Transcript aedcthw1_introduction to terms etc

Applied Electricity DC
The Electric Circuit
PURPOSE:
This section introduces the simple electric circuit and develops skills in measuring
and determining voltage, current and resistance.
TO ACHIEVE THE PURPOSE OF THIS SECTION:
At the end of this section the student will be able to:
• Describe electromotive force in terms of pressure.
• Describe current in terms of flow.
• Describe resistance in terms of opposition to flow.
• Define the units of EMF, current and resistance in relative units
• Draw a simple circuit diagram containing a battery, switch, load (lamp), a voltmeter and
an ammeter.
• Define the terms 'open-circuit' and 'closed-circuit'.
• Measure values of voltage using a single scale analog voltmeter and current using a
single scale analog ammeter
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Page 32-35
REFERENCES:
Electrical Principles for the Electrical
Trades. 5th Edition. Jenneson J.R
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1.PRESSURE AND FLOW
When studying or working with electric circuits three factors are always considered –
Electromotive force
voltage
• electric pressure, called _________________
or ______________________________
current
• electric flow, called ___________________________
resistance
• opposition to flow, called _________________________.
Before considering the electric circuit and the relationship between voltage, current and
resistance, consider the everyday phenomena known as pressure, flow and opposition.
Firstly consider what is meant by the term pressure.
A force acting on a given area
Pressure is defined as - ______________________________________________________.
A simple example would be the water pressure at a tap. See figure 1.
pressure
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greater
The greater the force applied, the ________________
the resulting pressure.
The movement of a quantity
Flow is defined as - _______________________________________________________.
For example, as shown in figure 2, the flow of water from a tap.
flow
What then is the relationship between pressure and flow?
Low pressure
greater
Small flow The greater the pressure the ________________ the flow.
High pressure
Large flow
increases
Increasing pressure ________________
flow.
decreases
Decreasing pressure ________________
flow.
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yes
Is it possible to have pressure without flow? ____________.
See figure 4.
no
Is it possible to have flow without pressure? ____________.
See figure 5.
Based on what has been determined so far, we can say –
Pressure determines flow
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2. OPPOSITION TO FLOW
So far we have established the relationship between pressure and flow. Now lets introduce a
third factor, that is, opposition to flow.
Perhaps the most common everyday occurrence of opposition to flow is the kinking of a
hose. See figure 6.
Hose kinked
reduced flow
reduced
Kinking the hose creates an opposition to flow and thus flow is _________________.
For a given pressure -
decreases flow
increasing opposition _____________
increases
decreasing opposition _____________ flow.
For a given opposition -
increases
increasing pressure ______________
flow
decreases flow.
decreasing pressure ______________
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Figure 7 illustrates the basic water circuit.
In this simple arrangement all of the concepts considered are seen in operation –
• the application of pressure causes flow
• the tap provides opposition to flow
• the circuit provides a path for flow to take place
• the amount of flow is determined by the size of the pressure applied and the level of
opposition.
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3. A CIRCUIT
A circuit is a closed path in which flow takes place. A circuit generally has • a source of pressure
• opposition to flow
• flow.
4. SIMPLE ELECTRIC CIRCUIT
The simple electric circuit, as shown in figure 8, consists of –
battery
• source of electrical pressure - e.g. _________________________
lamp
• opposition to electrical flow - e.g. ________________________
conductors
• path to allow flow to take place - e.g. _____________________________
switch
• on-off control of flow - e.g. __________________________.
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Increasing electrical pressure -
increases
___________________ electrical flow
and
Two batteries
Greater pressure
t
brighter
the lamp glows ___________________.
Figure 9
For a given electrical pressure, increasing opposition decreases
________________ electrical flow
and
dulls
the lamp brilliance __________.
Two lamps
Greater opposition
Figure 10
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In most circuits some form of electrical protection is provided. This protection could be in
the form of either a fuse or circuit breaker. In the space provided draw the circuit connections
for a circuit which consists of a fuse
• battery
• fuse
switch
• switch
• lamp.
+ battery
lamp
5. ELECTRICAL TERMS
In the simple circuits shown in figures 8, 9 and 10 it has been seen the concepts of pressure,
flow and opposition to flow exist electrically. When referring to an electric circuit particular
names are used to identify each of these quantities and abbreviations may be used to
represent them.
In an electric circuit –
voltage
• electrical pressure is known as _____________________
current
• electrical flow is known as _____________________
resistance
• opposition to electric flow is called ______________________.
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Table 1 lists the terms for electric pressure, electric flow and opposition to electric flow, plus
the abbreviations and units of measurement for each.
E
Volt
V
V
Volt
V
I
amperes
A
R
ohms
W
An example of the way in which these terms and abbreviations are used is Lamp 240 volt 60 watt
Hot Plate 240 Volt 1.2Kilowatt
Motor 415 Volt 750 Watt
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6. CIRCUIT DIAGRAMS
The circuit diagram is a simplified method of graphically showing the components that make
up circuit and the way in which they are interconnected.
The circuit diagram is drawn using •standard symbols to represent circuit components
•straight lines, drawn either horizontally or vertically, to represent circuit conductors.
Some standard symbols are shown below.
The circuit diagram for the circuit shown in figure 8 is shown below.
Figure 11
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In the space provided draw the neatly circuit diagram for the circuit arrangement shown in
figure 9.
Two terms that are commonly encountered when working with electric circuits are –
Prevents current flow
open circuit - ____________________________________
Open switch
Allows current flow
closed circuit - ___________________________________.
Closed switch
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7. MEASURING VOLTAGE AND CURRENT
Voltages associated with a circuit may be measured using an instrument called a voltmeter.
The voltmeter • measures the electrical pressure between two points
• must be connected across the two points between which the voltage is to be measured, this
is known as a parallel connection
• ideally will have very high internal resistance.
The standard symbol used to represent the voltmeter is
V
To measure the current flow in a circuit an instrument known as an ammeter is used.
The ammeter • measures current flow in a circuit
• must be connected so the circuit current flows through the meter, this is known as a series
connection
• should have very low internal resistance.
The standard symbol used to represent the ammeter is
A
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Draw the circuit diagram for a circuit which consists of a • battery
• fuse
• switch
• resistance
• voltmeter to measure the battery voltage
• ammeter to measure the circuit current.
A
V
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8. USING THE VOLTMETER
The following points should be considered when using a voltmeter:
1.Always connect the voltmeter in parallel with the circuit component across which the
voltage is to be measured. See the examples shown in figure 12.
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1.Always connect the voltmeter with correct polarity.
DC voltmeters are polarised, that is, positive (+) and negative (-).
Red is positive and black is negative.
Be sure to connect the positive (red) test lead to the positive end of the component across
which the voltage is to be measured.
If mistakes are made, the pointer will deflect backwards, that is, to the left of zero and the
meter possibly damaged.
2.The positive end of a component is the end into which the current flows.
3.When voltmeters with multiple ranges are used to measure unknown voltages, protect the
meter by switching to the highest range first, then slowly adjust the range down until a
voltage reading is indicated on the meter scale.
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9. READING THE VOLTMETER SCALE
Single Scale Voltmeter
The scale of a 0-10V DC voltmeter is shown in figure 13.
Figure 13
The meter is identified as a voltmeter by the upper case V shown below the scale and that it
is a DC meter by the straight line drawn immediately below the upper case V.
The meter scale is divided into major and minor divisions, with:
1
• 10 major divisions each representing __________
volt.
• each major division divided into 5 parts, giving minor divisions which each represent
0.2
__________
volts.
The indicated value if the pointer is at position:
a is 2V
b is 5V
c is 7.4V
d is 9.5
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Example:
Determine each of the values indicated by the pointer positions (a-h) shown on
the voltmeter scale of figure 3. Record your answers in the table provided
below.
Figure 14
1
2.6
4.1
5.3
6.6
7.5
9
9.8
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10. THE AMMETER
The following points should be considered when using an ammeter:
1.Always connect the ammeter in series with the circuit component through which the
current is to be measured. See the examples shown in figure 15.
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1.Always connect the ammeter with correct polarity.
DC ammeters are polarised, that is, positive (+) and negative (-).
Red is positive and black is negative.
Be sure to connect the ammeter so that current flows into the positive terminal (red) and out
of the negative terminal (black).
If mistakes are made, the pointer will deflect backwards, that is, to the left of zero and the
meter possibly damaged.
2.Trace out the path taken by circuit current in order to determine the correct ammeter
connections. Remembering, current flow is always from the source positive terminal through
the circuit to the source negative terminal.
3.When an ammeter with multiple ranges is used to measure unknown currents, protect the
meter by switching to the highest range first, then slowly adjust the range down until a
current reading is indicated on the meter scale.
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11. READING THE AMMETER SCALE
Single Scale Ammeter
The scale of a 0-2A DC ammeter is shown in figure 16.
Figure 16
The meter is identified as an ammeter by the upper case A shown below the scale and that it
is a DC meter by the straight line drawn immediately below the upper case A.
The meter scale is divided into major and minor divisions, with:
•8 major divisions each representing 0.25
__________ amperes.
•each major division divided into 5 parts, giving minor divisions which each represent
__________
amperes.
0.05
The indicated value if the pointer is at position:
a is 0.5A
b is 1.25A
c is1.55A
d is 1.825A
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Example:
Determine each of the values indicated by the pointer positions (a-h) shown on
the ammeter scale of figure 17. Record your answers in the table provided
below.
Figure 17
0.25
0.6
0.775
1.1
1.35
1.5
1.725
1.95
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Main Points lesson 1
Increase
Pressure
Increase
Flow
Voltmeter
Parallel
High Resistance
Increase
opposition
Decreases
Flow
Ammeter
Series
Low Resistance
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The End