Alternating Current (AC) MCQs Quiz | Class 10

Understanding Alternating Current (AC)

Alternating Current (AC) is a fundamental concept in electricity, essential for understanding how most power is generated and distributed. Unlike Direct Current (DC), which flows in only one direction, AC periodically reverses its direction and continuously changes its magnitude. This unique characteristic makes AC highly versatile for various applications, especially for long-distance power transmission.

Definition of Alternating Current

Alternating Current (AC) is an electric current that periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. The voltage in AC circuits also reverses along with the current. The most common waveform for AC is a sine wave, although other forms like triangular or square waves are also used. This periodic reversal allows AC to be efficiently transmitted over long distances and easily stepped up or down using transformers.

Key Characteristics of Alternating Current

• Periodic Reversal: The most defining characteristic is that AC changes its direction periodically. This means the current flows one way, then reverses to flow the opposite way, completing a cycle.
• Varying Magnitude: The magnitude of AC also changes continuously over time. For a sinusoidal AC, it starts from zero, increases to a maximum (peak) value, decreases to zero, then reverses direction, increases to a maximum in the opposite direction, and finally returns to zero.
• Frequency: This refers to the number of complete cycles AC completes in one second. It is measured in Hertz (Hz). For instance, in India, the standard frequency is 50 Hz, meaning the current reverses direction 100 times (50 cycles) every second. In many other countries (e.g., USA), it is 60 Hz.
• Waveform: Typically, AC has a sinusoidal (sine wave) waveform. This is because generators (alternators) naturally produce this smooth, periodic variation.
• Generation: AC is generated by alternators, which are devices that convert mechanical energy into electrical energy using the principle of electromagnetic induction. As a coil rotates in a magnetic field, the direction of the induced current periodically reverses.
• Transformation: A significant advantage of AC is that its voltage can be easily increased (stepped up) or decreased (stepped down) using transformers. This is crucial for efficient long-distance power transmission at high voltages and safe distribution at lower voltages.

AC vs. DC: A Quick Comparison

Quick Revision Points

• AC changes its direction and magnitude periodically.
• The standard AC frequency in India is 50 Hz.
• AC is generated by alternators based on electromagnetic induction.
• Transformers are used to step up or step down AC voltage.
• AC is preferred for long-distance power transmission due to reduced power loss at high voltages.
• The most common waveform of AC is sinusoidal.

Extra Practice Questions

1. What is the primary advantage of using AC over DC for long-distance power transmission?

   • It is easier to generate AC.
   • AC causes less heating effect.
   • AC voltage can be easily stepped up or down.
   • AC is safer to use.

   Answer: AC voltage can be easily stepped up or down.

2. In India, what is the standard frequency of Alternating Current (AC)?

   • 50 Hz
   • 60 Hz
   • 100 Hz
   • 20 Hz

   Answer: 50 Hz

3. Which device is commonly used to convert AC voltage to a higher or lower AC voltage?

   • Rectifier
   • Inverter
   • Transformer
   • Generator

   Answer: Transformer

4. The direction of current in an AC circuit:

   • Remains constant.
   • Periodically reverses.
   • Always flows from positive to negative.
   • Is determined by the load only.

   Answer: Periodically reverses.

5. What is the typical waveform of Alternating Current?

   • Square wave
   • Triangular wave
   • Rectangular wave
   • Sine wave

   Answer: Sine wave