Environmental Engineering Reference
In-Depth Information
substance breaks down. Coal, when it is burnt, releases heat. Food, when it is digested,
provides the body with heat and movement.
Potential energy is related to gravity. Because of the apparent pull that Earth exerts
upon objects within its gravitational field, material is drawn towards Earth's centre. Thus
objects lying at greater distances from its centre (for example, rocks on a hillside, water
at the top of a waterfall or the air near a mountain summit) possess more potential energy.
This energy is converted to kinetic energy when the rock, the water or the air descends to
lower levels; some energy is converted to heat through friction.
Thermal, kinetic, chemical and potential energy are important to Earth's system but
operate internally and so cannot be observed directly from space. To understand the
results of these different flows of energy, we must look more closely at them,
concentrating on the forms of energy that have significance for the physical geography of
Earth.
METHODS OF ENERGY TRANSFER
The types of energy we have considered so far do not have a uniform distribution over
the globe. Both Earth and the air experience major inequalities in energy receipts and
emissions. As a result of these differences, spatial transfers of energy take place, for
energy is redistributed to minimize the inequalities, or to maintain (or to achieve) an
equilibrium.
To understand how energy is transferred we need to consider a little further the
principles of energy transformation and modification. We have seen already that energy
can exist in a number of forms, and as a general principle energy will be transferred from
areas of high energy status to areas of lower energy status in an attempt to eradicate the
differences. Thus energy differences expressed by the level of temperature in two bodies,
such as the air and the soil, tend to be reduced over time as heat is transferred from the
hotter to the cooler body. In this way the soil is heated during the day when the air is
warm and loses heat energy back to the air at night when the atmosphere is cool (Figure
2.4).
In the case of thermal energy, three main methods of transfer can be identified:
radiation, convection and conduction. Radiation is the process by which energy is
transmitted through space, mainly by the mechanism of electromagnetic waves.
Convection involves the physical
HEAT AND TEMPERATURE
key concepts
We may think that heat and temperature are the same thing. Something that has a lot
of heat may be expected to have a high temperature, but it is not as simple as that.
Heat is a measure of the internal energy of a body or substance. It is due to the
velocity of vibration of the molecules of which that body is composed. If a metal rod
is heated at one end, it will gain heat energy and the molecular movement will be
faster. These molecules will collide with the cooler and more slowly moving
neighbouring molecules of the unheated part of the rod, passing on some of their
energy This process continues along the rod even though the molecules themselves
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