Frequency of AC MCQs Quiz | Class 10

Understanding Frequency of Alternating Current (AC)

Alternating Current (AC) is a type of electric current in which the direction of the flow of electrons reverses periodically. Unlike Direct Current (DC), which flows in only one direction, AC constantly changes its direction. The concept of frequency is central to understanding AC power systems.

Meaning of AC Frequency

Frequency of AC refers to the number of complete cycles that the current or voltage completes in one second. A ‘cycle’ for AC means the current starting from zero, increasing to a maximum in one direction, decreasing to zero, then increasing to a maximum in the opposite direction, and finally returning to zero. This entire process constitutes one complete cycle.

In simpler terms, it tells us how many times the current reverses its direction back and forth in a single second. For instance, if the frequency is 50 Hz, it means the current completes 50 such cycles, and its direction changes 100 times (twice per cycle) in one second.

Unit of Frequency

The standard international (SI) unit for frequency is the Hertz (Hz). One Hertz is defined as one cycle per second.

• 1 Hz = 1 cycle/second

Common frequencies used globally are 50 Hz and 60 Hz. India and most of Europe use 50 Hz, while North America (USA, Canada) and parts of Japan use 60 Hz.

Interpretation of AC Frequency

The frequency of AC has significant implications for electrical systems and devices:

• Appliance Design: Electrical appliances like motors, transformers, and lighting systems are designed to operate at a specific frequency. Using an appliance designed for 60 Hz on a 50 Hz supply (or vice versa) can lead to inefficient operation, overheating, or even damage. For example, a 50 Hz motor running on 60 Hz supply might run faster but draw less current, while a 60 Hz motor on 50 Hz might run slower and draw more current.
• Flicker in Lights: At very low frequencies, the periodic dimming and brightening of incandescent lights can be perceived as flicker. For example, 25 Hz AC light sources show noticeable flicker, which is why higher frequencies like 50 Hz or 60 Hz are chosen for household lighting.
• Time Period: Frequency (f) is inversely related to the time period (T), which is the time taken to complete one cycle. The relationship is given by the formula:

T = 1 / f

For a 50 Hz supply, the time period is 1/50 = 0.02 seconds. For a 60 Hz supply, it is 1/60 = 0.0167 seconds.

• Inductive Reactance: In AC circuits, components like inductors have a property called inductive reactance, which depends on frequency. Higher frequencies lead to higher inductive reactance, affecting current flow in inductive circuits.

Quick Revision Points

• Frequency is the number of complete cycles of AC per second.
• The SI unit of frequency is Hertz (Hz).
• In India, the standard AC frequency is 50 Hz.
• Frequency is inversely proportional to the time period (T = 1/f).
• The frequency affects the operation of electrical appliances and the design of power systems.

Extra Practice Questions

1. What is the time period of an AC supply having a frequency of 60 Hz?
2. Why is it generally not advisable to use a 60 Hz appliance directly with a 50 Hz power supply without appropriate conversion?
3. How many times does the voltage of a 50 Hz AC supply become zero in one second?
4. Explain the difference between AC and DC in terms of current direction and frequency.
5. What instrument is typically used to measure the frequency of an AC signal?