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Process control

Electro/Digital

Electricity

This is the first lesson on the subject of Electro/Digital. The lesson begins withOhms law, which gives the relationship between voltage, current and resistance.Several important aspects of these three electrical quantities will be described

The second half of the lesson addresses the testset. The testset consists of twoparts a voltage source for direct (DC) and alternating (AC) voltage and a meterfor DC and AC voltage and DC current.

Contents of the lesson

1 Ohms law

2 Electrical measurements

3 Testset

4 Measurement of DC current and DC voltage

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Lesson

1. Ohms lawOhms law is an important law relating to electricity. It gives the relationshipbetween voltage, current and resistance. The proportionalities discussed belowwere discovered by the physicist Ohm.

A potential difference of 1 volt (V) across a resistance of 1 ohm (W) results in anelectric current of 1 ampere (A).

Expressed as a formula, this is:V = I * R

1.1 Voltage

An electrical conductor contains free electrons. These electrons carry a negativeelectrical charge. The electrons repel one another because they are carrying thesame negative charge. The energy needed to move the electrons towards oneanother is referred to as voltage. As the voltage increases the electrons arepushed closer together, overcoming the forces of repulsion caused by the equalelectrical charge. When subjected to this voltage, the electrons tend to move.

The electrons move under the influence of the voltage from a point with highelectron density to a point with low electron density. A point with manyelectrons is more highly negatively charged than a point with fewer electrons. Inother words electrons move from negative to positive.

Before the existence of electrons was discovered, it was thought that electricityflowed from positive to negative. Only later was it discovered that negativeparticles were the carriers of electric current and that they flowed from negativeto positive. In order to avoid confusion, however, things were left the way theywere and we still use the convention that an electric current flows from positiveto negative.

Another way of looking at this is that if an electric current flows through aconductor, there has to be a voltage difference across the conductor. There needsto be a reference point in order to determine a voltage difference. In most casesthe voltage of the earth represents the neutral point for a voltage difference. Inelectronics this reference voltage is referred to as the earth potential.

We can therefore use the earth potential as a reference point for all othervoltages. The voltage difference between a measurement point and the earthpotential is referred to as the electrical potential of that point. The voltagedifference between two points with different potentials is referred to as thepotential difference. The unit for measuring potential or voltage is the volt (V).

- free electrons

- negative electrical charge

- reference voltage/earth potential

- electrical potential

- volt


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Question 1What is meant by earth potential?

1.2 Current

We refer to the movement of electrons under the influence of an applied voltageas current. The current can be considered as the number of electrons that movethrough a conductor per unit time. We are talking here about very large numbersof electrons per second.

In order to be able to describe an electric current using normal numbers, peoplehave defined the coulomb, where 1 coulomb = 6.25 * 1018 electrons. The unit ofelectric current is the ampere. There is a simple relationship between the ampereand the coulomb: 1 ampere (A) = 1 coulomb (C) per second (s).

An electric current made up of electrons that are always flowing in the samedirection is called a direct current (DC current). A battery supplies DC current,for example. An electric current that is continually changing direction is calledalternating current (AC current). An AC current dynamo supplies AC current,for instance.

Question 2In which direction do the electrons go through a conductor if there is anACcurrent? What is the direction of the electric current?

1.3 Resistance

Electrons can move more easily through some materials than others. Theelectrons are hindered to a greater or lesser extent. This hindrance is referred toas electrical resistance.

Resistance depends on the type of material and also the shape. A short thickpiece of wire gives less resistance than a long thin piece. Substances with veryhigh resistance are referred to as insulators, and substances with a very lowresistance are called conductors.

The unit of electrical resistance is the ohm (W ). The resistance can be expressedas the drop or decrease in voltage (in V) that occurs per ampere (A). Inelectronics, components are used that are specially intended to form a resistanceto an electric current. Such components are referred to as resistors.

Question 3Which factors affect electrical resistance?

- 1 coulomb- ampere

- DC and AC current

- electrical resistance

- insulators/ conductors

- ohm


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2. Electrical measurementsA large number of instruments are used to make electrical measurements. Wewill limit ourselves to the voltmeter and the ammeter.

2.1 Voltmeter

A voltmeter is an instrument for measuring the magnitude of the voltage, forexample the voltage across a resistor. Care must be taken to ensure that thevoltmeter is always connected to the resistor in parallel. The principle of thistype of measurement is shown in figure 1.

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Figure 1Voltage measurement

Figure 1 is a diagram of a simple closed circuit. The voltage source supplies DCcurrent. The long bar is the positive and the short bar is the negative. Here theelectric current flows clockwise. The voltmeter is connected to the resistor inparallel and therefore current passes through it too. This current has to be assmall as possible because otherwise it would not be possible to measure thevoltage across the resistor accurately. A voltmeter therefore has to have a veryhigh resistance. In the case of DC current, the positive terminal of the voltmeterhas to be connected to the positive terminal of the voltage source. The negativeterminal of the voltmeter is therefore connected to the negative terminal of thevoltage source.

2.2 Ammeter

An ammeter is an instrument for measuring the magnitude of the electriccurrent, for example the current through a resistor. Care must be taken to ensurethat the ammeter is connected to the resistor in series. The principle of this typeof measurement is shown in figure 2.

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Figure 2Current measurement

- voltmeter

- connected in parallel

- high resistance

- ammeter

- connected in series


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The electric current in the circuit shown in figure 2 is the same everywhere. Inthis situation it does not matter where the current is measured, as long as theammeter is connected to the resistor in series. In order to obtain an accuratemeasurement, it is essential that the ammeter does not have any practical effecton the current being measured. This means that the resistance of an ammeter hasto be extremely low. In the case of measurement of a DC current, the positiveterminal of the ammeter also has to be connected to the positive terminal of thevoltage source.

2.3 Voltage source

A voltage source is a device that can maintain a potential difference. The devicehas two terminals: a positive terminal and a negative terminal.

If we connect a resistor to this battery, the electrons will flow from the pole withthe negative charge to the pole with the positive charge. As a result of this, theamount of negative charge on the negative pole is reduced, and the deficiency ofnegative charge on the positive pole is also reduced. The voltage differencebetween the two poles becomes smaller as a result of the transport of electrons.The current through the resistor becomes smaller as a result of the reduction inthe potential difference. In this way the voltage between the poles of the voltagesource will ultimately drop to zero.

If we connect a resistor to a voltage source, the electrons will flow from the polewith the negative charge to the pole with the positive charge. If we also connecta lamp to the voltage source, the lamp will continue to burn just as brightly. Thepositive and negative poles of the voltage source therefore appear to maintaintheir voltage.

This is only possible if the electrons arriving at the positive pole are transportedinternally within the voltage source to the negative pole. The voltage sourcepumps the electrons from the positive pole to the negative pole so to speak.

Question 4How can a voltage source be described?

3. TestsetA testset consists of two sections:- a voltage source for DC and AC voltage;- a meter for DC and AC voltage and DC current.

We will discuss each of these separately.

- low resistance


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3.1 Direct current (DC) supply

Figure 3 shows the section containing the connections for the DC supply. Thesockets marked A and B are the terminals for a non-adjustable voltage of 6 V.

This voltage can be turned on and off using the topmost knob (E).

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Figure 3DC current supply section of the testset

The sockets marked B and C are the terminals for a supply that can be adjustedusing knob D. There are three options:- turn knob D to position V. The supply source is then a constant voltage

source. The magnitude of the voltage can be adjusted using knob E. Knob Ealso serves as an on/off switch for these terminals. The highest voltage thatcan be set is approximately 9 V;

- turn knob D to position mA2. The supply source is now a constant currentsource. The magnitude of this current can also be adjusted using knob E upto a maximum of 2 mA;

- turn knob D to position mA20. Here too a constant current source isobtained, but now the maximum current is 20 mA instead of 2 mA.

Question 5A testset consists of two sections. What are they?

3.2 Alternating current (AC) supply

Figure 4 shows the section of the testset containing the connections foralternating current (AC) supply; it will not be discussed further because it is notincluded in these lessons.

- position V

- position mA2

- position mA20


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Figure 4ACt supply section of the testset

4. Measurement of DC current and DC voltageThe meter can be set to different sensitivities to measure AC voltages, DCvoltages and DC currents. Figure 5 shows the layout of the instruments meterpanel. The meter has three scale divisions, of which we will only be using the0-3 and 0-10 divisions.

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Figure 5Measurement section of the testset

- scale divisions


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4.1 Measurement of DC current

In order to determine a DC current, set knob F to the mA position. Sockets Aand B are the terminals for the DC measurement. The measurement range can beset using knob E. If it is set to position 3, for example, the magnitude of thecurrent is read on the scale 0-3. We take the readings from the same meterwhen knob E is set to the 0.3, 30 or 300 positions, but now the meter reading hasto be multiplied by 0.1, 10 or 100 respectively. As far as positions 0.1, 1, 10 and100 are concerned, however, we read the values on the scale 0-10, multipliedby the correct factor where appropriate.

If the approximate magnitude of the current is not known beforehand, themeasurement is started with E in the highest position (300). If the result of themeasurement permits, the meter is made more sensitive by gradually reducingthe range using knob E.

Question 6At what measurement range is knob E set if a current of unknown strength has tobe measured?

4.2 Measurement of DC voltage

A DC voltage can be measured when knob F is set to the V position. In this casethe measurement range is set using knob G, and sockets C and D are used asterminals. Here too, the scale division 0-3 is used for the settings 30 mV, 0.3, 3and 30 V, and the scale division 0-10 is used for positions 10mV, 0.1, 1 and10 V.The two positions of knob F furthest to the right are not required for this lesson.

- measurement range


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SummaryOhms law gives the relationship between voltage, current and resistance. Apotential difference of 1 volt across a resistance of 1 ohm results in an electriccurrent of 1 ampere.

Expressed as a formula, this is:V = I * R

The energy needed to move these electrons towards one another is referred to asvoltage.

The electrons move under the influence of the voltage from a point with highelectron density to a point with low electron density. In other words electronsmove from negative to positive. However, the convention is that an electriccurrent flows from positive to negative.

In most cases the potential of the earth represents the neutral (or zero) point. Inelectronics this reference voltage is referred to as the earth potential.

The voltage difference between a measurement point and the earth potential isreferred to as the electrical potential of that point. The unit for measuringpotential or voltage is the volt (V).

The current can be considered as the number of electrons that move through aconductor per unit time.In order to be able to describe an electric current using normal numbers, peoplehave defined the coulomb, where 1 coulomb = 6.25 * 1018 electrons. The unit ofelectric current is the ampere. There is a simple relationship between the ampereand the coulomb 1 ampere (A) = 1 coulomb (C) per second (s).

An electric current made up of electrons that are always flowing in the samedirection is called a direct current.An electric current that is continually changing direction is called alternatingcurrent.

The resistance can be expressed as the drop or decrease in voltage (in V) thatoccurs per ampere (A). The unit of electrical resistance is the ohm (W).Resistance depends on the type of material and also the shape. Substances withvery high resistance are referred to as insulators, and substances with a very lowresistance are called conductors.

A voltmeter is an instrument for measuring the magnitude of the voltage, forexample the voltage across a resistor. Care must be taken to ensure that thevoltmeter is always connected to the resistor in parallel. A voltmeter thereforehas to have a very high resistance.


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An ammeter is the normal name for an instrument measuring the magnitude ofthe electric current, for example the current through a resistor. Care must betaken to ensure that the ammeter is connected to the resistor in series. Thismeans that the resistance of an ammeter has to be very low.

A voltage source is a device that can maintain a potential difference.

A testset is made up of two sections:- a voltage source for DC and AC voltage;- a meter for DC and AC voltage and DC current.


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TestExercisesDo not send in your answers for correction

1 A coil is connected to a voltage of 12 V. The current passing through thewire is 0.5 A. Calculate the resistance of the coil.

2. A soldering iron contains an element with a resistance of 110 W . What is themagnitude of the current that is created in the element if the soldering ironis connected to a 220 V supply?

3. The bulb of a torch uses a current of 0.3 A at a voltage of 4.5 V. What is theresistance of the bulb?

4. A quantity of charge amounting to 4500 Coulomb moves through aconductor during a period of 5 minutes. How large is the current?

5. How many light bulbs, each with a resistance of 44 W , can burn whenconnected to a voltage of 220 V? The current used by each lamp whenburning normally is 0.5 A.

6. According to convention, what have we assumed to be the direction of theelectric current in a conductor? How do electrons move through thisconductor?

7. A current of 4 A is flowing through a circuit. The resistance of the circuit ismade twice as large. What does the current then become?

Answers to the questions in the lesson1. In most cases the voltage of the earth represents the neutral point for a

voltage difference. In electronics this reference voltage is referred to as theearth potential.

2. The direction in which the electrons flow will change continually becausethe positive and negative poles are continually exchanged. The electriccurrent also changes direction continually but it will always be in theopposite direction to the flow of electrons.

3. The resistance depends on the type of material and also the shape.

4. A voltage source is a device that can maintain a potential difference.

5. A testset is made up of two sections:- a voltage source for DC and AC voltage;- a meter for DC and AC voltage and DC current.


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6. If the approximate magnitude of the current is not known beforehand, themeasurement is started with E in the highest position (300). If the result ofthe measurement permits, the meter is made more sensitive by graduallyreducing the range using knob E.

Answers to the exercises1. Ohms law gives the relationship between voltage, current and resistance.

Expressed as a formula, this is: V = I * R

Using the data results in 12 = 0.5 * R

It follows from this that R = 5.0

12 = 24 W

2. With Ohms law in the form of a formula:V = I * R

Putting in the data gives:220 = I * 110

It follows from this that I = 110220

= 2 A


Putting in the data gives:4.5 = 0.3 * R

It follows from this that R = 3.05.4

= 15 W

4. The following relationship exists between the ampere and the coulomb:1 ampere (A) = 1 coulomb (C) per second (s).

Putting in the data results in a current of )60*5(

4500 = 15 A


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Let the number of light bulbs be X. After putting the data into the formulawe obtain:220 = 0.5 * (X * 44)

It follows from this that:

5.0220

= (X * 44)s

or X = ( )4*5.0220

= 10

It is therefore possible to have 10 light bulbs burning normally.

6. The convention is that an electric current flows from positive to negativeThe electrons move under the influence of the voltage from a point withhigh electron density to a point with low electron density. In other words theelectrons move from negative to positive.

7. Ohms law gives the relationship between voltage, current and resistance inthe form of a formula. Because the current is inversely proportional to theresistance, a doubling of the resistance results in halving the current. Thecurrent was 4 A and it drops to 2 A.

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