The correct answer is D: “Both (b) and (c),” meaning that the stability of an arc in a vacuum depends on both the circuit parameters and the contact materials and their vapor pressure. Let’s explore this in detail and explain why the other options are not correct:

Stability of an Arc in Vacuum:

An arc in vacuum refers to an electric arc or discharge that occurs in a vacuum environment, where there is no air or other gases present. This phenomenon is commonly encountered in various electrical applications, such as vacuum interrupters in circuit breakers and vacuum tubes in electronics.

Explanation of Option D: (b) Circuit Parameters:

The stability of an arc in vacuum is influenced by various circuit parameters, including the voltage applied, current flowing through the arc, and the geometry of the electrodes. These parameters determine the conditions under which the arc operates.

For example, the arc voltage and current are critical factors that affect the size, temperature, and behavior of the arc. The voltage-current characteristics of the arc must be within specific limits to maintain stability and prevent undesired behaviors such as arc extinction or excessive overheating.

Contact Materials and Their Vapor Pressure:

The contact materials used in vacuum interrupters and vacuum tubes play a crucial role in the stability of the arc. These materials are typically metals or metal alloys that can withstand the high temperatures generated by the arc.

The stability of the arc is influenced by the vapor pressure of these materials at elevated temperatures. As the arc heats the contact materials, they may vaporize or release metal vapor into the vacuum, affecting the arc’s behavior. Controlling the vapor pressure of the contact materials is essential to maintain a stable and reliable arc.

Now, let’s explain why the other options are not correct:

Option A: The Contact Material Only:

Option A is not correct because it oversimplifies the factors that influence the stability of an arc in vacuum. While the contact material is indeed a critical factor, as mentioned in option (c), it is not the only factor.

Circuit parameters, such as voltage, current, and electrode geometry, also significantly impact the behavior and stability of the arc. Neglecting these parameters would provide an incomplete understanding of the arc’s stability.

Option B: The Circuit Parameters Only:

Option B is not correct because it also provides an incomplete perspective. While circuit parameters are important and directly influence the arc’s behavior, they alone cannot ensure the stability of the arc in vacuum.

The properties of the contact materials and their vapor pressure, as mentioned in option (c), are equally vital. The interaction between circuit parameters and contact materials is essential to maintaining a stable arc.

Option C: The Contact Materials and Its Vapor Pressure:

Option C correctly identifies one of the significant factors influencing the stability of an arc in vacuum—the contact materials and their vapor pressure.

However, it does not acknowledge the importance of circuit parameters (option b), which are equally critical. Both factors, circuit parameters and contact materials, must be considered together to fully understand and control the stability of an arc in a vacuum environment.

In summary, the stability of an arc in vacuum is a complex phenomenon influenced by multiple factors, with two of the most important being circuit parameters (such as voltage, current, and electrode geometry) and contact materials (including their properties and vapor pressure at elevated temperatures).

These factors interact to determine the behavior of the arc and its reliability in various applications, emphasizing the importance of considering both (b) and (c) factors to achieve stable and efficient vacuum arc operation.