Download Introduction: Sources of electric current

SOURCES OF ELECTRIC CURRENT
There are several different devices that can supply the voltage necessary to
generate an electric current. The two most common sources are generators
and electrolytic cells.
A Generators
Generators use mechanical energy, such as water pouring through a dam or
the motion of a turbine driven by steam, to produce electricity. The electric
outlets on the walls of homes and other buildings, from which electricity to
operate lights and appliances is drawn, are connected to giant generators
located in electric power stations. Each outlet contains two terminals. The
voltage between the terminals drives an electric current through the appliance
that is plugged into the outlet.
Power sources for generators can be steam from geothermal vents, or a
nuclear reactor. Hydroelectricity would also fall under this category.
B Electrolytic Cells
Electrolytic cells use chemical energy to produce electricity. Chemical
reactions within an electrolytic cell produce a potential difference between the
cell’s terminals. An electric battery consists of a cell or group of cells
connected together.
C Other Sources
Fuel Cells
There are many sources of electric current other than generators and
electrolytic cells. Fuel cells, for example, produce electricity through chemical
reactions. Unlike electrolytic cells, however, fuel cells do not store chemicals
and therefore must be constantly refilled.
Certain sources of electric current operate on the principle that some metals
hold onto their electrons more strongly than other metals do. Platinum, for
example, holds its electrons less strongly than aluminum does. If a strip of
platinum and a strip of aluminum are pressed together under the proper
conditions, some electrons will flow from the platinum to the aluminum. As the
aluminum gains electrons and becomes negative, the platinum loses
electrons and becomes positive. This was observed in the lab activity we did.
The strength with which a metal holds its electrons varies with temperature. If
two strips of different metals are joined and the joint heated, electrons will
pass from one strip to the other. Electricity produced directly by heating is
called thermoelectricity.
Photovoltaic Cells
Some substances emit electrons when they are struck by light. Electricity
produced in this way is called photoelectricity. When pressure is applied to
certain crystals, a potential difference develops across them. Electricity thus
produced is called piezoelectricity. Some microphones work on this principle.