Energy
Energy
is simply defined as the capacity to do work or supply heat. Work is defined as a dynamic
change such as an object moving a distance against a force.
Kinetic
energy
is the energy of a mass in motion.
Potential
energy
is defined as stored energy. Can you think of any examples
of potential energy?
What
are the ways in which energy is produced for our consumption?
Transfer
of Thermal Energy
A
common unit of energy is the calorie. A calorie is defined as the amount of heat
that is needed to raise the temperature of one gram of water one degree
centigrade. Modern convention uses a unit of energy called a joule. A calorie is
equal to 4.184 joules. A joule is approximately the energy needed to lift 2000 grams a distance of 10 centimeters.
Heat
is transferred by either direct transfer from one object touching another or by
the emission of radiation in the form of infrared radiation. Two results can occur as a result
of this transfer of thermal energy. A substance can undergo either a change in
temperature or a change in physical state.
When
a substance exchanges heat with its surroundings without changing its physical
state, the substance will undergo a change in temperature. This change in
temperature depends on a property called specific heat capacity.
Specific
heat capacity is the "heat needed to produce a given temperature change (in
Celsius or Kelvin) per
gram of substance" and is expressed in the units J/gK.
Specific heat capacity depends
on both the type of substance and the mass of the substance.
The "D"
means "change in" and "q" represents the quantity of heat.
DT
and q can have either negative or positive signs and Csp will always
have a positive sign. The convention used is “q” has a positive value if
heat is added to the substance from its surroundings and a negative value if the
substance loses heat to its surroundings. Heat capacity can be used just like
any other conversion factor except that you have to multiply by more measured
quantities.
The
specific heat capacity of water is 1 calorie/(gram x degree). How
many calories are needed to raise the temperature of 50 g of water 10 oC?
The
specific heat capacity of aluminum is 0.217 calorie/g deg. How many calories are
needed to raise the temperature of 50 g of aluminum 10 oC?
Notice
that considerably less heat was needed to heat the aluminum than to heat the
water. The
high specific heat capacity of water is very important as a heat moderator.
It takes a large quantity of heat to significantly change the temperature
of a large mass of water in an ocean or large lake.
It also is difficult to remove the heat from a large quantity of water.
The oceans and lakes thus moderate temperatures on land and the aquatic
life in these bodies of water or on the land by these bodies of water do not
experience dramatic, rapid temperature changes. Most of the hottest and coldest
regions on Earth are those regions that are distant from large bodies of water.
Our body temperature also remains fairly constant due to the fact that we
are made up of 70-80% water.
Changes of Physical State
When the physical state of a substance
is changed, heat is either absorbed or liberated but the temperature remains
constant.
The
heat required to melt ice without any increase in the temperature of the
system is equal to 80
calories/g. This is called the heat
of fusion
and it relates to the energy needed to break the hydrogen-bonds that are
holding the water molecules together in the solid state.
The
heat required to boil water without an increase in temperature is equal to 540
calories/g. This is called the heat
of vaporization
and it relates to the energy needed to break the hydrogen-bonds holding the
water molecules together in the liquid state.
The
energy of a hydrogen-bond in water is about 5% of the energy of the
hydrogen-oxygen chemical bond energy. Although the energy of the hydrogen-bond
is small it has a significant effect on the amount of energy that is required
to melt ice or boil water.
The
heat of fusion and the heat of vaporization can also be used as conversion
factors.
How
many calories will it take to melt 50 g of ice at 0 oC to water at
0 oC?
Problems
1.
How many calories are required to raise the temperature of 5 grams of water
from 10oC to 15oC?
2.
What are the methods by which electrical energy can be generated?
What are the major disadvantages associated with each method?
3.
How many calories will it take to boil 100 g of water if
the water is already at 100 oC (the boiling point of water)?
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Copyright © January 2001 by Richard C.
Banks...all rights reserved.