these are ‘p’ for pico, and ‘n’ for nano. We will look at these when we consider capacitors in Unit E2.
Measuring Current
To measure a current, the circuit has to be broken and the ammeter connected in the gap.
To complete this part of the exercise you will need access to a computer running circuit simulation software e.g. Crocodile Physics or Livewire.
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GCSE Electronics.
Unit E1 : Discovering Electronics
(a) Series circuit
Within your simulation software create the following circuit using the
circuit symbols shown.
In your circuit you will not get the labels alongside the circuit symbols these are included so that you can answer the questions about the circuit.
When you have successfully constructed the circuit on the computer the three ammeters will display the current flowing at each part of the circuit and both lamps should be on.
Note : The readings on each ammeter should be positive. If you have a negative value it means you have connected the ammeter incorrectly - check the circuit carefully and correct before answering the questions.
Complete the following to show your results.
Ammeter 1 = ....... mA Ammeter 2 = ....... mA Ammeter 3 = ....... mA
Looking at your results what conclusion can you make about the current flowing in a series circuit ?
........................................................................................................................................
........................................................................................................................................
Save your circuit as “Basic-Theory-Circuit1”.
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Topic 1.3 – Circuit Concepts.
(b) Parallel circuit
Within the simulator create the following circuit using the circuit
symbols shown.
Note: In your circuit you will not get the labels alongside the circuit symbols
these are included so that you can answer the questions about the
circuit. Please remember to change the battery voltage to 6V
When you have successfully constructed the circuit, the four ammeters will display the current flowing at each part of the circuit and both lamps should be on. Note : The readings on each ammeter should be positive. If you have a negative value it means you have connected the ammeter incorrectly - check the circuit carefully and correct before answering the questions.
Complete the following to show your results.
Ammeter 1 = ........ mA Ammeter 2 = ........ mA
Ammeter 3 = ........ mA Ammeter 4 = ........ mA
Looking at your results what conclusion can you make about the current flowing in a parallel circuit?
........................................................................................................................................
.......................................................................................................................................
Save your circuit as “Basic-Theory-Circuit2”.
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GCSE Electronics.
Unit E1 : Discovering Electronics
(c) Circuit drawn with voltage rails
If a supply has a voltage of, say, 9V, you will often find on circuit diagrams that the negative of the supply is marked as 0V and the positive as 9V, as shown below.
l
Notice the way the current I flows in the two circuits
You will see later on that using voltage rails can help to make the drawing of electronic circuits easier.
(d) Series /Parallel circuit
Consider the following circuit which contains 2 different signal lamps and a filament lamp connected to a 9V power supply. You will notice that a filament lamp has a different symbol to the signal lamp.
Complete the following table for this circuit using the values of I and I given. 12
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