Electric current can be generated from a variety of sources. These sources generally rely on different physical principles to create the flow of electrons.
A common source of electric currents is chemical cells and generators driven by moving water or vapor.
other sources of electricity includes:
- wind driven generators
- solar cells or panels
- thermocouples
- some crystals when under pressure(piezo electric effect)
Chemical cells, often referred to as galvanic cells or voltaic cells, are devices that convert chemical energy into electrical energy through spontaneous chemical reactions. The most common example of a chemical cell is the battery, which stores and uses electrical energy.
A chemical cell consists of two electrodes. One electrode is made of material that can undergo oxidation and is referred as the anode. The other electrode is made of material that can undergo reduction and is referred to as the cathode. These electrodes are usually placed in different solutions containing ions that can take part in the reactions.
The electrodes are immersed in an electrolyte. An electrolyte is a solution or paste that contains ions which can carry charge between the two electrodes. This electrolyte allows the movement of ions, completing the circuit and enabling the flow of electrons.
At the anode, oxidation occurs. oxidation is loss of electrons. At the cathode, reduction occurs (gain of electrons). The flow of electrons from the anode to the cathode through an external circuit is what generates electric current.
The difference in electric potential between the two electrodes creates a voltage, which is what drives the current. The voltage depends on the materials used for the electrodes and the nature of the electrolyte.
Chemical cells as source of electric current
Chemical cells that produces electromotive force as a result of chemical reactions is usually grouped into two categories. These categories are primary cells and secondary cells.
Primary cells
Primary cells are type of chemical cells that cannot be renewed once the chemicals are exhausted. Primary cells undergoes decays as a result of chemical reaction and once exhausted, they can only be replaced and not regenerated. Secondary cells are cells that can be renewed by recharging once chemical processes that generates current in them has been exhausted. In the next section we will be describing how various chemical cells are designed. Some of these cells includes the simple cell, the Leclanche’ cell and the dry cell.
Simple primary cells
The figure below shows a very simple chemical cell made of lemon, copper plate , zinc plates and conducting cables. The lemon juice acts as an electrolyte.
When the circuit is complete, the galvanometer deflects showing that current is flowing. Flowing current is a sign of existing e.m.f across the two metal plates. The galvanometer deflections drop after some time. This is because there are chemical processes in the setup that hinder further flow of current.
If similar plates were used, the galvanometer would not deflect, meaning that no current will flow. The two metals plates acting used as electrodes must have different rates of reaction when immersed in the electrolyte. Zinc is more reactive compared to copper. When these metals are immersed in an acidic medium like citric acid found in lemon, an e.m.f is set up at the other ends of the metal.
Making a simple primary cell
To make a primary simple cell, you will need the following apparatus:
- Zinc plates and copper plates
- A beaker containing dilute sulphuric acid
- bulb
- connecting wires
- an ammeter with a range of 0-100 mA
procedure
Clean the metal plates using a wire brush. Then dip them into the dilute sulphuric acid as shown in the setup below.
close the switch and observe the brightness of the bulb.
Record the ammeter reading. Observe if it remains constant over a period of time. Observe formation of gas bubbles on the plates.
Add potassium dichromate to the acid and observe what happens.
Observations
Bubbles of gas form around the zinc plate when the switch is open. No bubbles form around the copper plate. This indicates that zinc is reacting with the acid faster than copper. When the switch is closed, some readings are seen on the ammeter and bulb lights dimly. Bigger bubbles of gas forms around the copper plate when the switch is closed. The gas formed is found to be hydrogen gas. Zinc metal is seen to corrode due to the acid as reaction is takes place.
The current reduces with time and soon the bulb is observed going off. Addition of potassium dichromate makes the bulb relights.
Explanations on the working of simple chemical cell
Dilute sulphuric acid exists in the form of hydrogen ions (H+) and sulphate ions (SO4 2-) as represent in the chemical formula below:
The two metal plates also known as the electrodes when dipped in the dilute sulphuric acid carries electric charges into and out of the electrolyte.
The chemical action between zinc and dilute sulphuric acid liberates electrons which flows through the connecting wire and the bulb to the copper plate. The chemical equation below shows represents the process that releases electrons:
The hydrogen ions (H+) moves to the copper plate where they are neutralized by the electrons that had come from the zinc and acid reaction.
This produces hydrogen gas bubbles around the copper plate.
Copper receives more electrons from the reactions of zinc and the acid. This makes the zinc plate negative and copper plate positive. Conventionally, the direction of current is from positive plate to the negative plate .
The flow of current stopped due to the defects in the cell. The two defects in this simple cell are known as polarisation and local action. polarisation and local actions are the main defects of simple cells.
polarisation
This is the accumulation of bubbles around the copper plate. This accumulation causes an insulation to the flow of current and also sets up some local cells with copper whose electron flows tends to oppose the flow of electrons from the zinc plate. The overall effect is increase in the internal resistance of the cell hence reducing the flow of current.
Addition of potassium dichromate causes some of its oxygen atoms combine with the hydrogen atoms that has formed around copper to form water. that is:
This process boosts the current flow once more but causes the electrolyte to get more diluted.
local action
Local action is a process where the zinc plate corrodes due to it’s reaction with the dilute sulphuric acid. It is promoted by the impurities in the zinc plate. Local action can be minimized by use of pure zinc or coating the zinc metal with mercury in a process known as amalgamation.
The Leclanche’ cell
Leclance’ cell is an improvement from the simple cell. It is a cell where defects in simple cells have been minimized. The basic structure of the leclanche cell is as shown below.
From the diagram, the carbon rod (positive terminal) is covered with mixture of manganese (IV) oxide and carbon powder. The manganese (IV) oxide acts as a depolariser. It reacts with the hydrogen gas formed on the carbon rod to produce water hence slowing down defect of polarisation. This process is however slow hence large currents cannot be drawn out of this cell steadily for a long time. The carbon powder increases the effective area of the plate which reduces the opposition to the flow of current. remember that, the larger the area of conductor, the less the electrical resistance in a conductor.
The zinc plate is dipped in ammonium chloride solution, which converts zinc to zinc chloride when the cell is in operation. Local action defects has not been removed from this cell.
Leclanche cell is most suitable for devices that don’t need current to be drawn from the cell for a long time. For example operating electrical bells and telephone boxes. Leclanche cell has longer life compared to the simple cell.
The Dry Cell
Dry cell is a primary chemical cell without a liquid as an electrolyte. Instead of ammonium chloride solution used in the leclanche’ cell, ammonium chloride jelly is used.
The figure below shows the structure of a dry cell.
Manganese (IV) oxide and carbon powder are used as depolariser in the cell. The hydrogen gas produced at the positive terminal meets with oxygen atoms in the depolariser to form water. This makes the cell become wet after use.
The zinc case acting as the negative electrode corrodes due to it’s reaction with ammonium chloride forming zinc chloride. This makes local action remains a defect in a dry cell.
A dry cell, like other primary cells, cannot be renewed when chemical actions that produces current are complete. A new dry cell has an e.m.f. of about 1.5 V.
Large currents should not be drawn from the dry cell within a short time. Short circuiting the dry cell can also ruin it. A dry cell must be stored in dry places since it can be damaged by moisture through chemical process.
Dry cells are commonly used in torches, calculators and radio receivers.
Related topics
- Arrangement of components in a circuit: concise Detailed approach
- Arrangement of cells: Easy introduction
- Induced Electromotive Force
- Factors that affects magnitude of the induced e.m.f
- Transformers
- The Fleming’s Right Hand Rule
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