Exercise 3-5 Solution: Alternating Currents Video
Section A
1. At present, most of the electrical energy consumed in the United States is in the form of __________________ current. alternating
2. The AC developed at a power plant must be boosted in magnitude to __________
a. reduce line loss Answer line loss is due to the long runs of wire
b. increase current
c. decrease the resistance of the transformer windings
3. Transformers are used to __________.
a. increase AC voltages
b. decrease AC voltages
c. both a and d Answer
4. AC is a current that periodically changes in both _________________ and ______________________. magnitude and direction
5. The most common type of AC signal is called the _________________________ sine wave
6. Define a single cycle of alternating current________________________________
current begins at zero, reaches maximum potential in one direction, decreases back to zero, reaches maximum potential in the opposite direction, returns to zero
7. How many alterations are in 1 cycle of AC current? 2, one in positive direction and one in the negative direction
8. Define frequency___________________________
Number of cycles occurring in a time frame of one second. It may also be described as the number of recurrences of a periodic phenomenon in a given unit of time
9. The AC current supplied to most homes in the United States is ______________cycles. 60
10. There are ___________________ cycles in one kilocycle. 1,000
11. There are _________________________ cycles in one megacycle. 1,000,000
12. There are _____________________ cycles in one gigacycle. 1,000,000,000
13. What is another term commonly used for cycles per second of frequency?_________________
a. Alternations
b. Frequency
c. Hertz Answer
d. Period
14. Define the period of an AC wave____________________
Time required for one complete cycle of a regular, repeating series of events to occur
15. The period of a frequency is measured in ________________
a. volts
b. amps
c. degrees
d. time Answer
16. Give the formula for calculating the period of a signal___________________________ T = 1/F
17. In a pure AC sine wave, the signal will cross the zero volt point at ________________ degrees.
a. 90 and 180
b. 180 and 270
c. 0 and 180 Answer
d. 90 and 270
18. In a pure AC sine wave, the signal will be at its peak value at ___________________ degrees.
a. 90 and 180
b 180 and 270
c. 0 and 180
d. 90 and 270 Answer
19. Define instantaneous voltage________________________________________________________________
Amplitude of a voltage at a specific point in time
20. The average value of an AC sine wave is the average height of the wave. What formula is used to determine the average value?
avg = 0.637xpeak voltage
21. If the average value is known, what formula is used to determine the peak value of the sine wave?
peak value = avg value/0.637
22. The effective value may be thought of as the __________________ value of the sine wave. working
23. What formula is used to determine the effective value of a sine wave?
eff = 0.707 x peak
24. If the effective value is known, what formula is used to determine the peak value of the sine wave?
peak = eff x 1.414
25. Voltages are commonly stated as being in their ___________________
a. average values
b. effective values
c. peak values
26. What is the peak value of a 120 volt sine wave
peak = 120 x 1.414 = 169.68
27. What is the peak to peak value of a 120 volt sine wave
peak to peak = peak x 2 = 169.68 x 2 = 339.36
28. Define wavelength__________________________________________________________________
The physical length of one complete cycle in space and is determined by its frequency
29. List two formulas used to determine wavelength
a. in miles 186,000/Frequency
b. in meters 300,000,000/Frequency
30. What is the wavelength in meters of an FM station operating at a frequency of 100 MHz? 3 meters
31. Lower frequencies have __________________ wavelengths longer
32. Higher frequencies have __________________ wavelengths shorter
33. Draw two sine waves with unequal amplitudes, in phase.
34. Draw two sine waves 90 degrees out of phase.
35. Draw two sine waves 180 degrees out of phase. Note: This is called destructive interference
Section B
1. With a pure capacitive load, the current will ____________________ the voltage by 180 degrees. lead
2. With a pure inductive load, the current will _______________ the voltage by 90 degrees. lag
3. With a pure resistive load, the current and voltage will _______________
a. be out of phase by 90 degrees
b. be out of phase by 180 degrees
c. be in phase Answer
4. The phase of a circuit is determined by it's ____________
a. voltage
b. resistance
c. impedance Answer
d. current
5. State the formula used to determine the impedance of an AC circuit
Z = (R2 + X2)1/2
6. Define "reactance"
It is the opposition to the flow of alternating current. It is measured in ohms. When reactance is combined with resistance it is called impendence.
7. What unit of measurement is used for reactance?
a. volt
b. ampere
c. ohm Answer
d. watt
8. State the formula used to determine capacitive reactance.
XC = 1/(2pFC)
9. State the formula used to determine inductive reactance.
XL = 2pFL
10. Define "inductance"
It is the property of a circuit that tends to oppose any change of a current due to a magnetic field associated with the current. The unit of inductance is the Henry
11. Determine the total reactance of a series circuit which contains a coil with 100 ohms of inductive reactance and a capacitor which has 200 ohms of capacitive reactance.
100 ohm
12. Define "impedance"
It is the total opposition a circuit offers to the flow of alternating current. Impedance is derived by taking the square root of the resistance squared plus the reactance squared, It is measured in ohms.
13. State the formula used to determine the total impedance of an AC circuit.
Z = (R2 + X2)1/2
14. Determine the total impedance of a series circuit which contains a 60 ohm resistor, a coil with 160 ohms of inductive reactance, and a capacitor with 50 ohms of capacitive
reactance.
125.299 ohm
15. Determine the total impedance of a parallel circuit which contains a 60 ohm resistor, a coil with 150 ohms of inductive reactance, and a capacitor with 100 ohms of
capacitive reactance (all 3 devices are connected across a 10 volt source and are in parallel).
58.834 ohm
16. Draw a vector diagram of two voltages 90 degrees out of phase. Voltage A will lead voltage B by 90 degrees and will have a value of 18 volts. Voltage B has a value of 6
volts. Determine the resultant voltage of this circuit.
17. A transformer has two windings. They are calle3d the ________________ and ___________________ windings. primary secondary
18. What happens to the current in the secondary of a transformer when the voltage is doubled?
a. it remains the same
b. it doubles
c. it is reduced to one half Answer
19. The unit of measurement for inductance is ________________
a. hertz
b. volt
c. ohm
d. henry Answer
20. Since the inductance of a coil cannot change, its value is dependent upon its physical characteristics. _____True _____False True
21. Determine the inductive reactance of a coil which has an inductance of 0.5 henry and a frequency of 100 Hz.
2 x 3.1415927 x 100 x 0.5 = 314.15927 ohm
22. Determine the inductive reactance of a coil which has an inductance of 0.5 henry and is operated at a frequency of 1 kHz.
3.141.5927 ohm
23. As the frequency of a coil increases, its inductive reactance will ________________
a. increase Answer
b. decrease
c. remain unchanged
24. Determine the total inductive reactance of a circuit containing two uncoupled coils wired in series. Coil A has 50 ohms of inductive reactance and coil B has 75 ohms of
inductive reactance.
Total inductive reactance = 50 ohm + 75 ohm = 125 ohm
25. Determine the total inductive reactance of a circuit containing two uncoupled coils wired in parallel. Coil A has 100 ohms of inductive reactance and coil B has 80 ohms of
inductive reactance.
44.4444 ohm
26. Name three power losses in transformers.
a. ___________________ Eddy currents current induced in little "swirls" on large conductors
b. ___________________ Hysteresis maintains memory...
c. ___________________ Copper losses heat produced by copper windings
27. Why are transformer cores laminated?
a. to reduce eddy currents Answer
b. to reduce copper loss
c. to reduce hysteresis loss
28. A smaller diameter wire will induce ___________ copper loss in a transformer.
a. more Answer
b. less
29. Transformers will always produce more power than they receive. _____True _____False False
Section C
1. A capacitor has the ability to _______________electrical energy and ______________ the flow of direct current. store block
2. A capacitor may be considered as having a(n) ___________________ material sandwiched between two conducting plates. insulating or non-conducting
3. The capacitance of a capacitor depends upon four physical characteristics. What are they?
a. _______________________ plate area
b. _______________________ plate separation
c. _______________________ number of plates
d. _______________________ dielectric material
4. The nonconductive material placed between the plates of a capacitor is called the _________________ material. dielectric
5. If the distance between the plates of a capacitor is doubled, what will happen to its capacitance?
a. it will remain unchanged
b. it will double
c. it will reduce by one fourth
d. it will reduce by one half Answer
6. If the distance between the plates of a capacitor is doubled, what will happen to its voltage rating?
a. it will remain unchanged
b. it will double Answer
c. it will reduce by one fourth
d. it will reduce by one half
7. If the eplate area of a capacitor is doubled what will happen to its capacitance>
a. it will remain unchanged
b. it will double Answer
c. it will reduce by one fourth
d. it will reduce by one half
8. Capacitors are commonly conducted in parallel to
a. increase the number of plates Answer
b. increase the plate separation
c. increase the plte area
d. change the dielectric material
9. Capacitors are commonly conducted in series to
a. increase the number of plates
b. increase the plate separation Answer
c. increase the plate area
d. change the dielectric material
10. State the formula used to determine the capacitance of capacitors connected in parallel.
CTotal = C1 + C2 + ... + CN
11. State the two formulas used to determine the capacitance of capacitors connected in series.
Only the general formula is stated below - the other is merely a restricted version of this one
1/CTotal = 1/(C1 + C2 + ... + CN )
12. Define leakage current in a capacitor.
It is usually very small and is almost always undesired. It is the current which flows through the dielectric, or across the surface of the dielectic. If is capacitor is leaking current, it will generally run warm.
13. Define capacitor hysteresis.
This is a heat loss in the capacitor caused by movement of the molecules within the dielectric material
14. As the frequency increases, the current which appears to flow through a capacitor will
a. increase Answer
b. decrease
c. remain the same
15. With a capacitor using the three dot color code, the first dot represents the
a. first digit Answer
b. second digit
c. multiplier
d. dielectric constant
16. The second dot represents the
a. first digit
b. second digit Answer
c. multiplier
d. dielectric constant
17. The third dot represenst the
a. first digit
b. second digit
c. multiplier Answer
d. dielectric constant
18. What do the dots represent on a capacitor using the six dot color code?
a. first dot mica indicator
b. second dot 1st digit
c. third dot 2nd digit
d. fourth dot multiplier
e. fifth dot tolerance
f. sixth dot temperature coefficient
19. Name seven common types of capacitors
a. __________________ air
b. __________________ mica
c. __________________ ceramic
d. __________________ paper
e. __________________ plastic
f. __________________ tantalum
g. __________________ electrolytic
20. What type of capacitor would you expect to find in a circuit operating at a frequency of 250 MHz?
air, mica, or ceramic
21. What type of capacitor would you expect to find being used as a filter capacitor in a power supply?
electrolytic or polarized tantlum
20. What type of capacitor would you expect to find in a general purpose low frequency circuit?
ceramic, paper, plastic