Applied Electricity DC The Electric Circuit PURPOSE This

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Applied Electricity DC The Electric Circuit PURPOSE: This section introduces the simple electric circuit

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. 1 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

• 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 L

Page 32 -35 REFERENCES: Electrical Principles for the Electrical Trades. 5 th Edition. Jenneson

Page 32 -35 REFERENCES: Electrical Principles for the Electrical Trades. 5 th Edition. Jenneson J. R L

1. PRESSURE AND FLOW When studying or working with electric circuits three factors are

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 L

greater The greater the force applied, the ________ the resulting pressure. The movement of

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. increases flow. Increasing pressure ________ High pressure decreases flow. Large flow Decreasing pressure ________ L

yes Is it possible to have pressure without flow? ______. See figure 4. no

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 L

2. OPPOSITION TO FLOW So far we have established the relationship between pressure and

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 decreasing pressure _______ flow. L

Figure 7 illustrates the basic water circuit. In this simple arrangement all of the

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. L

3. A CIRCUIT A circuit is a closed path in which flow takes place.

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. _____________. L

Increasing electrical pressure __________ electrical flow and increases t the lamp glows __________. Two

Increasing electrical pressure __________ electrical flow and increases t the lamp glows __________. Two batteries Greater pressure brighter Figure 9 For a given electrical pressure, increasing opposition ________ electrical flow decreases and the lamp brilliance _____. dulls Two lamps Greater opposition Figure 10 L

In most circuits some form of electrical protection is provided. This protection could be

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 • 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 ___________. L

Table 1 lists the terms for electric pressure, electric flow and opposition to electric

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. 2 Kilowatt Motor 415 Volt 750 Watt L

6. CIRCUIT DIAGRAMS The circuit diagram is a simplified method of graphically showing the

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 L

In the space provided draw the neatly circuit diagram for the circuit arrangement shown

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 L

7. MEASURING VOLTAGE AND CURRENT Voltages associated with a circuit may be measured using

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 L

Draw the circuit diagram for a circuit which consists of a • battery •

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 L

8. USING THE VOLTMETER The following points should be considered when using a voltmeter:

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. L

1. Always connect the voltmeter with correct polarity. DC voltmeters are polarised, that is,

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. L

9. READING THE VOLTMETER SCALE Single Scale Voltmeter The scale of a 0 -10

9. READING THE VOLTMETER SCALE Single Scale Voltmeter The scale of a 0 -10 V 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 2 V b is 5 V c is 7. 4 V d is 9. 5 L

Example: Determine each of the values indicated by the pointer positions (a-h) shown on

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 L

10. THE AMMETER The following points should be considered when using an ammeter: 1.

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. L

1. Always connect the ammeter with correct polarity. DC ammeters are polarised, that is,

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. L

11. READING THE AMMETER SCALE Single Scale Ammeter The scale of a 0 -2

11. READING THE AMMETER SCALE Single Scale Ammeter The scale of a 0 -2 A 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. 5 A b is 1. 25 A c is 1. 55 A d is 1. 825 A L

Example: Determine each of the values indicated by the pointer positions (a-h) shown on

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. 725 1. 95 L

Main Points lesson 1 Increase Pressure Increase Flow Voltmeter Parallel High Resistance Increase opposition

Main Points lesson 1 Increase Pressure Increase Flow Voltmeter Parallel High Resistance Increase opposition Decreases Flow Ammeter Series Low Resistance L

The End

The End