Electromagnetic Oscillations and Alternating Current MCQ Quiz - Objective Question with Answer for Electromagnetic Oscillations and Alternating Current - Download Free PDF
Last updated on Jul 8, 2025
Latest Electromagnetic Oscillations and Alternating Current MCQ Objective Questions
Electromagnetic Oscillations and Alternating Current Question 1:
In the circuit shown below, R = 10 Ω, L = 5 H, E = 20 V, and the current i = 2 A is decreasing at a rate of -1.0 A/s. Find the voltage across the resistor (Vab) at this instant.
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 1 Detailed Solution
Calculation:
The potential difference (PD) across the inductor is given by:
VL = L × (di/dt)
Substitute the given values:
VL = 5 × (-1.0) = -5 V
The voltage across the resistor is:
VR = i × R = 2 × 10 = 20 V
The total voltage across the circuit is the sum of the voltage across the resistor and the inductor, and the applied voltage:
Vab = VE + VR + VL
Substituting the values:
Vab = 20 + 20 + (-5) = 35 V
Electromagnetic Oscillations and Alternating Current Question 2:
A series LCR circuit is connected to an AC source of 220 V, 50 Hz. The circuit contains a resistance R = 80 Ω, an inductor of inductive reactance XL = 70 Ω, and a capacitor of capacitive reactance XC = 130 Ω. The power factor of circuit is \(\rm \frac{x}{10}\). The value of x is :
Answer (Detailed Solution Below) 8
Electromagnetic Oscillations and Alternating Current Question 2 Detailed Solution
Calculation:
The formula for the cosine of the phase angle (φ) is:
cos(φ) = R / Z = R / √(R² + (XC - XL)²)
⇒ cos(φ) = 80 / √(80² + 60²)
⇒ cos(φ) = 80 / √(6400 + 3600)
⇒ cos(φ) = 8 / 10
Final Answer: x = 8 / 10
Electromagnetic Oscillations and Alternating Current Question 3:
An LCR series circuit of capacitance 62.5 nF and resistance of 50 Ω. is connected to an A.C. source of frequency 2.0 kHz. For maximum value of amplitude of current in circuit, the value of inductance is ______ mH.
(take π2 = 10)
Answer (Detailed Solution Below) 100
Electromagnetic Oscillations and Alternating Current Question 3 Detailed Solution
Calculation:
The amplitude will be maximum when there will be resonance condition
The value of resonance frequency is f = 1 / (2π√(L C))
⇒ 2000 Hz = 1 / (2π√(L × 62.5 × 10–9))
⇒ L = 1 / (4π2 × 20002 × 62.5 × 10–9) = 0.1 H = 100 mH
Electromagnetic Oscillations and Alternating Current Question 4:
In an LC oscillator, if values of inductance and capacitance become twice and eight times, respectively, then the resonant frequency of oscillator becomes x times its initial resonant frequency ω0. The value of x is:
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 4 Detailed Solution
Calculation:
The resonance frequency of an LC oscillation circuit is:
ω0 = 1 / √(LC)
L → 2L
C → 8C
ω = 1 / √(2L × 8C) = 1 / (4√(LC))
⇒ ω = ω0 / 4
So, x = 1 / 4
Electromagnetic Oscillations and Alternating Current Question 5:
A circuit with an electrical load having impedance 𝑍 is connected with an AC source as shown in the diagram. The source voltage varies in time as V(𝑡) = 300 sin(400𝑡) V, where 𝑡 is time in s. List-I shows various options for the load. The possible currents i(𝑡) in the circuit as a function of time are given in List-II.
Choose the option that describes the correct match between the entries in List-I to those in List-II.
List-I |
List-II |
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(P) |
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(1) |
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(Q) |
|
(2) |
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(R) |
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(3) |
|
(S) |
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(4) |
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|
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(5) |
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Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 5 Detailed Solution
Calculation:
Given Expressions for (P), (Q), (R), and (S):
(P): Given voltage: vz = 10 sin(400t) (volts)
Impedance: Z = R ⇒ Purely resistive
(Q): Z = jωL, ω = 400, L = x (inductive)
Voltage: V = 6 sin(400t - 53°) V
(R): Z = jωL + R = 50 + j10, ⇒ |Z| = √(502 + 102) = √2600
Voltage: V = 6 sin(400t + 53°) V
(Phase shift is positive, indicating a capacitive nature.)
(S): Z = jωL + R = 60 + j60, ⇒ |Z| = √(602 + 602) = 60√2
Voltage: V = 300 sin(400t) / 60√2 = 5 sin(400t)
Top Electromagnetic Oscillations and Alternating Current MCQ Objective Questions
In Alternating Current (AC), the direction and magnitude of the current varies
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 6 Detailed Solution
Download Solution PDFThe correct answer is Periodically.
Key Points
CONCEPT:
- The electric current flows in two ways: Alternating current and Direct Current.
- Direct current flows only in one direction.
- Alternating current: The electric current whose direction changes periodically is called electric current.
- Alternating current reverses its direction periodically.
- It also changes its magnitude periodically because of the induced electromagnetic force.
- For Alternating current both magnitude and direction change. The frequency of the alternating current in the Indian power supply is 50 Hz. The time period is 1/50 = 20 msec.
EXPLANATION:
- In Alternating Current (AC), the direction and magnitude of the current vary periodically. So option 2 is correct.
Step-up transformers are used for
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 7 Detailed Solution
Download Solution PDFCONCEPT:
Transformer:
- An electrical device that is used to transfer electrical energy from one electrical circuit to another is called a transformer.
There are two types of transformer:
1. Step-up transformer:
- The transformer which increases the potential is called a step-up transformer.
- The number of turns in the secondary coil is more than that in the primary coil.
2.Step-down transformer:
- The transformer which decreases the potential is called a step-down transformer.
- The number of turns in the secondary coil is less than that in the primary coil.
EXPLANATION:
- As in the step-up transformer, the number of coils in the secondary coil is more than that in the primary coil. So the electrical potential increases.
- Hence step-up transformer is used for increasing the voltage/potential. So option 4 is correct.
- In step-up transformer, the current in the secondary coil is less than that in the primary coil.
The transformer core is laminated in order to
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 8 Detailed Solution
Download Solution PDFThe correct answer is Minimize eddy current loss.
- Transformer cores are laminated in order to minimize core loss.
- By providing laminations, the area of each part gets reduced and hence resistance will get very high which limits the eddy current to a minimum value, and hence eddy current losses get reduced.
- The laminations provide small gaps between the plates. As it is easier for magnetic flux to flow through iron than air or coil, the stray flux or leakage flux that can cause core losses is minimized.
The resonant frequency of a RLC circuit is equal to __________.
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 9 Detailed Solution
Download Solution PDFCONCEPT:
- The ac circuit containing the capacitor, resistor, and the inductor is called an LCR circuit.
- For a series LCR circuit, the total potential difference of the circuit is given by:
\(V = \sqrt {{V_R^2} + {{\left( {{V_L} - {V_C}} \right)}^2}} \)
Where VR = potential difference across R, VL = potential difference across L and VC = potential difference across C
- For a series LCR circuit, Impedance (Z) of the circuit is given by:
\(\)\(Z = \sqrt {{R^2} + {{\left( {{X_L} - {X_C}} \right)}^2}} \)
Where R = resistance, XL =induvtive reactance and XC = capacitive reactive
CALCULATION:
- For a series LCR circuit, Impedance (Z) of the circuit is given by:
\(\)\(\Rightarrow Z = \sqrt {{R^2} + {{\left( {{X_L} - {X_C}} \right)}^2}} \)
- Inductive reactance,
⇒ XL = Lω
- Capacitive reactance
\(\Rightarrow X_c=\frac{1}{C\omega}\)
- Resonance will take place when XL = XC.
⇒ XL = XC
\(\Rightarrow L\omega =\frac{1}{C\omega}\)
\(\Rightarrow \omega =\frac{1}{\sqrt{LC}}\)
A step-up transformer converts ________ into a ________.
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 10 Detailed Solution
Download Solution PDFCONCEPT:
Transformer:
An electrical device that is used to transfer electrical energy from one electrical circuit to another is called a transformer.
There are two types of transformer:
Step-up transformer:
- The transformer which increases the potential is called a step-up transformer.
- The number of turns in the secondary coil is more than that in the primary coil.
Step-down transformer:
- The transformer which decreases the potential is called a step-down transformer.
- The number of turns in the secondary coil is less than that in the primary coil.
EXPLANATION:
- As in the step-up transformer, the number of coils in the secondary coil is more than that in the primary coil. So the electrical potential increases.
- A step-up transformer converts low voltage at high current into a high voltage at low current. So option 4 is correct.
- In a step-up transformer, the current in the secondary coil is less than that in the primary coil.
- The secondary coil is made of thin insulated wire while the primary coil is made of thick insulated wire.
The peak value of an A.C. is 2√2 A, its rms value will be:
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 11 Detailed Solution
Download Solution PDFCONCEPT:
- Root mean square value of Alternating current (Irms): The value of steady current, which would generate the same amount of heat in a given resistance in a given time, as is done by the alternating current. when passed through the same resistance for the same time.
- The r.m.s. value is also called effective value or virtual value of alternating current.
The relation between the peak value of the alternating current value of current (Io) and r.m.s. value of the current is given as:
\({I_{rms}} = \frac{{{I_o}}}{{√ 2 }}\)
CALCULATION:
Given that:
Peak current (I0) = 2√2 A
\({I_{rms}} = \frac{{{I_o}}}{{√ 2 }}\)
\({I_{rms}} = \frac{{{2\sqrt2}}}{{√ 2 }} = 2 A\)
The rms value is 2 A.
An ac voltage v = vm sinωt applied to a capacitor drives a current in the capacitor, i = ______________.
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 12 Detailed Solution
Download Solution PDFConcept:
The power factor of the inductor has lagging nature, whereas for capacitor its leading.
For a pure inductor \(\phi = {90^0}\) (The angle between voltage and current)
This shows that if we plot voltage vs current graph the output signal will be lagging by phase angle π/2
The given figure shows the power factor (ϕ) for pure R, L and C circuit
Explanation:
From the above explanation, we can see that for a pure capacitance circuit if AC voltage of v = vm sinωt then the driven current in the capacitor will be
I = im sin(ωt + ϕ)
Now as mentioned for pure inductor phase angle will be lagging by 90° or π/2.
i.e., ϕ = π/2 ⇒ i = im sin (ωt + π/2)
In a step-up transformer, the value of current in the secondary coil in comparison to primary coil is
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 13 Detailed Solution
Download Solution PDFConcept:
- A transformer is an electrical device which transfers electrical energy from one circuit to another
- It works based on the principle of electromagnetic induction
- It is used for increasing or decreasing the amount of voltage or current as per our requirement based on these transformers are classified into two types step-up (increasing) or step-down (decreasing) transformer.
There are two types of transformer:
1. Step-up transformer:
- The transformer which increases the potential is called a step-up transformer.
- The number of turns in the secondary coil is more than that in the primary coil.
2.Step-down transformer:
- The transformer which decreases the potential is called a step-down transformer.
- The number of turns in the secondary coil is less than that in the primary coil.
Explanation:
As in an ideal transformer, there is no loss of power i.e.
Pout = Pin
So, VsIs = VpIp
\(\therefore \frac{{{V_s}}}{{{V_p}}} = \frac{{{N_s}}}{{{N_P}}} = \frac{{{I_P}}}{{{I_S}}}\)
Where Ns = number of turns of the secondary coil, NP = number of turns of the primary coil, Vs = Voltage secondary coil, VP = Voltage primary coil, Is = Current in the secondary coil and IP = current in the primary coil.
In Step up transformer voltage is increased So, the current will be reduced. As there is no loss in power.
Additional Information
- A transformer can be used to either increase or decrease the voltage of a circuit.
- In other words, it can either step up (increase) or step down (decrease) the voltage.
- A transformer is necessary because sometimes the voltage requirements of different appliances are variable.
The current in the inductor __________________.
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 14 Detailed Solution
Download Solution PDFCONCEPT:
- Inductors: The coils of wire that are wound around any ferromagnetic material (iron cored) or wound around a hollow tube that increase their inductive value are called inductors.
- The inductance (L) is measured in Henry (H) and the instantaneous voltage in volts.
- Rate of instantaneous voltage is given by (v = L di/dt)
EXPLANATION:
- The given diagram is a simple inductor circuit with alternating current.
- From the phaser diagram, the Inductor current lags inductor voltage by 90° = π/2.
- The plot of current and voltage for this very simple circuit:
- From the current and voltage wave diagram, Inductor current lags inductor voltage by 90°. So option 1 is correct.
- Inductor current lags inductor voltage by 90°.
- Capacitor voltage lags current by 90°.
- In Resister only circuit, Voltage and Current are in the same phase.
- Or we can say there is no lag between current and voltage.
Value of current in an A.C. circuit is I = 2cos(ωt+θ). The value of Irms is:
Answer (Detailed Solution Below)
Electromagnetic Oscillations and Alternating Current Question 15 Detailed Solution
Download Solution PDFCONCEPT:
Alternating current:
- An alternating current can be defined as a current that changes its magnitude and polarity at a regular interval of time.
RMS value of current:
- RMS value of current is defined as the square root of the mean of the square of AC current for 1 complete cycle.
- It is equal to that value of DC current which generates the same heating effect as the AC current.
\(⇒ I_{rms}= \frac{I_{o}}{\sqrt{2}}\) -----(1)
Where Io = peak value of AC current
CALCULATION:
Given - I = 2cos(ωt+θ)
- The value of I will be maximum when cos(ωt+θ) is maximum,
- The maximum value of cos(ωt+θ) = 1
Therefore,
⇒ Io = 2A
So,
\(⇒ I_{rms}= \frac{I_{o}}{\sqrt{2}}=\frac{2}{\sqrt{2}}=\sqrt{2}A\)
- Hence, option 1 is correct.