Production Of Electric Current:

A cell is a chemical pack that produces current due to irreversible chemical reaction. A cell consist of positive anode and negative cathode that are separated by solution of various acid or salts.

Electrodes:

Electrodes are the positive and negative terminals of a cell. Current leaves the cell through the positive electrode and enters the cell through the negative electrode.

Types Of Electrodes:

There are two types of electrodes. They are :

1.The Anode

2.The Cathode

The Anode:

Anode the positive electrode / terminal of the cell. Current leaves the cell through the anode.

The Cathode:

The cathode is the negative terminal of the cell. Current enters the cell through the cathode.

Electrolyte:

Electrolyte is the chemical composition of the cell in which the electrodes are immersed. The electrolyte is in a molten form so the that the electrons would be able to move about and constitute current.

Simple Cell:

A simple cell consist 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 .

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.

Convectional Flow of Current:

In a cell, current flow from the positive anode to the negative cathode while electrons flow from the negative cathode to the positive anode. This opposite movement of current and electrons called convectional flow. Simple cell can however only supply current for a short time due to its defects that is discussed below:

Polarization:

Polarization defect is due to the formation of hydrogen gas bubbles on the copper anode of the cell. The hydrogen gas bubbles cover the copper anode and set up a back emf which oppose the forward emf of the cell. The back emf gradually reduces the current in the external circuit and eventually stop the chemical action of the cell which in turn stop the cell from working.

Polarization defect can be prevented by the addition of chemicals called depolarizers such as manganese dioxide and potassium dichromate to the electrolyte. The depolarizers oxidizes the hydrogen gas bubbles to form water.

Local Action:

Local action is the wearing away of the zinc plate into the acid. Local action is due to the present of impurities such as iron and carbon in the zinc. The impurities set up tiny cells around the zinc surface and produce hydrogen bubbles on the zinc surface. Local action defect can be prevented by the process of amalgamation. Amalgamation is the rubbing of the zinc plate with mercury. The mercury prevents the impurities from touching or coming in contact with the acid and therefore prevent local action.Polarization and local action are the main defects of simple cells.

The Leclanche’ Cell

In this cell carbon rod is used as the positive terminal and zinc as the negative electrode. The electrolyte is ammonium chloride solution (NH₄Cl). No polarization since it is reduced by use of manganese (IV) oxide (MnO₂) which oxidizes hydrogen into water. Local action still occurs. They are used in operating bells and telephone boxes.

The Dry Cell

It is referred to as dry because it contains no liquid. The ammonium solution is replaced with ammonium chloride jelly or paste, the manganese (IV) oxide and carbon powder are used as the depolarizer.

The hydrogen gas produced is oxidized to water which eventually makes the cell wet after use. They are used in torches, radios calculators etc.

Secondary Cells

They are also called storage cells since they store electrical charge as chemical energy.

Experiment: To charge and discharge a simple secondary cell

Procedure

Set up the apparatus as shown below.
Close the switch S1 and observe the changes in the plates if any.
Note how the ammeter reading varies with time.

Discussion

When charging oxygen is produced at the anode and hydrogen at the cathode.

The oxygen reacts with lead to form lead (IV) oxide which is deposited at the anode.

The hydrogen formed has no effect.

When discharging current flows in opposite direction with oxygen being formed at the cathode and hydrogen at the anode.

The colour of the positive electrode changes from brown to grey.

Lead-Acid Accumulator.

A 12V accumulator has six cells connected in series. Each cell has several plates forming lattice grid with positive plates carrying lead (IV) oxide and the negative plates having spongy lead. They are placed close to one another with an insulating sheet separating them. They are rated in ampere-hours i.e. 30 Ah means that it can supply 1 ampere for 30 hours or 2 amperes for 15 hours etc.

Example

A battery is rated at 30 Ah. For how long will it work if it steadily supplies a current of 3 A?

Solution

Q = I t, hence t = Q / I => 30 / 3 = 10 hours.

Alkaline Accumulators

Potassium hydroxide (KOH).Nickel hydr oxide (Ni (OH) forms the positive electrode while iron forms the negative electrode.

They are two types nickel cadmium (NiCd) and nickel iron (NiFe).

They are used in ships, hospitals and buildings where large currents are required for emergencies.

Advantages of alkaline accumulators over lead-acid accumulators

Large currents can be drawn from them
They require little maintenance
They are portable
They can remain discharged for a long time without getting ruined.

Disadvantages

They are very expensive
They have lower e.m.f per cell.physics

2.. Production Of Electricity from Heat Energy ( Thermoelectric Effect ):

Electricity can be produced from heat using a thermocouple. Electricity is when there is a difference in temperature between the hot junction and cold junction of the thermocouple.

A thermocouple consist of a copper and iron wires at one end and the free ends are connected to a galvanometer.

Thermoelectricity is the electricity that is produced by a thermocouple while the effect that produced the current is called thermoelectric effect.

Working Principle of Thermocouple:

A thermocouple has a cold junction and hot junction. When the hot junction is immersed in hot water so that there is a difference in temperature between the hot junction and the cold junction, current flow in the wire of the thermocouple. The current is detected and measured be a galvanometer connected to the free ends of the two different wires. The higher the difference in temperature between the hot junction and the cold junction, the larger the current produced by the thermocouple.

3..Production of Electricity from Mechanical Energy:

Electricity is produced from mechanical energy when a coil of insulated wire moves and cut across the magnetic field of a magnet. Current is induced in the coil and is tapped out through the split rings/ commutator and carbon brushes to which the ends of the coils are connected. The device is called generator or dynamo

An ac generator is a device that convert mechanical energy into electrical energy when a coil move and cut across the magnetic field of a magnet.

An AC generator consist of a coil placed in the magnetic field of a north and south poles of a magnet. The ends of the coil are connected to two slip rings. The rings are connected to two carbon brushes. Current is tapped out through the two carbon brushes by wires connected to them.

4.. Production Of Electricity from Solar Energy:

Electricity can be produced from solar energy. The device that is used is called photoelectric cell or photo cell.

A photo cell is a device that convert solar energy of the sun into electrical energy or electricity. It consist of a photosensitive surface metal such as potassium and alkaline metal that emits electrons when light incident on it.

Working Principle of Photo Cell:

A photo cell consists of a light sensitive metal surface. When solar ( light ) energy from the sun incident on the metal surface, the metal surface emits electrons by photoelectric effect. The emitted electrons then flow in the circuit and constitute the current which is detected and measured by a galvanometer connected in the circuit.

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