In a lead-acid cell, lead is called as

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Option B: negative active material

Explanation & Concepts:
In a lead-acid battery, spongy lead (Pb) acts as the negative active material. During discharge, this lead reacts with the electrolyte ($H_2SO_4$) to form lead sulfate ($PbSO_4$) and releases electrons to the external circuit.

Working Principle: The cell operates via a double-sulfation process. At the negative plate, metallic lead ($Pb$) undergoes oxidation, losing two electrons to become $Pb^{2+}$, which subsequently combines with the sulfate ions from the electrolyte to form insoluble $PbSO_4$. This release of electrons makes the lead plate the anode (negative terminal) during the discharge process.

Key Formulas: $Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^{-}$ — Negative plate discharge reaction; $PbO_2 + 4H^{+} + SO_4^{2-} + 2e^{-} \rightarrow PbSO_4 + 2H_2O$ — Positive plate discharge reaction

Key Points: Negative active material is spongy lead (Pb).; Positive active material is lead dioxide ($PbO_2$).; Electrolyte is dilute sulfuric acid ($H_2SO_4$).; During discharge, both plates convert to lead sulfate ($PbSO_4$).

Advantages: High cell voltage (approx 2V per cell).; Low internal resistance enabling high discharge currents.

Disadvantages: Heavy weight due to lead plates.; Sulfation risk if left in a discharged state for long periods.

Applications: Automotive starting, lighting, and ignition (SLI).; Uninterruptible Power Supplies (UPS) and standby power.

Additional Info: The positive plate is typically darker in color (dark chocolate brown) due to $PbO_2$, while the negative plate is greyish.. Option A (Positive active material) is incorrect as it refers to $PbO_2$.

Answer: B is correct — Spongy lead ($Pb$) serves as the negative electrode in a standard lead-acid cell electrochemical system.

Exam Tip: Always remember the mnemonic 'P-O-S-I-D' for Lead-Acid: Positive plate = Oxidized ($PbO_2$); Negative plate = Reduced/Spongy Lead ($Pb$).

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Option A: positive active material

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Option C: passive material

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Option D: none of the above

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