PHY4: ELECTRICITY (CELLS)

This unit is about cells in electricity both primary and secondary.

Cells

A cell is a device that converts chemical energy into electrical energy. A group of cells is called a battery.

Types of cells

There are two types of cells;

  1. Primary cells
  2. Secondary cells

Primary cells

It is one in which current is produced as a result of a non reversible chemical change taking place within its components. A primary cell cannot be recharged after its voltage has dropped but is thrown away.

PHY4: ELECTRICITY (CELLS) 1

Examples of primary cells are;

-A simple cell

-A dry cell

-Leclanche cell

Secondary cell

It’s one that can be recharged when its voltage has dropped by passing a current through it from another source of current.

PHY4: ELECTRICITY (CELLS) 2

Examples

Lead – acid cell

Nickel – cadmium alkaline cell.

Secondary cells are sometimes called storage cells or accumulators

A simple cell

A simple cell consists of copper and zinc plates joined with a conductor and placed in an electrolyte of dilute sulphuric acid.

ELECTRICITY 1

When the plates are joined by the conductor as shown above the zinc plate begins to dissolve in the acid and bubbles of hydrogen are formed on the copper plate.

Current of electrons flows through the wire from zinc to the copper plate.

Explanation

When pure acid is added to water, the sulphate [SO42-] group of atoms separates from the two hydrogen atoms taking two electrons with them. One from each hydrogen atom. The hydrogen atoms therefore a net positive charge.

H2SO4 2H+ + SO42-

Zinc toms go into solution to form zinc ions leaving two electrons for each atom on the plate.

electricity 2

The electrons left by the dissolving process on the zinc plate move along the conductor to the copper plate. As the zinc ions enter the solution. Hydrogen ions leave the solution and deposited onto the copper plate.

These hydrogen ions received on electrode from the copper to become neutral atoms and are liberated to form gas bubbles. The copper plate becomes positively changed since it loses electrons. This enables it to attract electrons through the conductor from the zinc plate. This movement of electrons through the conductor is called an electric current.

The conventional direction of an electric current is from the positive terminal towards the negative terminal and these represented by an arrow in current diagram.

electricity 3

Electro motive force [EMF]

The electromotive force of a cell is the potential difference across its terminal on an open circuit.

electricity 4

Defects/ disadvantages of a simple cell

  1. Polarization

This is the formation of layer of hydrogen bubbles on the copper plate. This layers weakens the current hence current flow reduces i.e. the gas partially insulates the plate and hence increase internal resistance of the cell.

The hydrogen layer sets up a reverse voltage in the cell in opposition to that did to the copper and zinc.

Prevention/ remedy

A depolarizing agent is added to the cell e.g. potassium dichromate or magnesium [iv] oxide is added to the acid to oxide the hydrogen to water.

  1. Local action

This is coming off of hydrogen bubbles from the zinc plate. It is caused by the presence of impurities in the plate

Prevention/ remedy

The zinc plate should be cleaned in sulphuric acid then rubbed with a small globule of mercury over the surface with a piece of cotton. The amalgam all over the surface and the impurities are covered and can no longer come into contact with the acid.

A dry cell

electricity dry cell

Secondary cells

  1. Lead – acid cell

In this cell both terminals are made of lead. Red lead oxide [Pb3 O4] as the positive plate and lead [11] oxide [PbO] as the negative plates. If both plates have large surface area and are close together the internal resistance will be low and hence a large current will be produced. The electrolyte used is dilute H2SO4 [aq]. Car battery may consist of six lead – acid cells.

Care of lead – acid cells

  1. The level of H2SO4[aq] should be inspected regularly and any less made up with distilled water
  2. Acid should never be added except when spillage has occurred
  3. The cell should be charged regularly
  4. It should not be left discharged for a long time
  5. Do not short circuit

2. Alkaline batteries / cells

The positive plate is made of nickel hydroxide. The negative plate is made of cadinium or iron. Caustic potash [potassium hydroxide] is the electrolyte. They are more expensive than lead acid cells and have a much longer life span. They can be left in a discharged condition for a long time without harm.

-they are not affected by large current.

Disadvantages

  • The e.m.f of the cell is only 1.25V

 

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ASSIGNMENT : ELECTRICITY (CELLS) assignment MARKS : 10  DURATION : 1 week, 3 days

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