Resistive Capacitive And Inductive Circuits

Question 1

In an A.C circuit, what happens if frequency is reduced?

Answer 1

Inductive elements may be damaged.

Lower frequency will decrease the reactance of the inductor which will increase its current, possibly damaging it.

Question 2

What is the relationship between the voltage and the current in an A.C circuit containing resistance and inductance?

Answer 2

Current lags voltage by up to 90°.

Remember ‘CIVIL’. But lag is not quite 90 degrees because of the resistance in the circuit.

Question 3

In this circuit.

Answer 3

Current leads voltage.

Remember ‘CIVIL’.

Question 4

Power factor relates to.

Answer 4

True power and apparent power.

Power factor = True Power / Apparent Power.

Question 5

If current lags voltage by 90°, the circuit is.

Answer 5

Inductive.

Remember ‘CIVIL’.

Question 6

What does the following circuit represent?

Answer 6

Differentiator.

An RC circuit with the output taken across the resistor is a differentiator.

Question 7

A low frequency supply.

Answer 7

May damage inductive components.

Lowering the frequency of the current through an inductive component will reduce its reactance and increase the current, possibly burning out the component.

Question 8

In a parallel R, L, C circuit, if the value of the capacitor is quadrupled, then the value of the impedance would.

Answer 8

Reduce.

Increasing the capacitor will decrease the capacitive reactance (and therefore the impedance).

Question 9

In a capacitive circuit, if the frequency is increased.

Answer 9

The current increases.

The current increases because the reactance decreases.

Question 10

In the circuit shown, what happens to the total circuit impedance if L is tripled?

Answer 10

Increases.

If the inductance of the inductor is tripled, then its reactance will triple which will increase the impedance of the circuit.

Question 11

In a tuned circuit at resonance, the circuit will be.

Answer 11

Resistive.

In a tuned circuit, the inductive reactance and the capacitive reactances cancel each other, leaving just pure resistance.

Question 12

A circuit has a resistance of 50 ohms and an inductance of 0.2 Henry. If it is connected to a 200 volt 50 Hz supply the reactance will be.

Answer 12

62.84 ohms.

Inductive reactance = 2πfL. 2 * 3.14 * 50 * 0.2 = 62.84 ohms.

Question 13

In a vector diagram showing the magnitude and direction of the inductance, capacitance and resistance in an AC circuit.

Answer 13

It is impossible to show values of this kind using vectors.

Question 14

At resonant frequency the phase difference between the voltage and current of an AC supply feeding an inductive and capacitive network is.

Answer 14

0 degrees.

Question 15

An increase in which of the following factors will cause an increase in the inductive reactance of a circuit?

Answer 15

Inductance and frequency.

Question 16

When a circuit with a series inductance and capacitance is at resonant frequency.

Answer 16

The impedance of the circuit is such that the voltage is in phase with the current.

Question 17

A pure capacitor, inductor and resistor are connected in series and the voltage drops across each are 10V and the total current flowing in the circuit is 1A. How much true power is the circuit consuming?

Answer 17

10W.

Get the formula for resonant frequency.

Question 18

If a capacitor 1µF and an inductor 10mH are connected in parallel what is their approximate resonant frequency?

Answer 18

1.6 kHz.

Get the formula for resonant frequency.

Question 19

Current in an inductive circuit will.

Answer 19

Lag voltage.

Remember CIVIL.

Question 20

The impedance of a series tuned circuit at resonance is.

Answer 20

Minimum.

Question 21

In an inductive resistive circuit, 1 amp flows from a supply of 100V dissipating 50 Watts of power. The phase angle is.

Answer 21

60°.

Power Factor = TP/AP = 50/100 = 0.5.

Question 22

In the circuit shown, impedance will be.

Answer 22

5 ohms.

Draw the vector diagram and find the resultant by Pythagoras.

Question 23

In the circuit shown, if the frequency is increased from zero to resonant frequency, the circuit current would.

Answer 23

Increase.

In a series resonant circuit, the impedance is a minimum.

Question 24

The true power developed in this circuit is.

Answer 24

12W.

True power is only in the resistor = I2 R = 2x2x3 = 12W.

Question 25

In a series LCR circuit at resonance, R = 100 ohms, L and C are each 10 ohms. The total impedance is.

Answer 25

100 ohms.

Resultant reactance (XL-XC) is 10-10 = 0 ohms.

Question 26

In a high Q parallel resonant circuit, what happens to impedance if capacitance is increased by a factor of 4?

Answer 26

It reduces by 75%.

Impedance cannot be any higher in a parallel circuit at resonance.

Question 27

A parallel circuit at resonance has its impedance at a.

Answer 27

Maximum.

Parallel circuit - impedance is maximum at resonance.

Question 28

The formula for impedance in a series circuit is the root of.

Answer 28

R2 + (XL-XC)2.

Question 29

In this circuit applied voltage is.

Answer 29

V = IXC.

V = I * XC (from Ohm’s Law).

Question 30

In a parallel LCR circuit, if the capacitance is increased by 4 times, what happens to the impedance of the capacitor?

Answer 30

Decreases by 75%.

The impedance of the capacitor alone would reduce to 1/4.

Question 31

If the frequency to a capacitor increases, the current through it.

Answer 31

Is zero.

No current goes ‘through’ a capacitor regardless of its frequency.

Question 32

If in an electrical circuit there is a difference between the capacitive reactance and the inductive reactance, what would the power factor of the circuit be?

Answer 32

Less than unity.

Power Factor is always less than one.

Question 33

In a Series LC circuit if the value of the capacitance is increased then the power factor will be.

Answer 33

Less than 1.

The power factor is always less than 1 unless the circuit is at resonance.

Question 34

What is the current in the circuit with a resistance of 8 ohms, inductive reactance of 12 ohms, capacitive reactance of 18 ohms?

Question 35

If the value of the capacitance is increased, what will the power factor be?

Answer 35

Less than 1.

The power factor is always less than 1 unless the circuit is at resonance.

Question 36

What is the current in a circuit with a resistance of 8 ohms, inductive reactance of 12 ohms, capacitive reactance of 18 ohms with 20 volts applied?

Answer 36

2 amps.

Calculate the total impedance Z = √(36 + 64) = 10 and then use formula V = Z * I to find current.

Question 37

In a circuit containing a resistor, capacitor, and inductor where resonant frequency is met, what will the impedance of the circuit be?

Answer 37

Resistance across the resistor.

The XL and XC cancel at resonance, leaving only the R.