Table of Contents
Electric current is simply electric charge in motion. Electric cells are chemical devices, which are capable of causing an electric current to flow. This produces electric force, which pushes the current along.
Electrons flow from the negative terminal or cathode of the cell to the positive terminal or anode
TYPES OF ELECTRIC CELLS
Electric cells are divided into two namely: the primary cells and the secondary cells
PRIMARY CELLS – These are those cells in which current is produced as a result of an irreversible chemical charge.
SECONDARY CELLS – These cells are those which can be recharged when they run down by passing current backwards through them.
There are three components in a cell. Viz:
- The anode (positive electrode)
- The cathode (negative electrode)
- The electrolyte
THE SIMPLE PRIMARY CELL (VOLTAIC CELL)
A simple cell can be made by placing two different electrodes (metals) in an electrolyte. Two wires are then used to connect these metals to a voltmeter. If a deflection is noticed it means that the cell creates a voltage.
Defects of a simple cell
The two major deflects of a simple all are polarization and local action
- Polarization – This cell defect is characterized by the release of “hydrogen bubbles.” The bubbles collect at the positive electrode and insulate it. This show down and eventually stops the working of the cell.
This defect can be corrected either by occasionally brushing the plates, which is highly inconvenient, or by using a depolarizer e.g. manganese oxide.
- Local action – This occurs when impure zinc is used. The impurities in the zinc result in the gradual wearing away of the zinc plates. This can be prevented by cleaning the zinc with H2SO4 and then rubbed with mercury. The mercury amalgamates the zinc by covering the impurities thereby preventing it from coming into contact with electrolyte.
Leclanche cells are of two types; the wet and the dried types.
The wet Leclanche cell consists of a zinc rod at the cathode in solution of ammonium chloride contained in a glass vessel. The anode is a carbon rod contained in a porous pot and is surrounded by manganese chloride as a depolarizer. An e.m.f. is set up by the zinc, the carbon and the electrolyte, which drives a current from zinc to carbon through the cell. The e.m.f of a Leclanche cell is 1.5v. Its defect is that when the cell has worked for sometimes, the rate of hydrogen production becomes greater than the rate at which it is oxidized by the manganese dioxide, hence the formation of polarization. Therefore the cell must be allowed to rest from time. The wet Leclanche cells are restricted to intermittent current supply because they do not give continuous service. They are too heavy to carry about without spilling the liquid.
For the dry Leclanche cell, the defect of heaviness is overcome. The ammonium chloride electrolyte is a jelly-like material and not aligned solution. The positive electrode is a carbon rod surrounded by a packed mixture of manganese dioxide and powered carbon, inside a zinc container, which is the negative electrode.
The dry can be carry about easily e.g. torch batteries, and transistor radio batteries. Due to local action, they deteriorate after sometime.
THE DANIEL CELL
This is also a primary cell invented to counter the problem of polarization. The zinc rod is the negative electrode while the positive electrode is the container. The electrolyte is dilute tetraoxosulphate (vi) acid contained in a porous pot around the zinc rod, and the depolarize is copper tetraoxosulphate (vi) in the surrounding copper container. The Daniel cell is much more efficient than the Leclanche cell. The e.m.f. is of a constant value of l.08V.
Secondary cells are of two main types – lead acid accumulator, and the alkaline or nickel-iron (Ni-Fe) accumulation.
The lead-acid accumulator is the most common one. It consists of lead oxide as the positive electrode, lead as the negative electrode and tetraoxosulphate (vi) acid as the electrolyte. During the discharge, when the cell is given out current both plates gradually charge to lead tetraoxosulphate (vi) while the acid gradually becomes more dilute and the density decreases. When fully charge the relative density and e.m.f. of the cell are 1.25 and 2.2v respectively. But when discharge they are reduced to 1.5 and less than 2.0v respectively. The rod density of the cell should not be allowed to drop 1.15 before it is recharged.
Maintenance of lead acid accumulators
- The liquid level must be maintained by using distilled H2O
- The cell should be charge appropriately
- If the cell is not in use for a long time, it should be discharge from time to time or the acid remove and the cell dried
- The battery should be kept clean so that current dose not leaks away across the casing between the terminals.
The alkaline or Ni-Fe accumulators – The positive electrode is made of nickel hydrogen while the negative plate is either of iron or calcium. The electrolyte is potassium hydroxide dissolve in water. This cell last longer and lead acid cells, keep their charge longer and they require less maintenance. They are used for emergencies in factories and hospitals. They are expensive and bulky with a small e.m.f value, about 1.25v
- What is a cell?
- Highlight the defects of cells
- Briefly differentiate between primary cell and secondary cells
- During an activity, 15 coulombs of charge passed through an ammeter in 2second what is the reading of the ammeter? (a) 2A (b) 5A (c) 8A (d) 10A
- The energy transformation taking place when a cell supplies current to a bulb is from (a)light energy to heat energy (b) mechanical energy to light energy (c)solar energy to electrical energy (d) chemical energy to light energy
- Which of a-d below is correct? (i) Ordinary torch battery is an example of primary cell; (ii) accumulations have very high interne resistance (a) (i) only (b) (ii) only (c) (iii) only (d) (i) and (ii) only
- Which of the following statement is not true? (a) the chemical action in a primary cell is irreversible (b) lead-acid accumulation can be recharged (c) lead-acid accumulator has large internal resistance (d) a secondary cell can be recharged
- The defect in simple cell which result in a back e.m.f and increase in internal resistance is known as (a) local action (b) reduction (c) polarization (d) oxidation
- The rheostat could serve the following except. (a) as a variable resistor (b) as a potential divider (c) as a means of varying the current in a circuit (d) as a converter of solar energy to electrical energy
- Which of the following devices convert sheet energy to electric current? (a) Photo cell (b) battery (c) voltmeter (d) thermocouple
- Which of the following devices coverts mechanical energy to electric current (a) battery (b) photocell (c) thermopile (d) dynamo
- Using suitable diagram, explain the simple cell
List two defects of a simple cell