### Correct Option: A. Ionization of gas molecules #### Detailed Explanation: When a gas is subjected to an electric field, the primary phenomenon that increases its electrical conductivity is **ionization of gas molecules**. Let's break down why this is the case: 1. **Understanding Conductivity**: - Electrical conductivity in gases refers to the ability of the gas to conduct electric current. This is largely dependent on the presence of charged particles (ions and electrons) that can move freely under the influence of an electric field. 2. **Ionization Process**: - Ionization occurs when gas molecules gain enough energy to lose or gain electrons, resulting in the formation of positive ions (cations) and free electrons. This can happen through various means, such as: - **Collisions with high-energy particles**: When gas molecules collide with energetic particles (like electrons or photons), they can gain enough energy to become ionized. - **Application of an electric field**: In the presence of a strong electric field, electrons can be stripped from gas molecules, leading to ionization. 3. **Effect of Ionization on Conductivity**: - Once ionization occurs, the newly formed ions and free electrons can move in response to the electric field. This movement constitutes an electric current, thereby increasing the conductivity of the gas. The more ions and free electrons present, the higher the conductivity. 4. **Examples of Ionization**: - A common example of ionization in gases is seen in neon lights, where a high voltage causes the gas to ionize, allowing it to conduct electricity and emit light. #### Why Other Options Are Incorrect: - **B. Thermal expansion of the gas**: - Thermal expansion refers to the increase in volume of a gas when it is heated. While heating a gas can increase the kinetic energy of its molecules, it does not directly lead to an increase in conductivity. In fact, thermal expansion can lead to a decrease in density, which may reduce the likelihood of collisions necessary for ionization. - **C. Decrease in pressure**: - A decrease in pressure generally leads to a lower density of gas molecules. While this can affect the mean free path of the molecules (the average distance a molecule travels before colliding with another), it does not inherently increase the number of charged particles available for conduction. In fact, lower pressure can reduce the likelihood of ionization events. - **D. Increase in humidity**: - While increasing humidity (the amount of water vapor in the air) can affect the conductivity of air, it does so by introducing water molecules, which can also ionize. However, this is not the primary mechanism for increasing conductivity in gases subjected to an electric field. The effect of humidity is secondary compared to the direct ionization of gas molecules. ### Summary of Key Points: - **Ionization of gas molecules** is the primary phenomenon that increases electrical conductivity in gases under an electric field. - Ionization creates free electrons and positive ions, which can move and carry electric current. - Other factors like thermal expansion, pressure changes, and humidity do not directly enhance conductivity in the same way as ionization does. - Understanding the role of ionization is crucial for applications in fields like plasma physics, gas discharge tubes, and atmospheric electricity.