Energy used in Generating Electricity – Chemical Energy
A continuous flow of current can be achieved by generation of current from;
- Chemical energy.
- Heat energy.
- Solar energy.
- Mechanical energy.
Electric current is produced from the chemical energy through the use of cells that convert the chemical energy into electrical energy.
An electrical cell is an “electrical power supply”. What a cell does is convert stored chemical energy into electrical potential energy, allowing a current to flow from the positive terminal to the negative one via an external circuit.
A cell consists of electrodes that are separated by solutions of various acids or salts, called electrolytes.
Electrodes (Cathode & Anode):
Electrodes are the positive and negative terminals of a cell that are separated by a solution of various acids or salts.
The positive terminal is called a cathode and the negative terminal is called an anode. This current can be thought of as the flow of positive charges, even though it is usually electrons, which have a negative charge that carries current in a copper wire circuit. Since the current is the flow of positive charge, the negative electrons flow in the opposite direction to this (conventional) current.
Within the cell itself, the charge flows from the negative terminal to the positive terminal as it gains electrical energy.
An electrolyte is the chemical composition of the cell in which the electrodes are immersed. The electrolyte is in a molten form so that the electrons would be able to move about and constitute a current.
A simple cell consists of a copper rod as the positive anode and a zinc plate as the negative cathode immersed in a container that is filled with dilute sulphuric acid (tetraoxosulphate (vi) acid).
Working Principle of a Simple Cell:
When the switch is closed so that the copper and the zinc plates are connected together, the zinc slowly dissolves in the acid by chemical reaction. Electrons and bubbles of hydrogen gas are produced. The electrons flow through the wires from the zinc to the copper and produce current while the bubbles of hydrogen gas are formed on the copper anode of the cell.
A bulb connected between the copper and zinc terminals will light up, confirming the current flow.
Defects of Simple Cells:
Simple cells can only supply current for a short period of time. This is due to its defects that which are known as;
(ii) Local Action.
These defects reduce the practical value and performance of these cells.
Polarization is a defect that occurs in simple electric cells due to the accumulation of hydrogen gas bubbles on the copper anode (positive electrode) of the cell. The hydrogen gas bubbles cover the copper anode and set up a back emf which opposes the forward emf of the cell generated between the zinc and copper. The back emf gradually reduces the current in the external circuit and will eventually stop the chemical action of the cell.
Prevention of Polarization: Polarization can be minimized using a depolarizer such as manganese oxide or potassium dichromate. These are added to the electrolyte.
ii. Local Action:
In local action, embedded impurities, such as iron and carbon, in a zinc electrode can act as positive electrodes and create electric currents between zinc and this positive electrode. The impurities set up tiny cells around the zinc surface and produce hydrogen bubbles on the zinc surface. This leads to the dissolution of the zinc plate into the solution (acid), thus the zinc is continuously wasted.
Prevention of Local Action: Can be minimized by rubbing mercury over the surface of the zinc plate. This is a process called amalgamation. The reason this is done is that the mercury on the zinc plate prevents impurities from coming in contact with the acid, which prevents local action.