We have learned that chemistry is concerned with the properties of matter and with the energy changes that matter undergoes. We have discussed properties related to the mass and volume of a sample of matter.
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A D V E R T I S E M E N T
In this section we examine properties related to energy. Energy is measured either in joules (J) or in calories (cal), where the conversion factor relating the two units is:
4.184 J = 1 cal
The terms kilojoule (kJ), 1000 J, and kilocalorie (kcal), 1000 cal, are also commonly used. The large calorie (Calorie) used in nutrition is equal to one kilocalorie.
The amount of heat energy associated with a particular sample is dependent on its temperature, its mass, and its composition. Let us consider temperature before discussing its relationship to the energy of a sample.
Some measurement units are defined below:
Ampere (amp): A unit of electrical current or rate of flow
of electrons. One
volt across one ohm of resistance causes a current flow of one ampere.
Amperes are used by utilities and electrical engineers to measure electrical
Joule (J): A unit of electrical energy equal to the work
done when a current of one ampere passes through a resistance of one
ohm for one second (synonymous with watt-second).
Ohm: A measure of the electrical resistance of a material
equal to the resistance of a circuit in which the potential difference of 1 volt
produces a current of 1
ampere. Ohms are used by utilities and electrical engineers to measure the
resistance of wires conducting electricity.
Volt: A unit of electrical force equal to the amount of
electromotive force that will cause a steady current of one
ampere to flow through a resistance of one ohm. High-voltage electricity
moves faster than low-voltage electricity, as seen in the difference between
high-voltage transmission lines used to move electricity quickly throughout a
region and lower-voltage distribution lines used to move electricity directly to
Voltage: The amount of electromotive force, measured in
volts, that exists between two points. Voltage is used to describe the
amount of power produced by a generator.
Watt (W): The rate of energy transfer equivalent to one
ampere under an electrical pressure of one volt. One watt equals 1/746
horsepower, or one
joule per second. It is the product of voltage and current (amperage). The
term "watt" (in addition to the larger measurements of
megawatt) is commonly used to describe the capacity of an electric
generator. For example, a 1,000-watt photovoltaic system has the capacity to
produce 1,000 watts of power at any given time, though it may not consistently
produce this much.