Contents
The study of Physics Topics involves the exploration of matter, energy, and the forces that govern the universe.
In this chapter we shall discuss only the chemical cell.
What are the Differences Between Primary and Secondary Cells?
1. Primary cell: In this type of cell chemical energy is converted into electrical energy. Since we get electrical energy with out the help of any electrical source, it is called a primary cell.
Example: Vokaic cell, Leclanche cell, dry cell, Daniell cell.
The active components of this cell gradually decay if it sends current in the external circuit for some time and ultimately the cell becomes inactive. The electro-chemical reactions taking place inside this cell are irreversible. This implies that the inactive components cannot be made active by a reversible process. The cell is to be abandoned unless the components are replaced.
2. Secondary cell: In this type of cell too, chemical energy is converted into electrical energy. But unlike a primary cell, it can be reused several times. For this external current source is needed to recharge it.
Example: Lead-acid accumulator, Alkali accumulator.
The active components of this cell gradually decay as the cell sends current in the external circuit. But here the electro-chemical reactions are reversible. With the help of reverse reactions the inactive components of the cell can be made active again. To start reverse reactions, current from an external source of electricity, having electromotive force greater than that of the cell, is allowed to pass through the cell. This cell acts in two steps:
1. Discharging: The act of sending current in the external circuit by this cell is called discharging of the cell. During discharging chemical energy is converted into electrical energy.
2. Charging: The act of activating the practically inactive cell by sending a current in it from an external source in the reverse direction is called charging of the cell. During charging, electrical energy is converted into chemical energy.
During charging, electrical energy is converted into chemical energy which is stored in the cell, and conveniently can be converted into electrical energy. Hence this cell is also called storage cell or accumulator.
3. Standard cell: As long as this cell is active its electro-motive force does not change. So comparing with the constant value of the emf of this cell, emfs of other cells can be determined or different electrical instruments can be calibrated. This cell is almost never used as a source of electricity, At present the internationally recognised standard cell is the Weston cadmium cell. Emf of this cell at 20C is 1.01830V.
Symbol of a cell: All cells are shown by drawing two parallel lines of unequal lengths [Fig.]. The long line represents the positive pole and the short line represents the negative pole of the cell.
Elementary Idea of Secondary Cell or Storage Cell
1. Lead-acid Accumulator: It is a secondary cell. French scientist Gaston Plante discovered it in 1859.
Discussion:
Positive electrode: The active element of this electrode is lead dioxide (PbO2).
Negative electrode: The active element of this electrode is metallic lead, taken in a spongy form.
Electrolyte: Dilute sulphuric acid of specific gravity 1.25 is taken as electrolyte in a thick glass or bakelite vessel. The plates are dipped in the acid. Only the terminals of the two electrodes remain outside the vessel.
Electromotive force of the cell: When a fully charged cell begins to discharge, its emf is 2.2 V. But after a short while the emf comes down to 2.0 V and remains constant for a long time. At last, when the cell is fully discharged the emf of the cell comes down to about 1.8 V. To ascertain whether the cell has been fully charged and is in active state, or it has been fully discharged, a voltmeter inserted in the external circuit can well serve the purpose. The ingredients of the cell need not be inspected.
It is to be noted that during the time of discharge, lead sulphate is formed at the two electrodes and the active elements i.e., Pb, PbO2 and H2SO4 gradually decay.
Specific gravity of sulphuric acid: The cell contains sulphuric acid of specific gravity 1.25. During discharging, the specific gravity of sulphuric acid solution decreases. When the cell is fully discharged the specific gravity of the acid solution falls to 1.18. During charging, sulphuric acid is regenerated. When the cell is fully charged the specific gravity of sulphuric acid comes back to its normal value 1.25 . So, even by measuring the specific gravity of sulphuric acid, the condition of the cell can be determined. However, to determine the condition of the cell, measurement of the emf of the cell with a multimeter is a better option.
Use: The internal resistance of the cell is very low i.e., current flows through the cell almost without any resistance. So this cell is used for getting a steady current of high magnitude in the external circuit for a long time.
It is used
1. in cars, buses, trains and even in laboratories, in inverters, for generating electricity in houses during power-cuts.
2. Alkali Accumulator: The active elements of this cell are iron (negative plate), nickel hydroxide (positive plate) and caustic potash solution (KOH) (electrolyte). This is also called nickel-Iron accumulator or nife cell. This cell is also known as Edison cell after the name of its discoverer Thomas Alva Edison.
Disadvantages: In comparison to lead-acid accumulator:
- Its internal resistance is high.
- Emf is low (1.3 V).
- Efficiency is small.
Advantages:
- No internal disturbance occurs on heavy jerking.
- It can be left unused for a long time in a fully charged or in a fully discharged condition.
- No damage is done if it is overcharged or over-discharged.
Capacity and Efficiency of a Secondary Cell
Capacity: Capacity of a secondary cell is defined as the amount of electricity (charge) which the cell is capable of supply-ing in the external circuit before being completely discharged.
Unit of capacity: Ampere hour (A ᐧ h) is the unit of capacity.
1 A ᐧ h = 1 A × 1h = 1 A × 3600s = 3600 A ᐧ s = 3600 C
So, a cell having capacity I A h can supply a charge of 3600 C. Obviously A ᐧ h is a big unit.
Example: By the statement that capacity of a secondary cell is 50 A ᐧ h, we mean that the cell can supply a current of 1 ampere for 50 hours or 2 amperes for 25 hours. However, the capacity expressed in ampere hour is always an approximate value only.
Efficiency: The amount of charge given to a secondary cell during charging cannot be get back totally during discharging. The ratio of the charge obtained to the amount given is called ampere-hour efficiency. In case of lead-acid accumulator it is 0.9 or 90%.
Again the amount of external energy supplied to a secondary cell during charging cannot be get back totally in the form of electrical energy during discharging. The fraction of the supplied energy obtained from the cell is called energy efficiency or watt-hour efficiency of the cell. It is given by,
During charging of a lead-acid accumulator the external source used has an average emí of 2.2 V. But during discharging the average emf obtained from the accumulator is 2.0 V. So, the energy efficiency of a lead-acid accumulator is,
η = \(\frac{2.0}{2.2}\) × 0.9 = 0.8 = 80%
Numerical Examples
Example 1.
A battery is charged at potential of 15 V for 8 h by means of a current of 10 A. While discharging it supplies a current 5 A for 15 h at a potential difference of 14 V. Calculate the watt-hour efficiency of the battery. [CBSE ‘04]
Solution:
Watt-hour efficiency or energy efficiency
\(=\frac{\text { energy obtained during discharging }}{\text { energy supplied during charging }}\)
= \(\frac{14 \times 5 \times 15}{15 \times 10 \times 8}\) = \(\frac{7}{8}\) = 0.875 = 87.5%
Differences between Primary and Secondary Cells
Primary cell | Secondary cell |
1. Electrical energy is obtained from chemical energy. | 1. The electrical energy obtained from the external source is stored as chemical energy. Afterwards electrical energy is obtained from this chemical energy. |
2. There is no alternative but to change the active elements when they are exhausted. So, this cell is an irreversible one. | 2. The exhausted elements can be made active by sending current from an external source. So, this cell is a reversible one. |
3. Internal resistance is comparatively large. | 3. Internal resistance is very small. |
4. A continuous large current for a long period may not be obtained from this cell. | 4. A continuous large current for a long period may be obtained from this cell. |
5. Generally this cell is used as a voltage source. | 5. Generally this cell is used as a current source. |