Ch.19 and 20

Electrical isolation

An advantage of using a transformer with a separate winding secondary is

Leads

For an inductor the induced voltage VL always BLANK the current by 90 degrees

Leads the current I by 90 degree

For an inductor, the induced voltage VL always

Mutual inductance

the ability of one coil to induce a voltage in another coil nearby is referred to as

False

The amount of inductive reactance equals two li times the frequency plus the inductance

1 uH

For the most part, inductor values below what value are not practical for use in electronic circuits

Leakage flux

For two coils with mutual inductance, any magnetic lines that do not link the two coils result in

Increases

How is mutual inductance affected when primary and secondary inductances are increased and coupling is made tighter

False

The amount of current in an ac circuit with only inductive reactance is equal to the applied voltage divided by the inductance

Apparent power

The amount of power the transformer can deliver to a resistive load is called

False

The combined parallel inductive reactance will be greater than the lowest branch reactance

True

The constant factor 2 pie indicates the circular motion from which a sine wave is derived

Infinity ohms

The dc resistance between the primary and secondary windings of a transformer should always measure

Coefficient of coupling

The fraction of total flux from one coil linking another coil is the

Inductance

The general use of BLANK is to provide minimum reactance for relatively low frequencies but more for higher frequencies

It opposes a change in current

The polarity of a coil's counter end is always such that

Opposes

The polarity of an inductor's counter end is always such that it BLANK a change in current

True

The rate of flux change is greatest when the alternating current passes through zero

Transformer

What component uses mutual inductance to step up or step down an alternating voltage

Variac

What is a common piece of test equipment used to increase the ac voltage slowly while monitoring the operation of the equipment being repaired

Eddy current

What is a current that flows in a circular path through the cross section of a transformer core that results in a loss

Impedance matching

What is it called when a transformer is used to achieve maximum transfer of power from a generator to a load when the generator and load impedances are not the same

Tight

A high value of coefficient coupling is called BLANK coupling

False

A steady dc current can induce a voltage in coil

False

An induced voltage across a coil is actually the result of inducing electrons to move in a conductor; however there is no induced current

False

In a series circuit containing two inductors and a voltage source, the current is less in the inductor with the higher inductance

Ohms

Inductive reactance is measured in

False

The base unit for inductance is the gilbert

Less than the lowest branch reactance

The combined parallel inductive reactance branch will be

Henry

The symbol for inductance is L, and the unit is the BLANK

Increases

With iron core coils the hysteresis and eddy-current losses BLANK with frequency

True

Although L can be determined when XL and f are known it's value is determined strictly by its physical construction

Wire

BLANK -wound resistors have enough inductance to be evident at ratio

Air core

BLANK coils are used in rf circuits because higher frequencies need less inductance for the required inductive effect

Inductance

BLANK is the ability of a conductor to produce voltage when the current varies

Inductive

BLANK reactance is equal to two time pi times frequency times inductance

True

Counter emf or back emf is also known as bucking voltage

1/T

Frequency is equal to

628 ohms

How much is the XL for a coil with negligible resistance that has 62.8 V across it with 0.1 A of current

True

If the number of turns of a coil is doubled in the same area and length , the inductance is increased by four times

True

In a parallel circuit with two inductors and voltage source, the voltage is the same across both inductors and is equal to the voltage source

False

Induced voltage is the result of flux cutting across an insulator

True

Inductance is the ability of a conductor to produce induced voltage when the current varies

False

Inductance is the characteristic that opposes any change in voltage

Ohms

Inductive reactance is measured in BLANK

Sine wave ac

Inductive reactance only applies to

Below

Inductor values BLANK 1 uH are generally not practical for use in electronic circuits

Mutual coupling

Mutual inductance is also called

True

One Henry is the amount of inductance that allows one volt to be induced when the current changes at the rate of one ampere per second

True

Since reactance is an opposition in ohms , the values XL in series or parallel are combined the same way as ohms of resistance

True

The higher the frequency of the alternating current and the greater the inductance the XL opposition will be increased

True

The inductive reactance is the ohms of opposition that an inductance has for sine-wave current

True

The inductive reactance of a coil is result of the inductor's counter end with a varying current

Open

The most common trouble in coils is an BLANK winding

Turns ratio

The ratio of the number of turns in the primary to the number in the secondary is the

Volt-amperes

The unit of apparent power is

Henry

The unit of mutual inductance is the

The same way as ohms of resistance

The values of XL in series or in parallel combined

False

There is inductive reactance for steady direct current

Direct

There is no inductive reactance for steady BLANK current

Laminated steel

To minimize transformer core losses while maintaining high flux density, the core can be made of

False

Too much current flowing through a coil will increase its permeability and inductance

True

Very often XL can be determined from voltage and current measurements

True

XL is measured in ohms

True

A coil had more inductance than the equivalent length of straight wire because the coil concentrates magnetic flux

Capacitance

A coil has some BLANK between windings

True

A typical air-core rf inductor is called a choke

True

Air-core for rf applications have induced values in mH and uH

True

The ability of a conductor to induce voltage in itself when current changes, is its self -inductance

Self inductance

The ability of a conductor to induce voltage in itself when the current changes is its BLANK

Equal to

The amount of current in an ac circuit with only inductive reactance is BLANK the applied voltage divided by XL

0 ohms

What is the inductive reactance for steady direct current

120 mH

What is the mutual inductance for two 400 mH could L1 and L2 that have a coefficient of coupling of 0.3

1 A

What is the primary current for a transformer with a 1:10 turns ratio that has 720 V across 7200 ohms in the secondary

Lenz

What law or theorem states that the reaction VL opposes its cause, which is the change in i?

Inductive reactance

What results from an inductor's counter end with a varying current

Alternating

When BLANK current flows in an inductance the amount of current is much less than the dc resistance alone would allow

Reflected impedance

When a transformer is used to change or transform a secondary load impedance to a new value as seen by the primary, the secondary impedance is called

The current is less than the dc resistance alone would allow

When alternating current flows in an inductance

True

When alternating current flows in an inductance, the amount of current is much less than the dc resistance alone would allow

Capacitance Inductor

When it is necessary to measure the value of an inductor, a BLANK analyzer

Higher than the primary

When the secondary of a transformer has more turns than the primary the secondary voltage is

Hysteresis loss

Which of the following is a transformer core loss

120 Hz

Which of the following is not a typical frequency rating for a power transformer

Provide electrical isolation

Which of the following is not an advantage of using an auto transformer

Block dc

Which of the following is not an example of how XL is generally used

Auto transformer

With an BLANK the secondary is not electrically isolated

Increase with frequency

With iron-core coils, the hysteresis and eddy-current losses

Is equal to the power in the primary

With zero losses assumed for the transformer the power in the secondary

6.28fL

XL equals