Free Printable EE 125 - MIDTERM EXAM

1.

Resonance in general, occurs when

a)

the applied frequency is a maximum

b)

the applied frequency is greater than the natural frequency

c)

the applied frequency is equal to the natural frequency

d)

the applied frequency is less than the natural frequency

2.

A circuit is in resonance when

a)

the impedance is an maximum

b)

the impedance is zero

c)

the reactance is maximum

d)

the current is maximum

3.

An LRC circuit has a "natural frequency" which is dependent on ...

a)

inductance and capacitance

b)

inductance resistance and capacitance

c)

resistance and capacitance

d)

inductance and resistance

4.

What is reactance?

a)

a measure of how much a capacitor or an inductor, increases a changing current

b)

a measure of how much a capacitor or an inductor, restricts a current

c)

a measure of how much a capacitor or an inductor, converts a changing current to heat

d)

a measure of how much a capacitor or an inductor, restricts a changing current

5.

What is the reactance of an inductor?

a)

1/wL

b)

L/w

c)

wL

d)

w/L

6.

What is reactance of a capacitor?

a)

wC

b)

w/C

c)

C/w

d)

1/wC

7.

What is the unit for capacitative reactance?

a)

Farad

b)

Ohm

c)

Henry

d)

Ohm Farad

8.

What is impedance?

a)

resistance

b)

the sum of resistance and total reactance

c)

the vector sum of resistance and total reactance

d)

the sum of capacitative and inductive reactance

9.

What is the difference between impedance and reactance

a)

reactance depends on frequency but impedance does not

b)

reactance involves resistance as well, but impedance does not

c)

impedance is the vector sum of resistance and reactance

d)

there is no difference

10.

What are the symbols for impedance, reactance and resistance?

a)

X, Z, R

b)

X, Y, R

c)

Z, X, R

d)

Z, R, X

11.

What is the formula for finding impedance?

a)

$\sqrt{\left(R^2\ -\ \left(wL\ -\ \frac{1}{wC}\right)^2\right)}$

b)

\sqrt{\left(R^2\ +\ \left(wL\ -\ \frac{1}{wC}\right)^2\right)}

c)

$\sqrt{\left(R^2\ +\ \left(wL\ +\ \frac{1}{wC}\right)^2\right)}$

d)

$\sqrt{\left(R^2\ -\ \left(wL\ +\ \frac{1}{wC}\right)^2\right)}$

12.

An LRC resonates when the applied frequency is equal to

a)

$\sqrt{LC}$

b)

$\frac{1}{\sqrt{LC}}$

c)

$\frac{2\pi}{\sqrt{LC}}$

d)

$\frac{1}{2\pi\sqrt{LC}}$

13.

Current in an LRC circuit is

a)

in phase with V_R

b)

in phase with Vs

c)

in phase with V_L

d)

in phase with Vc

14.

If the inductive reactance is smaller than the capacitive reactance, then the current

a)

lags the supply voltage

b)

leads the supply voltage

c)

is in phase with the supply voltage

d)

leads the resistor voltage

15.

if the inductive reactance is less than the capacitive reactance then the circuit will move into resonance if

a)

the inductance is decreased

b)

capacitance decreased

c)

capacitance is increased

d)

the frequency is decreased

16.

if the inductive reactance is less than the capacitative reactance, the circuit will move into resonance if

a)

the overlapping area of the plates increases

b)

the distance between the plates increases

c)

the overlapping area of the plates decreases

d)

the dielectric between the plates decreases

17.

The use of inductor?

a)

Passive component that store food in a fridge when electric current flows through it.

b)

Passive component that store energy in a magnetic field when electric current flow through it

c)

Passive component that store electric charge in an AC circuit.

d)

Passive component that limit the current flow.

18.

What is period

a)

Time required for a sine wave to complete one full cycle.

b)

It is equal to frequency.

c)

Unit = second

d)

T = 1/f

19.

In purely capacitor circuit

a)

Voltage lead current by 90 degree

b)

Voltage lag current by 90 degree

c)

Current lead voltage by 90 degree

d)

Current lead voltage by -90 degree

20.

if $v=V_{0_{\ }}Sin\ \left(314t+13^0\right)$ , the voltage in polar form will be

a)

$\frac{V}{\sqrt{2}}\angle13^0$

b)

$\frac{V_0}{\sqrt{2}}\angle13^0$

c)

$V_0\angle13^0$

d)

$v\angle13^0$

21.

For a single phase 25 Hz ac system, supply voltage is $100\angle25^0$ . the equivalent sinusoidal volatge is

a)

$v=141.4\sin\left(314t+25^0\right)$

b)

$v=100\sin\left(157t+25^0\right)$

c)

$v=141.4\sin\left(157t+25^0\right)$

d)

$v=100\sin\left(314t+25^0\right)$

22.

The power factor is defined as

a)

cosine of the phase difference between voltage and current

b)

ratio of resistance with impedance

c)

ratio of active power with apparent power

d)

All of above

23.

power factor is 0.95 lagging indicates

a)

Current lags to voltage by $\cos^{-1}\left(0.95\right)$

b)

Voltage lags to current by $\cos^{-1}\left(0.95\right)$

c)

any one may correct

d)

both are correct

24.

For a series R-L circuit having inductive reactance 3 ohm and resistance $\sqrt{3}$ . The power factor will be

a)

0.5

b)

1

c)

0

d)

$\sqrt{3}$

25.

1. Voltage in sinusoidal form is $v=230\sin\left(314t+46^0\right)$ . The value after converting into rectangular form is

a)

110.06+j115.41

b)

112.98+j116.99

c)

112.98-j116.99

d)

116.99+j112.98

26.

If 0.5 ohm resistance 10 mili Henry inductance and 1000 micro Farad are connected in series with 50 Hz single phase AC supply. The load power factor will be

a)

0.99

b)

0.999

c)

0.996

d)

0.991

27.

$v=40\angle60^0\$ voltage is connected across series R and L. A current $i=10\angle30^0$ is flowing in the circuit. Assuming 50 Hz single phase AC supply, the value of R and L will be

a)

3.464 ohm and 6.37 mH

b)

3.56 ohm and 9.37 mH

c)

3.464 ohm and 6 mH

d)

3.464 ohm and 16.37 mH

28.

if apparent power is 100 kVA and real power is 60 kW. the reactive power will be

a)

80 kVAR

b)

40 kVAR

c)

160 kVAR

d)

80 kW

29.

√(-300)

a)

10i√3

b)

15√15

c)

-10√3

d)

2i√13

30.

Write an equation for the cosine signal with amplitude 3, period 2pi, phase shift of pi, and vertical shift of 5.

a)

y = 5cos(t - pi) + 3

b)

y = 3cos(t - pi) + 5

c)

y = 3cos(2t - pi) + 5

d)

y = 5cos(2t - pi) + 3

31.

A sinusoidal voltage is expressed as $v=20\ \sin\ \left(314.16\ t\ +\frac{\pi}{3}\right)\ V.$ Its frequency and phase angle, respectively, are

a)

314.6 Hz, 60 $\degree$

b)

60 Hz, 60 $\degree$

c)

50Hz, 60 $\degree$

d)

50 Hz, -60 $\degree$

32.

In an AC circuit , if $V=100\angle60\degree\ \&\ I=10\angle30\degree$ . What will be the real power?

a)

86W

b)

8.6W

c)

866W

d)

866 KW

33.

In an AC circuit , if $V=100\angle60\degree\ \&\ I=10\angle30\degree$ . What will be the reactive power?

a)

500 VAR

b)

5 VARW

c)

50 VARW

d)

500 KVAR

34.

IN a RL circuit, V_R=2V and V_L=3V. The magnitude of the total voltage is

a)

5V

b)

6V

c)

1V

d)

3.6V

35.

Given $v=200\sin377t\ and\ i=8\sin\left(377t-30\degree\right)$ for an AC circuit. What will be the power factor and True Power of the circuit?

a)

0.866 leading and 692.82W

b)

0.866 lagging and 692.82W

c)

0.866 leading and 800 W

d)

0.866 lagging and 800 W

36.

Calculate the line voltage and Line current in this balanced Y-Y system:

a)

Eline = 1.38 KV

Iline = 0.5312

b)

Eline = 13.8 KV

Iline = 5.312 A

c)

Eline = 7.967 KV

Iline = 5.312 A

d)

Eline = 7.967 KV

Iline = 0.5312

37.

Calculate phase voltage and phase current in this balanced Y-Y system:

a)

Ephase(source) = 7.967 kV

Iphase(source) = 5.312 KA

b)

Ephase(source) = 7.967 V

Iphase(source) = 5.312 A

c)

Ephase(source) = 7.967 kV

Iphase(source) = 5.312 A

d)

Ephase(source) = 7.967 V

Iphase(source) = 5.312 A

38.

Calculate the total power consumed in this balanced Y-Y system:

a)

1.2696 kW

b)

126.96 W

c)

12.696 kW

d)

126.96 kW

39.

In a three-phase system, the voltages are separated by

a)

45⁰

b)

90⁰

c)

120⁰

d)

180⁰

40.

In a three-phase system, when the loads are perfectly balanced, the neutral current is

a)

zero

b)

one-third of maximum

c)

two-thirds of maximum

d)

at maximum

41.

In a -connected source driving a -connected load, the

a)

load voltage and line voltage are one-third the source voltage for a given phase

b)

load voltage and line voltage are two-thirds the source voltage for a given phase

c)

load voltage and line voltage cancel for a given phase

d)

load voltage, line voltage, and source phase voltage are all equal for a given phase

42.

Three ideal inductors are connected in star. The combination is supplied from a balanced three phase ac supply. The power consumed by the circuit will be

a)

infinite

b)

$\sqrt{3}\times VL\times IL\times\cos\theta$

c)

zero

d)

$3\times Vph\times Iph\times\cos\theta$

43.

Three ideal inductors are connected in star. The combination is supplied from a balanced three phase ac supply. The power factor of the circuit will be...

a)

1

b)

0.5

c)

0.866

d)

0

44.

Three practical inductors are connected in delta. The combination is supplied from a balanced three phase ac supply. The power factor of the circuit will be...

a)

1

b)

0

c)

$\infty>pf>0$

d)

$0<pf<1$

45.

Three pure resisrors are connected in delta. The combination is supplied from a balanced three phase ac supply. The power factor of the circuit will be...

a)

1

b)

0

c)

$\infty>pf>0$

d)

$0<pf<1$

46.

three impedances (10+j 8) ohms are connected in star across a 400 V 3 phase supply. Determine the line and phase current

a)

18 A and 18 A respectively

b)

10.39 A and 18 A respectively

c)

18 A and 10.39 A respectively

d)

31.17 A and 18 A respectively

47.

three impedances (10+j 8) ohms are connected in star across a 400 V 3 phase supply. Determine the total power consumed?

a)

3248.66 W

b)

9746 W

c)

5626.94 W

d)

16,880 W

48.

3 phase load consisting of three equal impedances connected in delta across a 400 V 3-phase ac supply. The line current drawn is 10A at 0.7 lagging. Determine the total reactive power.

a)

49.47 KVAR

b)

4.947 KVAR

c)

4.850 KVAR

d)

48.5 KVAR

49.

3 phase load consisting of three equal impedances connected in delta across a 400 V 3-phase ac supply. The line current drawn is 10A at 0.7 lagging. What would be the phase current if the loads were connected in star?

a)

10 A

b)

3.33 A

c)

5.77A

d)

17.32 A

50.

3 phase load consisting of three equal impedances connected in delta across a 400 V 3-phase ac supply. The line current drawn is 10A at 0.7 lagging. What would be the total active power if the loads were to be connected in star?

a)

48.5 KW

b)

4.85 KW

c)

16.17 KW

d)

1.617 KW

51.

The voltage VAB=50 $\angle$ 30 $\degree$ volts. The voltage VBA = ?

a)

$50\angle-180\degree$

b)

$50\angle-150\degree$

c)

$50\angle-30\degree$

d)

$50\angle-210\degree$

52.

A balanced three phase load consists of three coils, each of 4 ohm resistance and 0.02H inductance. Determine the total active power when the coils are connected in star. The supply is a 415 V, 50 Hz, 3-phase ac.

a)

12.41 W

b)

37.23 W

c)

37.23 KW

d)

12.41 KW

53.

A balanced three phase load consists of three coils, each of 4 ohm resistance and 0.02H inductance. Determine the total active power when the coils are connected in delta. The supply is a 415 V, 50 Hz, 3-phase ac.

a)

12.41 W

b)

37.23 W

c)

37.23 KW

d)

12.41 KW

54.

calculate the current flowing into each terminal and in each phase of the winding of a 3 phase delta connected motor developing an output of 250 hp at 2300 V between the terminals at a p.f. of 0.75 & an efficiency of 85 %

a)

42.4 A & 24.48 A

b)

42.4 A & 73.44 A

c)

73.44 A & 42.4 A

d)

24.48 A & 42.4 A

55.

Three similar coils are connected in delta across a three phase AC supply. The watt-meter readings are 12KW & 7KW . Calculate the power factor

a)

unity power factor

b)

zero power factor

c)

0.91 lagging

d)

0.91 leading

EE 125 - MIDTERM EXAM

Total questions: 55

Worksheet time: 29mins

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