Advantage of AC over DC MCQs Quiz | Class 10
This quiz tests your understanding of the advantages of Alternating Current (AC) over Direct Current (DC), particularly focusing on concepts like transmission efficiency. It is designed for Class X Science (Code 086), Unit IV: Effects of Current. Answer all 10 multiple-choice questions, then submit to view your score and download a detailed answer PDF.
Understanding the Advantages of AC over DC
Alternating Current (AC) and Direct Current (DC) are two fundamental forms of electric current. While both are essential for various applications, AC holds significant advantages, especially concerning power transmission over long distances and voltage transformation. Understanding these advantages is crucial in modern electrical systems.
Key Advantages of Alternating Current (AC)
- Ease of Voltage Transformation: The most significant advantage of AC is its ability to be easily stepped up or stepped down using a transformer. This is vital for efficient power transmission.
- Step-Up for Transmission: Generators produce AC at relatively low voltages. For long-distance transmission, AC voltage is stepped up to very high levels (e.g., 132 kV, 400 kV) using step-up transformers.
- Step-Down for Distribution: Near consumption points, step-down transformers reduce the high transmission voltage to safer and usable levels (e.g., 230V in homes).
- Reduced Power Loss during Transmission: Power loss in transmission lines is primarily due to heating, given by the formula P_loss = I²R, where ‘I’ is the current and ‘R’ is the resistance of the wires.
- By stepping up the voltage for transmission, the current (I) can be significantly reduced for the same amount of power (P = V*I).
- A lower current dramatically reduces the I²R losses, as the loss is proportional to the square of the current. This makes long-distance transmission much more efficient and economical.
- Ease of Generation: AC generators (alternators) are generally simpler and more robust to construct and operate compared to DC generators.
- Cost-Effectiveness: Due to efficient transmission and the simplicity of transformers, AC distribution systems are more cost-effective for large-scale power grids.
- Simple Motor Design: AC motors, especially induction motors, are simpler, more robust, and require less maintenance than DC motors for many industrial applications.
Limitations of Direct Current (DC) for Transmission
Historically, DC was first used for power distribution. However, it faces severe limitations for long-distance power transmission:
- Difficulty in Voltage Transformation: DC voltage cannot be easily stepped up or stepped down using simple transformers. Doing so requires complex and expensive electronic converters, leading to higher costs and energy losses.
- Higher Power Loss: Without the ability to efficiently step up voltage, transmitting DC over long distances at practical voltages results in much higher currents and thus greater I²R power losses. This makes DC transmission highly inefficient for extensive grids.
- Limited Transmission Distance: Due to higher losses, DC transmission is typically limited to shorter distances or specialized applications (e.g., submarine cables, high-voltage DC (HVDC) links which convert AC to DC for very specific, very long point-to-point transmission to avoid certain AC issues, but this requires significant conversion equipment at both ends).
Transmission Efficiency Concept
The concept of transmission efficiency is paramount in power distribution. It refers to the percentage of generated electrical power that successfully reaches the consumer. High efficiency is achieved by minimizing power losses during transmission.
To transmit a given amount of power (P), if the voltage (V) is increased, the current (I) must decrease (since P = V × I). As power loss in wires is proportional to I²R, reducing the current by a factor of X reduces the power loss by a factor of X². This quadratic relationship makes high-voltage AC transmission exceptionally efficient.
AC vs DC: A Quick Comparison
| Feature | Alternating Current (AC) | Direct Current (DC) |
|---|---|---|
| Direction | Reverses periodically | Flows in one constant direction |
| Voltage Transformation | Easy using transformers | Difficult and inefficient |
| Power Loss (long distance) | Minimal (due to high voltage, low current) | Significant (due to high current) |
| Generation | Efficient (alternators) | Can be complex (dynamos) |
| Applications | Power grids, homes, factories | Batteries, electronic devices, solar panels |
Quick Revision Points
- AC can be easily stepped up or down using transformers, unlike DC.
- High voltage AC transmission significantly reduces power loss (I²R) over long distances.
- Power loss is proportional to the square of the current, making low-current transmission highly efficient.
- AC is the backbone of modern power grids due to its transmission advantages.
- DC is used for short-distance applications, electronics, and batteries.
Extra Practice Questions
- Why is stepping up voltage essential for efficient long-distance power transmission?
- Name the device used to change the voltage of an AC supply.
- Explain the relationship between current, voltage, and power loss in transmission lines.
- What are the primary challenges of transmitting DC power over hundreds of kilometers?
- Give two common applications where DC is preferred over AC.
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