Transcript Slide 1

Sources of electrical energy
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
FLVbattery
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
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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