AC Systems Lesson 3

In a direct current (DC) circuit, the current flows in ? direction, but in an alternating

current (AC) circuit, current flows in ? direction(s).

a.

a negative / a positive

b.

a positive / a negative

c.

a single / both

d.

the forward / the reverse

An AC waveform will cross the zero axis, but a DC waveform will not cross it.

Select one:

True

False

The most common frequency in the United States is ? Hz, which means the voltage

waveform completes ? cycles in ? second(s).

a.

50 / 50 / 1

b.

50 / 50 / 50

c.

60 / 60 / 1

d.

60 / 60 / 60

When comparing AC to DC, it is easier to transform an AC voltage.

Select one:

True

False

Why is AC used in power distribution systems more often than DC?

I. AC can be easily transformed to raise or lower levels of voltage and current for a given

amount of power.

II. AC is easier to produce than DC.

III. AC is safer than DC.

IV. AC transformers allow very high voltage values to deliver large amounts of power at

low current levels.

a.

I. and II.

b.

I. and III.

c.

I. and IV.

d.

II. and IV.

In order to lower power loss by reducing the current, one must raise the ? by the same

proportion as one lowers the current to deliver the same amount of power to the user.

This is expressed mathematically as P = ? .

a.

current / I/E

b.

power / I x R

c.

resistance / E/I

d.

voltage / E x I

When distributed by transmission lines, power loss is due to the ? of the wire that

generates heat in the conductor. The heat is generated as a result of ? flowing through

the conductor.

a.

diameter / power

b.

diameter / voltage

c.

length / power

d.

resistance / current

The power loss in a conductor can be expressed mathematically as ? .

a. P = E x I

b. P = E^2/R

c. P = I^2 x R

d. P = I^2/R

The power loss of a transmission line can be reduced by either reducing the ? of the

wire or the amount of ? .

a.

diameter / current

b.

length / voltage

c.

resistance / current

d.

resistance / voltage

Reducing the resistance of the wire by one-half will reduce power loss by ? , but

reducing the current by one-half will reduce the power loss by ? .

a.

25% / 50%

b.

50% / 25%

c.

50% / 50%

d.

50% / 75%

AC is converted from one level of voltage for transmission and then reduced to a

different level of voltage for utilization by the end-user by using ? .

a.

converters

b.

inverters

c.

rectifiers

d.

transformers

What would be the ratio of power loss of a transmission line carrying 1,000 amperes at

1,000 volts as compared to 10 amperes at 100 kilovolts? Assume a resistance of 1 ohm

on the transmission line.

a.

100 : 1

b.

1,000 : 1

c.

10,000 : 1

d.

100,000 : 1

When the current and the voltage sine waves are ? , they both have the same

frequency, crossing the reference line at the same time and going in the same direction.

a.

in-phase

b.

out-of-phase

c.

single-phase

d.

three-phase

Alternating current is used to supply over 90% of the world's electrical power

requirements.

Select one:

True

False

The variable of time in the production of electrical current has a major influence on the

effects of AC current.

Select one:

True

False

The rules of Ohm's Law, as learned in DC Theory, do not apply to AC circuits.

Select one:

True

False

The tendency of AC current to flow in the surface area of a conductor and not evenly

throughout the conductor is referred to as ? .

a.

AC effect

b.

frequency effect

c.

sine effect

d.

skin effect

Skin effect is caused by induced ? from the AC current flow, which cause the electrons

to be repelled toward the outer surface of the conductor.

a.

circular currents

b.

eddy currents

c.

negative currents

d.

positive currents

Skin effect increases the effective resistance of the conductor.

Select one:

True

False

Skin effect is smaller at high frequencies.

Select one:

True

False

Each time the generated voltage within a generator alternator reverses polarity, the

current in an external circuit connected through slip rings to the generator alternator

? .

a.

decreases in strength

b.

increases in strength

c.

reverses direction

d.

stays the same

In an AC generator, what changes to cause polarity changes?

a.

The armature

b.

The direction at which the conductor cuts the flux field changes as it rotates.

c.

The left hand rule

d.

The slip rings

As a generator armature makes one full revolution, it goes through ? of mechanical

rotation.

a.

180°

b.

270°

c.

360°

d.

450°

The AC voltage waveform is in the positive direction for some period of time and in the

negative direction for some period of time. What is the name given to this waveform?

a.

alternating wave

b.

inverted wave

c.

hertz wave

d.

sine wave

The plotted resulting waveform as the conductor rotates in a magnetic field follows ? .

a.

the angle of rotation

b.

the cosine of the angle of rotation

c.

the sine of the angle of rotation

d.

the tangent of the angle of rotation

At what degrees on the sine wave is the voltage equal to zero?

a.

0°, 90°, 270°

b.

0°, 90°, 360°

c.

0°, 120°, 360°

d.

0°, 180°, 360°

When is the voltage on an AC sine wave at the maximum positive and maximum

negative value?

a.

60°, 120°

b.

90°, 270°

c.

90°, 180°

d.

180°, 360°

When a conductor cuts the magnetic flux lines at ? , the maximum voltage will be

induced into the conductor.

a.

60°

b.

90°

c.

180°

d.

360°

Sine waves are so named because the voltage at any point along the wave form is equal

to the maximum, or peak, value times the ? of the angle of rotation.

a.

cosine

b.

degree

c.

sine

d.

tangent

A(n) ? occurs each time the AC generator finishes one complete rotation (360° travel).

a.

alternation

b.

cycle

c.

frequency

d.

hertz

The measure of the number of cycles per second of the voltage produced by an AC

generator is called the ? of that voltage and is measured in ? .

a.

alternations / rpm

b.

cycles / frequency

c.

frequency / hertz

d.

period / cycles

What is the frequency of a waveform that has gone through 60 positive and negative

alternations in one second?

a.

60 cycles

b.

60 hertz

c.

120 cycles

d.

120 hertz

The time required for one cycle is called the ? .

a.

alternation

b.

frequency

c.

hertz

d.

period

What is the period for a frequency of 60 Hz? (Round the FINAL answer to four decimal

places of the base unit.)

Answer:

What is the period for a frequency of 1MHz?

Answer:

What is the period for a frequency of 100 Hz? (Round the FINAL answer to two decimal

places.)

Answer:

A sine wave has a maximum voltage (peak) of 144 V. What is the voltage at 45° of

rotation? (Round the FINAL answer to one decimal place.)

Answer:

Peak-to-peak, Peak, RMS, and Average are four forms of measurement of voltage and

current associated with sine waves.

Select one:

True

False

A measurement taken from zero to the highest value obtained in either the positive or

negative half of a sine wave would determine ? .

a.

the average value

b.

the peak-to-peak value

c.

the peak value

d.

the RMS value

The peak-to-peak value is equal to ? time(s) the peak value.

a.

0.707

b.

1.41

c.

2

d.

4

The current waveform shown is an example of a(n) ? .

a.

a 2-amp negative AC current

b.

a 2-amp negative DC current

c.

a 2-amp positive AC current

d.

a 2-amp positive DC current

Another name for the effective value is the ? .

a.

the average value

b.

the peak-to-peak value

c.

the peak value

d.

the RMS value

The ? is the value of AC voltage that produces the same amount of power as a like

value of DC voltage.

a.

the average value

b.

the peak-to-peak value

c.

the peak value

d.

the RMS value

The peak voltage is measured and found to be 170 volts. What is the effective voltage?

(Round the FINAL answer to the nearest whole number.)

Answer:

The effective voltage is 480 volts. What is the peak voltage? (Round the FINAL answer to

the nearest whole number.)

Answer:

The waveforms will have the same heating effect on a resistor.

Select one:

True

False

What is the peak voltage value of an AC power source with an effective value of 50

volts? (Round the FINAL answer to the nearest whole number.)

Answer:

VAC

What is the peak-to-peak value of the AC voltage wave?

Answer:

If the AC voltage input to a full-wave rectifier is 100 volts peak to peak, what is the

average DC voltage value? (Round the FINAL answer to the nearest whole number.)

Answer:

When referring to values of AC voltages or currents, the letters RMS stand for Root Mean

Square.

Select one:

True

False

Give the RMS value for 120 VAC effective.

Answer:

Give the peak value for 120 VAC effective. (Round the FINAL answer to the nearest whole

number.)

Answer:

VAC

Give the average value for 120 VAC effective. (Round the FINAL answer to the nearest

whole number.)

Answer:

Give the peak-to-peak value for 120 VAC effective. (Round the FINAL answer to the

nearest whole number.)

Answer:

The average DC voltage value of a full-wave rectified AC sine wave can be found by

multiplying the peak value by 0.637 or by using the formula .

Select one:

True

False

Power in a circuit is the amount of work done per unit of ? .

a.

current

b.

time

c.

voltage

d.

wattage