Explanation:
Arc welding is a process in which two metal pieces are joined together using an electric arc. The electric arc is created by an electric current passing through an electrode and the workpiece. One of the important factors in arc welding is penetration, which refers to the depth of fusion between the two metal pieces being welded.

DC – Electrode Positive:
- In DC – Electrode Positive polarity, the electrode is connected to the positive terminal of the power supply, and the workpiece is connected to the negative terminal.
- In this polarity, the electrons flow from the workpiece to the electrode, resulting in a concentrated heat at the tip of the electrode.
- The intense heat generated at the electrode tip allows for better penetration as compared to other polarities.
- The high temperature created by the concentrated heat facilitates the melting of both the electrode and the workpiece, leading to a deeper fusion between the two metal pieces being welded.

DC – Electrode Negative:
- In DC – Electrode Negative polarity, the electrode is connected to the negative terminal of the power supply, and the workpiece is connected to the positive terminal.
- In this polarity, the electrons flow from the electrode to the workpiece, causing the heat to be concentrated at the workpiece.
- The heat generated at the workpiece is comparatively lower than in DC – Electrode Positive polarity, resulting in reduced penetration.
- However, this polarity is often used for welding thin materials, as the lower heat concentration helps prevent burn-through.

AC:
- In AC (alternating current), the polarity of the electrode continuously switches between positive and negative.
- As a result, the heat distribution is not as concentrated as in DC – Electrode Positive polarity, leading to reduced penetration.
- AC is commonly used for welding aluminum and magnesium alloys, as it helps prevent arc blow and improves the stability of the arc.

Conclusion:
In arc welding, penetration is minimum for DC – Electrode Negative polarity. DC – Electrode Positive polarity provides better penetration due to the concentrated heat generated at the electrode tip. AC polarity offers intermediate penetration levels, making it suitable for specific applications.