Electric Cells

An electric cell is a device that converts chemical energy into electrical energy. Cells are categorized into two main groups: Primary Cells and Secondary Cells.

1. Primary Cells and Their Defects

Primary cells are cells in which the chemical reactions are irreversible. Once the active chemicals are exhausted, the cell cannot be recharged. The simplest example is the Simple Voltaic Cell.

Defects of the Simple Voltaic Cell

• Local Action: This occurs due to impurities (like iron or carbon) in the zinc electrode. These impurities create tiny local cells on the surface of the zinc, causing it to dissolve even when the cell is not in use. Correction: Amalgamating the zinc rod with mercury.
• Polarization: This is the accumulation of hydrogen gas bubbles around the copper (positive) electrode. These bubbles insulate the electrode and create a back-emf, reducing the current. Correction: Adding a chemical depolarizer, such as manganese (IV) oxide (MnO2) or using a Daniel cell.

2. Common Primary Cells

• Daniel Cell: Uses a porous pot to separate the zinc (in zinc sulphate) from the copper (in copper sulphate). The copper sulphate acts as a liquid depolarizer.
• Leclanché Cell (Wet): Uses ammonium chloride as the electrolyte and MnO2 as the depolarizer. It is suitable for intermittent use.
• Dry Cell (Leclanché type): The most common battery for torches and clocks. It uses a paste of ammonium chloride instead of a liquid.

3. Secondary Cells (Accumulators)

Secondary cells can be recharged by passing a current through them in the opposite direction. They are used when high currents or long-term use is required.

• Lead-Acid Accumulator: Consists of lead (IV) oxide (anode) and lead (cathode) in dilute sulphuric acid. It has a high EMF (2V) and low internal resistance but is heavy and requires maintenance.
• Nickel-Iron (NiFe) Accumulator: Uses Nickel hydroxide and Iron in Potassium Hydroxide (KOH) electrolyte. It is lighter and more durable than lead-acid but has a lower EMF (1.2V).
• Lithium-Ion: High energy density, used in mobile phones and laptops.
• Solar Cells: These are non-chemical cells that convert light energy directly into electricity using the photovoltaic effect.

4. Arrangement of Cells

Cells can be arranged to achieve different voltage or current requirements:

• Series Arrangement: Positive terminal of one to negative of the next. Total EMF (E_total) = nE. Total internal resistance (r_total) = nr. Used to increase the total EMF.
• Parallel Arrangement: All positive terminals connected together. Total EMF = E (of one cell). Total internal resistance = r/n. Used to provide a larger current for a longer period.

5. Maintenance and Efficiency

Maintenance includes topping up lead-acid batteries with distilled water, avoiding deep discharges, and cleaning terminals to prevent corrosion. The efficiency of a cell is generally the ratio of the energy supplied by the cell to the chemical energy consumed.