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May be useful. You May Like This Books. How to Do Nothing Jenny Odell. The Innovators Walter Isaacson. Navy Equipment Encyclopedia Progressive Management. Human Compatible Stuart Russell. The Becoming Nora Roberts. Illustration for Part One Test Questions 3 and 4. A voltage of mV is the equivalent of a , mV. From Fig. Test, it is apparent that for this particular component, a the resistance does not change as the current through it changes, at least within the range shown.
Suppose there are 5 resistors, each with a value of , are con- nected in series. What is the net resistance of the combination? Suppose you are drawing a schematic diagram, and you have to make two lines representing wires cross on the page. You want to indicate that they are connected. You can do this by a drawing the two lines and letting them cross, but not putting a dot at the point of intersection. How many elementary charge units ECU are there in a volt? Nevertheless, in a conventional conductor, the particles responsible for the current are a negatively charged ohms.
Two objects will tend to be attracted toward each other if a they are both electrically neutral. A resistance of 20 M is the equivalent of a 0. Suppose that 5 resistors, each with a value of , are connected in parallel. What is the net voltage of the combination? Illustration for Part One Test Questions 14 through In the circuit shown by Fig. Test, suppose the battery consists of 1. How many cells are there in this battery? Test, what does the meter read, assuming it is calibrated properly and is of the correct range to give a meaningful reading of the DC in this circuit?
Suppose several more neon lamps, all identical to the one there now, are connected in parallel with the battery in the circuit of Fig. What will happen to the current drawn from the battery? How much power is dissipated by the neon lamp in the circuit of Fig.
How much energy is dissipated by the neon lamp in the circuit of Fig. A button cell would most likely be used as the main source of power for a a wristwatch. Suppose you let a 60 W bulb stay aglow continuously. Energy in your location costs 10 cents per kilowatt hour. How much does it cost to leave this bulb on for 10 days and 10 nights? PART 1 Test 95 Suppose that 16 resistors, each with a value of , are connected in a 4-by-4 series-parallel matrix.
Suppose a single solar cell is capable of supplying exactly 81 mW of DC power when exposed to direct sunlight. If 3 of these cells are connected in series, the resulting set of solar cells can theoretically provide a 9 mW of DC power in direct sunlight. If 3 of these cells are connected in parallel, the resulting set of solar cells can theoretically provide a 9 mW of DC power in direct sunlight.
Which of the following sources of DC power can make use of hydro- gen as the chemical from which the power is derived? Electrical current is expressed in terms of a the rate of change in voltage between two points per unit time.
Illustration for Part One Test Questions 26 through PART 1 Test 97 Test, the switches are all a open. Suppose one of the components at the top of the circuit in Fig. Test1- 3 is removed, leaving an open circuit in its place. Suppose that the component at the extreme lower right of the circuit in Fig.
Test is removed, leaving an open circuit in its place. Suppose two electrically charged objects are brought near each other, and an electrostatic force is observed that tends to push them apart. If the objects are brought closer and closer together, the electrostatic force will a become attractive once the objects come within a certain dis- tance of each other. PART 1 Test 99 b decrease in direct proportion to the separation distance, as that distance decreases.
A frequency of , Hz is the equivalent of a 0. Which of the following equations is true pertaining to the circuit in Fig. The variables I1, I2, I3, and I4 represent currents, all in amperes. Suppose that in the circuit of Fig. Test, all the resistors have values of What is I4?
Illustration for Test Questions 36 and This is called a alternating voltage. In an AC voltage, the polarity of the voltage between two points reverses periodically.
How a Wave Alternates In an AC wave, the alternation rate can be expressed in terms of how long a complete cycle takes, or as the number of complete cycles per unit time. The instantaneous amplitude of the AC from a utility outlet constantly and rapidly changes, in contrast to the instantaneous amplitude of the DC from a battery, which does not.
The way in which the instantaneous amplitude varies depends on the shape of the graph of the wave. This wave shape is also called the waveform. THE CYCLE When an AC wave is drawn as a graph of current, voltage, or power versus time, a cycle is the part of the wave between any point on the graph and the same point on the next alternation.
Figure shows some points that are commonly used to mark the instant where a single cycle starts and ends. The AC wave in this illustration is a sine wave, representing the most perfect possible AC wave.
It gets this name because it looks like a graph of the sine function in trigonometry. The time required for a single cycle to complete itself corresponds to the distance in the graph between these two points. Two successive wave troughs are also shown in Fig. Again, the time for a cycle is portrayed as the distance between two successive wave troughs.
These points correspond to a momentary absence of any current or voltage. Figure B shows examples of a positive-going zero point and a negative-going zero point. PERIOD The length of time between successive wave crests, troughs, positive-going zero points, or negative-going zero points is called the period of the AC wave. Period is denoted by the uppercase italic letter T. The standard unit for the period is the second, abbreviated by the lowercase, non-italic letter s.
In most electrical and electronics circuits, T is a fraction of a second. Some radio signals have periods less than 0. Period is often expressed in fractions of a second such as milliseconds ms , microseconds ms , nanoseconds ns , and picoseconds ps.
High frequencies were expressed in kilocycles, megacycles, or gigacycles, representing thousands, millions or billions thousand-millions of cycles per second.
Now, the standard unit of frequency is known as the hertz, abbreviated Hz. The hertz is exactly the same thing as the cycle per second. What is this frequency, f, in hertz? Express the answer as a number written out in full, and also as a number in power-of notation.
Express the answer in seconds, and also in nanoseconds. Here are some of the simplest and the most common forms. A sine wave, if displayed on an oscilloscope or plotted as a graph, looks like the heavy, wavy curve in Fig. A wave of this sort is sometimes said to be sinusoidal. The transitions between the negative and the positive polarities take place instantaneously, so in theory they should not show up. A square wave. The slope of the line indicates how fast the amplitude is changing.
Such waves are called sawtooth waves because of their appearance. What is the period of the wave shown? There are 3 horizontal divisions between successive crests or between successive troughs. Express the answer in hertz, and also in megahertz. A An example of a sawtooth wave. This has a slow rise and a fast decay. B A sawtooth wave with a fast rise and a slow decay. Therefore, the frequency of the wave shown in Fig.
The Strength of the Wave Depending on the quantity being measured, the amplitude of an AC wave can be expressed as amperes for current , volts for voltage , or watts for power.
In many waves, the posi- tive and negative peak amplitudes are the same. Peak is often abbreviated pk. You might see the amplitude of an AC wave expressed as 4. Figure A illustrates the concept of peak current or voltage for a pure AC sine wave. The positive or negative peak amplitudes of an AC wave do not depend on the period or the frequency.
If the positive and negative peak amplitudes are the same, then the peak-to-peak amplitude is twice the peak amplitude. Peak current has a direction assigned to it, and peak voltage has a polarity. But peak-to-peak quantities do not have direction or polarity. Thus, after calculating the peak-to-peak current or voltage, you should remove any directional expression or polarity sign. The peak-to- peak amplitude of an AC wave does not depend on the period or the frequency.
An irregular AC wave sometimes has an average amplitude of zero, but often this is not the case. The average amplitude for an irregular wave depends on the shape of the wave, and also on the positive and negative peak amplitudes. It does not depend on the period or the frequency. If an AC sine wave has a DC component along with it, then the average amplitude of the wave is the same as the DC component.
Situations of this kind are rare in ordinary electrical circuits, but they occasionally arise in electronic devices. This is almost never the same thing as average amplitude! A Peak pk amplitude for a sine wave. B Peak-to-peak pk—pk amplitude for a sine wave. This is abbreviated rms. For a perfect sine wave with no DC component, the rms value is equal to about 0. For a perfect square wave, the rms value is the same as the peak value.
The pk—pk value is twice the rms value and twice the peak value. For other AC waveforms, the relationship between the rms value and the peak value depends on the shape of the wave. The rms amplitude of an AC wave, like peak, and peak-to-peak, and average amplitudes, does not depend on the period or the frequency.
Voltage is on the vertical scale, and time is on the horizontal scale. Each vertical division represents 5 V. What is the approximate positive peak voltage? The approximate negative peak vol- tage? Illustration for Problems through SOLUTION The average voltage is 0 V, assuming the positive and negative peak voltages are equal and opposite, and assuming as we are told in Problem that the wave is a sine wave.
Electric generators are the most common means of producing utility AC. Another scheme involves the conversion of DC to AC using specialized switches and transformers. A generator can consist of a rotating magnet inside a coil of wire, or a rotating coil inside a magnet or pair of magnets. The output of such a generator is always AC.
Small portable gasoline-powered generators, capable of producing a few kilowatts, can be purchased in department stores. Larger generators can produce enough electricity to supply a house or building. When AC is generated by a rotating magnet in a coil of wire, or by a rotating coil of wire inside a powerful magnet, AC voltage appears between the ends of the length of wire.
The voltage depends on the strength of the magnet, the number of turns in the wire coil, and the speed at which the magnet or coil rotates. The AC frequency depends only on the speed of rotation. Normally, for utility AC, this speed is revolutions per minute rpm , or 60 revolutions per second rps.
This produces an output frequency of 60 Hz. In some countries the rotation speed is rpm or 50 rps, producing an AC output frequency of 50 Hz. The electrical power that comes out of a generator can never be more than the mechanical power driving it.
In fact, there is always some energy lost, mainly as heat in the generator hardware. If you connect a generator to a stationary bicycle, your legs might provide enough power to run a small radio, but nowhere near enough to supply a household. The ratio can be multiplied by to get a percentage. Small power inverters can be obtained in department stores. The chopper opens and closes a switching transistor. This interrupts the battery current, producing pulsating DC.
The transformer converts the pulsating DC to AC, and also steps up the voltage. If the battery is rechargeable, solar panels can be used to replenish its charge and provide a long-term source of utility power. The output of a low-cost power inverter is generally not a good sine wave, but more closely resembles a square wave.
Square-wave choppers are inexpensive and easy to manufacture. For this reason, low-cost power inverters do not always work well with appliances that need a nearly perfect, Hz sine-wave source of AC power.
Sophisticated and expensive inverters produce fairly good sine waves, and have a frequency close to 60 Hz. A power inverter for converting low-voltage DC to utility AC. His colleagues argued that AC would work better, and they prevailed. But there is at least one advantage to DC in electric power trans- mission. This becomes apparent in long-distance power lines. A mechanical power level of 1 hp is equivalent to approximately W. Suppose it takes 2 hp of mechanical energy to turn a generator that puts out W of AC electricity.
As a percentage? Suppose you see an AC sine wave on an oscilloscope, and the oscilloscope screen looks like Fig. You have the scope set so each horizontal division represents exactly 5 ms and each vertical divi- sion represents exactly 1 V. The period of this wave is approximately a 15 ms b 30 ms c 3 ms d 6 ms 2. If each horizontal division in Fig. Illustration for Quiz Questions 1 through 4.
If the period of an AC sine wave doubles, and if nothing else about it changes, then the frequency a doubles. If the period of a perfect square AC wave doubles, and if nothing else about it changes, then the ratio of its rms amplitude to its peak-to- peak amplitude a doubles. A generator requires kW of mechanical power to operate, and is capable of producing , W of AC power.
The average amplitude of an irregular AC wave a is always zero. If an AC sine wave has a period of approximately 10 ms, then an AC square wave of the same frequency has a period of approximately a 10 ms.
The instantaneous amplitude of an AC wave a has a constant frequency. Phase Phase is an expression of a point in time during an AC cycle. Phase can also express the relative time displacement between two AC waves. The same point on the next cycle is given the value Halfway through the cycle is , a quarter of the way through the cycle is , and three-quarters of the way through the cycle is Figure illustrates this concept for a sine wave.
If two sine waves are in phase coincidence, the peak-to-peak amplitude of the resultant wave, which is also a sine wave, is equal to the sum of the peak- to-peak amplitudes of the two constituent waves. The phase of the resultant is the same as that of the constituent waves.
In this situation, people sometimes say that the waves are out of phase. Degrees of phase in a sine wave are expressions of the portion of the cycle that has passed since its starting time.
At A, graphic illustration of sine waves in phase coincidence. At B, graphic illustration of sine waves in phase opposition. If two sine waves have the same amplitude and are in phase opposition, they cancel each other out. The instantaneous amplitudes are equal and opposite at every point in time.
This is the sort of AC available at standard wall outlets intended for small appliances such as lamps, TV sets, and computers. In most parts of the United Stated, the rms voltage is standardized at V, but it can vary some- what above and below this level depending on power demand at the time, your location, and to some extent at the whims of your local electric utility.
The positive and negative peak voltages are 1. The peak-to-peak voltage is about twice the positive or negative peak voltage, or V pk—pk. This is called a 3-phase AC.
Figure shows what a 3-phase AC electrical waveform looks like when plotted as a graph of instantaneous voltage versus time.
There are advantages to 3-phase AC over single-phase AC. Each wave provides its own energy, independent of the other two. This helps to distribute the power demand around the various rooms and appliances. Call them wave X and wave Y. There is no DC component on either wave. The peak-to-peak voltage of wave X is V, and the peak-to-peak voltage of wave Y is V. The positive peak voltage? The negative peak voltage?
The rms voltage? Graphic illustration of 3-phase AC. Call them, again, wave X and wave Y. Absolute value is symbolized by placing vertical lines on either side of a quantity. Transformers work by means of a phenomenon called inductive coupling. In electrical applications, transformers usually consist of wires wound around cores made of laminated iron thin slabs of iron glued together. The secondary is the winding from which electricity is taken.
In a step- down transformer, the primary has more turns than the secondary. In a step-up transformer, the primary has fewer turns than the secondary. Figure A shows the schematic symbol for a step-down transformer, and Fig. The solid, parallel, vertical lines indicate that the transformers have laminated iron cores. The size, or gauge, of the wire used for a primary winding depends on the amount of current it must carry.
The higher the current demand, the larger must the wire be. For a given amount of power, a step-up transformer must use larger primary-winding wire than a step-down transformer. A transformer that steps up the voltage by a large factor must have a primary that consists of very heavy wire.
At A, schematic symbol for a step-down AC transformer. At B, schematic symbol for a step-up AC transformer. A step-up transformer turns ratio less than can have a smaller secondary-winding wire than a step-down transformer turns ratio larger than for a given amount of power. The secondary winding of a transformer can have intermediate connections, called taps, for obtaining various AC output voltages.
Still larger ones provide the utility power to which we are accustomed in our homes and businesses. The largest transformers handle thousands of volts and thousands of amperes, and are used at power-transmission stations. Some of these are as big, and as heavy as a fully loaded truck. Some power is also lost in the core. Conductor losses occur because of the ohmic resistance of the wire that makes up the windings. Let Epri and Ipri represent the primary-winding voltage and current in a hypothetical transformer, and let Esec and Isec represent the secondary- winding voltage and current.
In a perfect transformer, the product EpriIpri would be equal to the product EsecIsec. But in a real-world transformer, EpriIpri is always greater than EsecIsec. The reason for this is the fact that a transformer is a DC-blocking device. The blocking of DC is an important characteristic of transformers that you should remember.
Alternating current can pass through a transformer, but direct current cannot. Therefore, if the input wave to a transformer has a DC component, that component does not appear in the output wave.
This property of transformers can be useful in the design of certain electrical devices and systems. There is no DC component. This voltage, Epri-rms, appears across the entire primary winding. The primary-to-secondary turns ratio, T, is The transformer has a secondary with a tap in the center Fig. What is the rms output voltage, Esec-rms, that appears across the entire secondary winding, that is, between terminals X and Z?
Assume the input is a sine wave. What is the rms voltage that appears between terminals Y and Z? Assume that the input is a sine wave with no DC component. This splits the vol- tage into two equal parts. The primary- to-secondary turns ratio, T, is What is the peak-to-peak output voltage, Esec-pk—pk, that appears across the entire secondary winding?
The voltage across the primary winding is V rms, and the current through the primary winding is 2. The voltage across the secondary winding is In most house- hold and business appliances, the current is in phase with the voltage. These phenomena are beyond the scope of this elementary course. Let Erms stand for the voltage of the AC source in volts rms , let Irms stand for the current through the load in amperes rms , and let R stand for resistance of the load in ohms.
If quantities are given in units other than volts rms, amperes rms, and ohms, you must convert to these units and then calculate. What is the rms AC voltage of the AC source? What is the power consumed by the load?
If quantities are given in units other than these units, you must convert them before you plug any numbers into the above formulas. Suppose the AC voltage source supplies 12 V rms and the ammeter shows 4 A. We can plug in the numbers directly. How much energy is consumed by the load during this period of time? Give the answer in watt-hours, and then in kilowatt-hours. The circuit is activated for 90 min, and during this time the ammeter reads mA rms.
How much energy is consumed by the load? The lower atmosphere acts as an insulator, and the earth and the ionosphere together form a giant electrical storage device or a capacitor. The charge in this atmospheric capacitor is stupendous, and the insulating atmosphere frequently breaks down. Lightning discharges tend to be concentrated in areas of precipitation, particularly in rain storms. Lightning is common in tropical storms and hurricanes, although some have a lot more than others.
Lightning can also take place in heavy snow squalls and blizzards especially mountain storms , and it can even strike, once in a while, from a partly cloudy sky while the sun is shining and no precipitation is falling.
Sand storms and erupting volcanoes have also been known to produce lightning. Its capricious nature is the basis of legends and myths that go back to the beginnings of civilization. There are four types of lightning that are common in and near thunderstorms. A stroke begins when the charge between a cloud and the ground, or between a cloud and some other part of the cloud, becomes so large that the electrons begin to advance through the air from the negative charge pole.
This process can take from a few milliseconds to several tenths of a second. Once the stepped leader has established the circuit by ionizing a channel through the atmosphere, one or more return strokes immediately follow, attended by massive current that can peak at over , A. The main danger to people is from burns and electrocution. People can be harmed by lightning, and electronic equipment damaged, despite precautions.
However, the danger can be minimized. Lightning can take place at any time. If thunder can be heard, lightning is taking place within a few kilometers of your location, whether it can be seen or not. Disconnect all antennas and ground them.
For more information, consult a competent engineer or electrician. In an AC sine wave, Consider a circuit with 3-phase AC. A transformer is supplied with AC electricity. The transformer is con- nected to a load. The voltage across the primary winding is V rms, and the primary draws 1 A rms. The voltage across the secondary is 12 V rms, and the load draws 8 A rms. Assume that no power is dissipated anywhere except in the load and the transformer. Suppose an AC transformer has turns in the primary winding and turns in the secondary winding.
This is a a step-up transformer. If an AC sine wave with a voltage of V rms is applied to the primary winding, what is the peak-to- peak AC voltage across the secondary winding? Suppose the switch is closed. What is the load resistance? Assume that the timer has no resistance. Illustration for Quiz Questions 6 through 9. Suppose the switch is closed in the circuit of Fig. What is the power drawn by the load? Assume the timer has no resistance. What is the energy consumed by the load?
The switch is opened after the timer reading has gone from an initial value of 0 h, 0 min to All of the following can cause losses in a transformer, except a resistance in the wire that makes up the windings. Several new schematic symbols will come up. If you have any doubt about your ability to safely build or work with a power supply, leave it to a professional.
This is usually done by means of one or more heavy-duty semiconductor diodes, following a power transformer. The peak voltage in the reverse direction, called the peak inverse voltage PIV or the peak reverse voltage PRV across the diode, can be as much as 2.
Most engineers like to use diodes whose PIV ratings are at least 1. Second, the output voltage can drop considerably when the supply is required to deliver high current.
The main advantage of a half-wave circuit is that it costs less than more sophisticated circuits. The center tap is connected to an electrical ground, also called the chassis ground. This produces voltages and currents at the ends of the winding that are in phase opposition with respect to each other.
The PIV across the diodes can be as much as 2. Therefore, the diodes should have a PIV rating of at least 4. If you compare Fig. But because the transformer is more sophisticated, the full-wave center-tap circuit costs more than a half-wave circuit that delivers the same output voltage at the same rated maximum current.
It is diagrammed in Fig. The output wave- form is similar to that of the full-wave center-tap circuit Fig. The PIV across the diodes in the bridge circuit is about 1. Therefore, each diode needs to have a PIV rating of at least 1. The bridge circuit does not require a center-tapped transformer secondary. The bridge is also easier on the diodes than half-wave or full-wave center-tap circuits.
In practice, a voltage-doubler power supply works well only when the load draws a low current. The best way to build a high-voltage power supply is to use a step-up transformer, not a voltage-doubling scheme. Nevertheless, a voltage-doubler supply can be, and sometimes is, used when the cost of the circuit must be mini- mized and the demands placed on it are expected to be modest. It works on the entire AC cycle, and so it is called a full-wave voltage doubler.
This circuit subjects the diodes to a PIV of 2. Therefore, they should be rated for PIV of at least 4. Proper operation of this type of circuit depends on the ability of the capacitors to hold a charge under maximum load. This means that the capacitors must have large values, as well as being capable of handling high voltages. A full-wave voltage-doubler power supply. In a half wave circuit, the PIV rating of the diodes should be at least 4.
That means they must be rated for at least 4. What must be the minimum PIV rating of the diodes in this case? In a full-wave bridge circuit, the PIV rating of the diodes should be at least 2. This is half the PIV in the half-wave situation: 2. In a voltage-doubler circuit, the PIV rating of the diodes should be at least 4.
This is the same as the PIV in the half-wave situation: 4. A good component for this purpose is known as an electrolytic capacitor. This type of capacitor is polarized, meaning that it must be connected in the correct direction in the circuit. Each unit is also rated for a certain Fig. In the example of Fig. The diode must have an adequate power rating to prevent it from burning out.
The limiting voltage depends on the particular Zener diode used. Figure is a diagram of a full-wave bridge DC power supply including a Zener diode for voltage regulation. Note the direction in which the Zener diode is connected in this application: with the arrow pointing from minus to plus. Read more. Electricity Class 10 Cbse Pdf. Are you looking for electricity class 10 cbse pdf pdf, word document or powerpoint file formats for free?
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