Understanding AC Current Concepts

In an AC circuit, what is the term used to describe the opposition to the flow of alternating current due to inductance and capacitance?

• Resistance

• Reactance

• Impedance

• Conductance

Answer explanation

In an AC circuit, the term 'impedance' refers to the total opposition to the flow of alternating current, which includes both resistance and reactance (from inductance and capacitance). Thus, the correct answer is impedance.

What is the formula for calculating the average power in an AC circuit?

Answer explanation

The average power in an AC circuit is calculated using the formula P = VI\cos\phi, where V is the voltage, I is the current, and \phi is the phase angle. This accounts for the power factor, making it the correct choice.

In an AC circuit, if the voltage leads the current by 90 degrees, what type of component is likely present?

• Resistor

• Inductor

• Capacitor

• Transformer

Answer explanation

In an AC circuit, if the voltage leads the current by 90 degrees, it indicates the presence of an inductor. Inductors cause the current to lag behind the voltage, resulting in this phase difference.

What is the peak voltage of an AC supply if the RMS voltage is 230V?

• 230V

• 325V

• 115V

• 460V

Answer explanation

The peak voltage (V_peak) of an AC supply is calculated using the formula V_peak = V_RMS × √2. For an RMS voltage of 230V, V_peak = 230V × 1.414 ≈ 325V. Thus, the correct answer is 325V.

Which device is used to change the voltage level in an AC system?

• Rectifier

• Transformer

• Transistor

• Diode

Answer explanation

A transformer is used to change the voltage level in an AC system by stepping it up or down, making it the correct choice. Rectifiers convert AC to DC, while transistors and diodes serve different functions.

What is the frequency of the standard AC supply in Great Britain?

• 50 Hz

• 60 Hz

• 100 Hz

• 120 Hz

Answer explanation

The standard AC supply frequency in Great Britain is 50 Hz. This is the frequency used for electrical power distribution, distinguishing it from other regions like the USA, which uses 60 Hz.

• 6000 m

• 3000 m

• 1500 m

• 750 m

Answer explanation

Wavelength (λ) is calculated using the formula λ = speed / frequency. Here, λ = (3 × 10^8 m/s) / (50 Hz) = 6000000 m / 50 = 3000 m. Thus, the correct answer is 3000 m.

In an AC circuit, what is the phase difference between voltage and current in a purely capacitive circuit?

• 0 degrees

• 45 degrees

• 90 degrees

• 180 degrees

Answer explanation

In a purely capacitive circuit, the current leads the voltage by 90 degrees. This means that when the voltage reaches its maximum, the current is already at its maximum, confirming the phase difference of 90 degrees.

What is the unit of reactive power in an AC circuit?

• Watt

• Volt-Ampere

• Volt-Ampere Reactive

• Joule

Answer explanation

The unit of reactive power in an AC circuit is Volt-Ampere Reactive (VAR). Unlike real power measured in Watts, reactive power represents the power that oscillates between the source and reactive components.

Which of the following is true for a transformer in an AC system?

• It can increase or decrease DC voltage.

• It can only increase AC voltage.

• It can only decrease AC voltage.

• It can increase or decrease AC voltage.

Answer explanation

A transformer can change AC voltage levels by increasing (step-up) or decreasing (step-down) the voltage, making the correct choice "It can increase or decrease AC voltage." The other options incorrectly limit its function.