Sources of electrical energy

The driving force in electronic circuits In Chapter 6, the idea of electromotive force was explained. The electromotive force (emf) is the force that causes charges to move and it is measured in volts. Any device that can separate electric charges can act as a source of emf. Some other form of energy is changed into electrical energy when this occurs. Table 8.1 shows the most important of these. Table 8.1: Energy changes in common sources of electromotive force (emf).

Chemical cells Basically, a chemical cell is a container which holds two different electrodes and a liquid called an electrolyte. Figure 8.1 shows a simple cell consisting of a zinc and a copper plate placed in sulphuric acid.

The electrolyte breaks down into ions and these react with the electrodes. The chemical reaction which occurs causes a build-up of electrons on one of the electrodes and a deficiency of electrons on the other. Although this type of cell is not used as a commercial source of emf it illustrates how chemical cells work. There are two main types of chemical cells commonly used. These are called primary cells and secondary cells. Primary cells The dry cell is the most common primary cell and is similar to the simple chemical cell described above. The electrodes are usually zinc (anode) and manganese dioxide (cathode) with carbon as the unreactive electrode. The electrolyte is a moist paste containing ammonium chloride and zinc chloride. This cell produces 1.5 V. It is easy to handle, is very robust and can maintain a constant potential difference while it is operating FLVbattery

A problem with dry cells is their relatively short shelf life + high cost

Electrical energy is only used when the cell is connected into a circuit so that current can flow. However, from the time cells are made, other unwanted chemical reactions occur which result in a decrease in efficiency of the cell. Normally, an unused cell or battery’s minimum acceptable performance is taken to be 85% of what it was when it was first produced. The time for this to occur is known as the shelf life of the cell or battery. The shelf life of dry cells varies according to their size and the temperature where they are stored. Typically, a AAA cell would have a shelf life of 3-4 months, whereas a D cell will last from 9-10 months. Several dry cells may be placed together to form a battery. The small 9.0 V batteries which are commonly used actually contain six very small dry cells connected together. This is shown in Figure 8.7. As the chemicals in a dry cell are used up, the work that can be done by the cell is reduced. Eventually the cell must be discarded because dry cells cannot be recharged. Other types of dry cells now commonly used are the alkaline energiser cell, the mercury cell and the silver oxide cell. Mercury and silver oxide cells are often called button cells and are often used as watch and calculator batteries

Secondary cells Many chemical cells used today are secondary cells. These do not produce energy in the same way as primary cells, but rather they store energy. Secondary cells contain two electrodes and an electrolyte as before, but these do not react by themselves as they do in primary cells. Secondary cells must first be charged by passing current through them. This produces a chemical reaction which stores electrical energy. As the cell is used, the reaction is reversed and the stored energy is used. When a cell becomes flat, electrical energy must again be supplied to reverse the chemical reaction which stores the energy. The most common type of secondary cell is the lead-acid accumulator. These are used as car batteries and in an increasing number of locations in conjunction with solar cells and wind generators. Details of the lead-acid accumulator are given in Figure 8.3 and Table 8.2 FLV

Generators Generators are the most important means of producing electricity. If a piece of wire is moved through a magnetic field, the charges in the wire experience a force. This force is the emf and if the ends of the wire are connected in a circuit, current will flow This process is called electromagnetic induction. The energy used to move the conductor through the magnetic field is changed into electrical energy. Figure 8.5 shows a bicycle generator that uses this principle to generate electricity and provide power for the light. The huge generators in power stations produce electricity using the same principle.

Piezoelectricity Crystals of certain materials, such as quartz, produce small potential differences across themselves if they are put under stress. The potential difference produced depends upon the pressure being applied. The voltages produced are very small, but they can be easily increased if the currents produced by them are fed into an amplifier. Piezoelectric crystals are used in gas lighters. (squeeze a handle + produce a spark) Piezoelectric crystals are used to make loudspeaker parts (tweeters) Thermocouples A device called a thermocouple can be made by joining wire of two different metals together at one end. When the unconnected ends of the wires are used to complete a circuit a current is produced because of the emf. This occurs only if a temperature difference exists between the joint of the two metal ends and the circuit ends of the wires. Thermocouples are useful to measure very high and low temperatures in specialised situations.

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