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 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

Define the terms 'open-circuit' and 'closed-circuit'. 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. 5th Edition. Jenneson J.R L

electric flow, called ___________________________ 1.PRESSURE AND FLOW When studying or working with electric circuits three factors are always considered – electric pressure, called _________________ or ______________________________ electric flow, called ___________________________ 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. Pressure is defined as - ______________________________________________________. A simple example would be the water pressure at a tap. See figure 1. voltage Electromotive force current resistance A force acting on a given area pressure L

For example, as shown in figure 2, the flow of water from a tap. greater The greater the force applied, the ________________ the resulting pressure. Flow is defined as - _______________________________________________________. For example, as shown in figure 2, the flow of water from a tap. The movement of a quantity flow What then is the relationship between pressure and flow? Low pressure Small flow greater The greater the pressure the ________________ the flow. Increasing pressure ________________ flow. Decreasing pressure ________________ flow. increases High pressure Large flow decreases L

Based on what has been determined so far, we can say – 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

For a given pressure - increasing opposition _____________ flow 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 - increasing opposition _____________ flow decreasing opposition _____________ flow. For a given opposition - increasing pressure ______________ flow decreasing pressure ______________ flow. decreases increases increases decreases L

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

4. SIMPLE ELECTRIC CIRCUIT 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 – source of electrical pressure - e.g. _________________________ opposition to electrical flow - e.g. ________________________ path to allow flow to take place - e.g. _____________________________ on-off control of flow - e.g. __________________________. battery lamp conductors switch L

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

In an electric circuit – 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 - battery fuse switch lamp. fuse switch + 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 – electrical pressure is known as _____________________ electrical flow is known as _____________________ opposition to electric flow is called ______________________. voltage current resistance L

Hot Plate 240 Volt 1.2Kilowatt 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 L

The circuit diagram is drawn using - 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

open circuit - ____________________________________ 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 – open circuit - ____________________________________ closed circuit - ___________________________________. Prevents current flow Open switch Allows current flow Closed switch L

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 fuse switch resistance voltmeter to measure the battery voltage ammeter to measure the circuit current. A V L

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, 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 0-10V 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: 10 major divisions each representing __________ volt. each major division divided into 5 parts, giving minor divisions which each represent __________ volts. 1 0.2 The indicated value if the pointer is at position: a is 2V b is 5V c is 7.4V d is 9.5 L

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

The following points should be considered when using an ammeter: 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, 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 0-2A 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 __________ amperes. each major division divided into 5 parts, giving minor divisions which each represent __________ amperes. 0.25 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 L

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 L

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

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