Electricity

Static Electricity

Charged particles:

The charge is developed in nonmetals like plastic, wool, nylon, polyester, ebonite, etc. in insulating materials due to friction. These materials when rubbed against each other develop the charge. When we put off cyclic or polyester dress, we hear the clicking sound due to the transfer of charges. The dress materials get charged due to the friction of the body.

Electric Cells

A cell is a device where a potential difference between two conductors is maintained by a conversion of chemical energy into electrical energy. A cell consists of two metal rods called electrodes dipped in a solution called electrolyte.

When the cell is in pen circuit, the potential difference between electrodes is called its electromotive force (e. m. f). The strength of the cumulative e.m.f., which a cell can produce in a given circuit, depends on the e.m.f. The e.m.f of cells depend on

1. materials of its electrodes.
2. the nature of its electrolyte.

Simple cell

This cell was invented by Alessandro Volta in 1800. It consists of two plates, one copper and other of zinc, both are dipped in dilute sulphuric acid taken in the glass container. When two plates are connected by, a copper wire provided with a bulb in between, the bulb glows showing the flow of electric current. The copper and zinc plates are called the electrodes of the cell. Zinc is the negative electrode as it supplies the electron and copper is the positive electrode. Dilute sulphuric acid is the electrolyte. The electron current flows from zinc to copper. The potential difference between the terminals of the cell is 1.08V. Therefore, it is not suitable for use in devices like torches, calculators, transistors, etc.

The reaction inside the cell can be shown as

Zn → Zn⁺ + 2e^-
H₂SO₄ → 2H⁺ + SO₄^--
2H ⁺ +2e^- → H₂
Zn + H₂SO₄ → ZnSO₄ + H₂

Defects of simple cell

We cannot draw current for a long time as this cell has defects. There are two types of defects in the voltaic cell. These are:

1. Polarization
2. Local action

Polarization

It is caused by the deposition of the hydrogen gas on the surface of the copper plate in the form of small bubbles. They prevent the hydrogen ions from reaching the copper plate and thus the current given by the cell, decreases gradually. Polarization stops functioning of the cell. Polarization can be avoided by brushing off the hydrogen bubbles or by putting a chemical called potassium dichromate in acid. Potassium dichromate oxide changes hydrogen to water.

Local action

The combined zinc used in the cell contains certain impurities like carbon, iron, etc. Because of this local current is set up between zinc and these impurities in addition to the main current. These local currents cannot be utilized and thus constitute wastage of energy. Local action reduces the life of the cell. Amalgamating is the coating of zinc plate with mercury. Mercury covers these impurities present in the zinc plate. Local action can be avoided by amalgamating the surface of the zinc plate.

Dry cell

It was invented by Laclanche Georges in 1865. A dry cell is made without using the liquid. It consists of a zinc container, which also serves as the negative terminal. The electrolyte is a paste of ammonium chloride. A carbon rod with a brass cap serves as the positive electrode. It is surrounded by a mixture of powdered manganese dioxide and carbon particles.

Hydrogen is formed in the chemical reaction that takes place in the cell. The manganese dioxide reacts with the hydrogen formed and prevents it from collecting on the zinc electrode. In this way, the presence of manganese dioxide prevents the polarization. A porous seal at the top prevents the chemicals from dying out. The cell is dry from outside, but the chemicals are wet. The local action cannot be removed completely so it may decrease the life of the cell.

The electrons flow in a wire connecting the two terminals from the zinc to the carbon electrode. The dry cell, being compact, light and portable, is very convenient to use. Dry cells cannot be charged and reused and therefore they are called primary cells. Cells used in vehicles for lighting purposes or as anemergency light in our home called lead-acid cell. They can be charged and used again. Therefore, these cells are called secondary cells.

The action of the cell can be described as follows:

Zn → Zn⁺⁺ + 2e^-
2NH₄Cl → 2NH₃ + 2H⁺ + 2Cl^-
2N⁺⁺ + 2Cl^- → ZnCl₂
Zn + 2NH₄Cl → 2NH₃ + ZnCl₂+ 2H⁺ + 2e^-
The electrons so released are collected by zinc rod making it as negative potential with respect to the electrolyte.
The ammonia gas (NH3) so produced escapes and hydrogen ions react with manganese dioxide (MnO2) as follows the forming water.
H⁺ + MnO₂ → MnO₃ + H₂O+ 2e⁺⁺

Thus, the positive charge is transferred to the carbon rod, which gains positive potentials with respect to the electrolyte.

Advantages of dry cell over simple cell

1. The electrolyte is in the form of the paste. They do not contain liquid; therefore, it is easier to carry from one place to another place.
2. They are compact.
3. They are cheaper than other cells.

Connection of cells

In our daily life, we use more than two cells for different purposes. These cells are connected for use and this arrangement is called combination or connection of cells.

Cells in series:

When a number of cells are connected in series, the negative pole of one cell is connected to the positive pole of the cells and so on. When the internal resistance the cells are connected in series, to get the maximum current.

Cells in parallel combination:

When a number of cells are connected in parallel, all the positive poles are connected to one point and all the negative poles are connected to the other. When the external resistance is negligible in comparison to the internal resistance, the cells are connected in parallel so as to keep the current constant.