intro
MCQs 1:
What is the peak value of an AC mains voltage of 220V?
Topic: Peak Value in AC Voltage
Correct Answer: C) 311.0 V
- The peak value of an AC voltage can be calculated using the Root Mean Square (RMS) value, which is given as 220V in the question.
- To find the peak value, multiply the RMS value by the square root of 2.
- Therefore, 220V * √2 gives the peak value, which equals 311V.
- Hence, the correct answer is C, "311.0 V".
MCQs 2:
What is the power dissipated when a sinusoidal AC current passes through a resistor with resistance R and the peak current is Ip?
Topic: Power Dissipation in AC Circuit
Correct Answer: B)
- Power dissipated in an AC circuit with resistance R and peak current Ip can be calculated using the formula for power dissipation in resistor, P = I²R.
- Keep in mind the RMS value of current is used in this formula, so the peak current must be converted to RMS.
- Hence, the correct answer is B.
MCQs 3:
Observe the variation of instantaneous current (I) and instantaneous EMF (ϵ) in a given circuit. Which statement below is correct?
Topic: Phase Relationship between Voltage and Current in AC Circuit
Correct Answer: B) The voltage leads the current by π/2.
- When analyzing the phase relationship in an AC circuit, observe the initial conditions at t=0.
- In this case, the phase of the voltage is zero while the phase of the current is (-π/2), meaning the voltage leads by π/2.
- Hence, the correct answer is B, "The voltage leads the current by π/2".
Question 4:
A metal wire is heated using an alternating current with a peak value of 14A. To achieve the same heating effect, what should be the value of a constant current 'i'?
Topic: Heating Effect of AC and DC Currents
Correct Answer: D) About 10A
- The heating effect of AC and DC currents can be compared using their RMS values.
- In the given scenario, the RMS value of the AC current equals the DC current required to produce the same heating effect.
- Hence, the correct answer is D, "About 10A".
Question 5:
What is the resonance frequency in an R-L-C series circuit?
Topic: Resonance Frequency in R-L-C Series Circuit
Correct Answer: D) 1/2π√LC
- Resonance frequency in a series R-L-C circuit can be calculated using the formula 1/2π√LC.
- The resonance frequency is the frequency at which the impedance of the circuit is minimum and the circuit can respond at its maximum.
- Hence, the correct answer is D, "1/2π√LC".
Question 6:
What does an ammeter measure in an AC circuit?
Topic: Measuring Current in AC Circuit
Correct Answer: B) RMS value of current
- In an AC circuit, an ammeter is used to measure the RMS (Root Mean Square) value of the current.
- The RMS value provides a meaningful measurement of the magnitude of the alternating current.
- Hence, the correct answer is B, "RMS value of current".
Question 7:
A bulb is first connected to a DC, and then to an AC of the same voltage. When will the bulb shine the brightest?
Topic: Comparison of Brightness in AC and DC Currents
Correct Answer: D) Equally with both
- The brightness of a bulb is directly proportional to the power consumed, which is in turn proportional to the reciprocal of the resistance (1/R for the bulb).
- With the same applied voltage, the brightness will be the same for both AC and DC as the resistance remains unchanged (RAC = RDC).
- Therefore, the correct answer is D, "Equally with both".
Question 8:
What is the RMS voltage of the depicted waveform?
Topic: RMS Voltage in AC Circuit
Correct Answer: A) 10 V
- The RMS or root mean square voltage is a useful measure of the "effectiveness" of an AC voltage in driving current through a load resistor.
- For a constant waveform, such as in this case, the RMS value remains the same as the constant value.
- Therefore, the correct answer is A, "10 V".
Question 9:
How does the voltage of an AC source change over time according to the equation V = 50 sin(200πt), where t is in seconds and V is in volts?
Topic: Changing Voltage in AC Circuit
Correct Answer: B) The peak voltage of the source is 50 V
- In an AC circuit, the voltage changes over time according to a sine function V = V₀ sin(ωt), where ω = 2πf.
- By comparing V = 50 sin(200πt) with the above, we get V₀ = 50V and frequency f = 100Hz.
- Therefore, the correct answer is B, "The peak voltage of the source is 50 V".
Question 10:
What is the power dissipation in a pure inductor?
Topic: Power Dissipation in Pure Inductor
Correct Answer: A) Zero
- Power dissipation in a circuit is the result of resistance to current flow, where more resistance leads to higher energy loss, calculated by P = IVcosθ.
- For an inductor, the voltage leads the current by 90°, hence P = IVcos90°, and since cos90° is zero, the power dissipation is also zero.
- Hence, the correct answer is A, "Zero".
Question 12:
What is the value of a constant current that produces heat at the same average rate in a resistor as an alternating current does?
Topic: Heat Generation in AC Circuit
Correct Answer: C) The root mean square (r.m.s) current
- The heat generated in a resistor by a current is proportional to the square of the current (I^2R).
- The r.m.s value of current (Irms) is the effective value of an AC current, which produces the same heat as a DC current in the same resistor.
- Hence, the correct answer is C, "The root mean square (r.m.s) current".
MCQs 13:
Which of the following is not a factor on which the resistance of an Alternating Current (AC) circuit depends?
Topic: Resistance in AC Circuit
Correct Answer: A) Frequency
- The resistance in an AC circuit is a factor of the material, length, and cross-sectional area of the conductor.
- It is not dependent on the frequency, so changing the frequency will not alter the resistance of the circuit.
- Therefore, the correct answer is A, "Frequency".
MCQs 14:
What does the 220V rating on household AC appliances signify?
Topic: Voltage Rating on AC Appliances
Correct Answer: D) Root mean square voltage
- The voltage rating on an AC appliance relates to the effective voltage, which is the root mean square value.
- The root mean square value is the peak value divided by the square root of 2.
- Hence, the correct answer is D, "Root mean square voltage".
MCQs 15:
In which type of circuit is a choke coil typically utilized as a resistance?
Topic: Use of Choke Coil in Circuits
Correct Answer: B) Alternating Current (AC) circuits
- A choke coil, or choke, is an inductor used to block higher-frequency AC while passing lower-frequency or DC.
- It is typically used as a resistance in AC circuits.
- Hence, the correct answer is B, "Alternating Current (AC) circuits".
MCQs 16:
How is the maximum voltage or current that an alternating current achieves in one cycle referred to?
Topic: Characteristics of Alternating Current
Correct Answer: A) Peak value
- In an alternating current (AC), the voltage or current varies with time.
- The maximum or highest value attained during one cycle is referred to as the Peak Value.
- Hence, the correct answer is A, "Peak value".
MCQs 17:
How does the current behave in relation to the voltage in a capacitor circuit?
Topic: Current and Voltage Relationship in Capacitor Circuit
Correct Answer: B) The current precedes the voltage by π/2.
- Within a capacitor circuit, the relationship between current and voltage isn't simultaneous; in fact, one precedes the other.
- In this case, the current is ahead of the voltage by π/2.
- Therefore, the answer is B, "The current precedes the voltage by π/2".
MCQs 18:
Can the current flow in the direction indicated in the provided figure?
Topic: Current Flow in a Circuit
Correct Answer: A) Definitely.
- Current direction can be dictated by the type of source in the circuit, and the phase difference between currents in different branches.
- If the source in the depicted circuit is AC, and there's a phase difference of π between the 15A and 5A currents, then the current can flow in the indicated direction.
- Thus, the correct answer is A, "Definitely".
Question 19:
What is the frequency of the alternating current in Pakistan?
Topic: Frequency of Alternating Current in Pakistan
Correct Answer: A) 50 Hz
- The frequency of an alternating current is the rate at which it changes direction, measured in hertz (Hz).
- In Pakistan, the standard frequency for household current is 50 Hz, meaning the current changes direction 50 times per second.
- Thus, the correct answer is A, "50 Hz."
Question 20:
How is the root mean square value of alternating voltage represented?
Topic: Root Mean Square Value of Alternating Voltage
Correct Answer: A) 0.707 V0
- The root mean square (RMS) value of any waveform, including alternating voltage, provides a measure of the magnitude of the waveform.
- It is calculated as the square root of the average of the squares of all the instantaneous values during one cycle.
- For a sine wave, such as alternating voltage, the RMS value is 0.707 times the peak value (V0).
- Hence, the correct answer is A, "0.707 V0."