Language. Recall that energy can also be transferred between a system and
its environment as work W via a force acting on a system. Heat and work, unlike
temperature, pressure, and volume, are not intrinsic properties of a system. They
have meaning only as they describe the transfer of energy into or out of a system.
Similarly, the phrase “a $600 transfer” has meaning if it describes the transfer to
or from an account, not what is in the account, because the account holds money,
not a transfer.
Units. Before scientists realized that heat is transferred energy, heat was
measured in terms of its ability to raise the temperature of water. Thus, the
calorie (cal) was defined as the amount of heat that would raise the temperature of 1 g of water from 14.5ЊC to 15.5ЊC. In the British system, the corresponding unit of heat was the British thermal unit (Btu), defined as the amount of
heat that would raise the temperature of 1 lb of water from 63ЊF to 64ЊF.
In 1948, the scientific community decided that since heat (like work) is
transferred energy, the SI unit for heat should be the one we use for energy —
namely, the joule. The calorie is now defined to be 4.1868 J (exactly), with no reference to the heating of water. (The “calorie” used in nutrition, sometimes called the
Calorie (Cal), is really a kilocalorie.) The relations among the various heat units are
1 cal ϭ 3.968 ϫ 10
Ϫ3
Btu ϭ 4.1868 J.
(18-12)
The Absorption of Heat by Solids and Liquids
Heat Capacity
The heat capacity C of an object is the proportionality constant between the heat
Q that the object absorbs or loses and the resulting temperature change ⌬T of
the object; that is,
Q ϭ C ⌬T ϭ C(T f Ϫ T i ),
(18-13)
in which T i and T f are the initial and final temperatures of the object. Heat
capacity C has the unit of energy per degree or energy per kelvin. The heat
capacity C of, say, a marble slab used in a bun warmer might be 179 cal/CЊ, which
we can also write as 179 cal/K or as 749 J/K.
The word “capacity” in this context is really misleading in that it suggests analogy
with the capacity of a bucket to hold water. That analogy is false, and you should not
think of the object as “containing” heat or being limited in its ability to absorb heat.
Heat transfer can proceed without limit as long as the necessary temperature difference is maintained.The object may, of course, melt or vaporize during the process.
Specific Heat
Two objects made of the same material — say, marble — will have heat capacities
proportional to their masses. It is therefore convenient to define a “heat capacity
per unit mass” or specific heat c that refers not to an object but to a unit mass of
the material of which the object is made. Equation 18-13 then becomes
Q ϭ cm ⌬T ϭ cm(T f Ϫ T i ).
(18-14)
Through experiment we would find that although the heat capacity of a particular marble slab might be 179 cal/CЊ (or 749 J/K), the specific heat of marble itself
(in that slab or in any other marble object) is 0.21 cal/g иCЊ (or 880 J/kg иK).
524
CHAPTE R 18 TE M PE RATU R E, H EAT, AN D TH E FI RST L AW OF TH E R M ODYNAM ICS
We are led then to this definition of heat:
Heat is the energy transferred between a system and its environment because of
a temperature difference that exists between them.
its environment as work W via a force acting on a system. Heat and work, unlike
temperature, pressure, and volume, are not intrinsic properties of a system. They
have meaning only as they describe the transfer of energy into or out of a system.
Similarly, the phrase “a $600 transfer” has meaning if it describes the transfer to
or from an account, not what is in the account, because the account holds money,
not a transfer.
Units. Before scientists realized that heat is transferred energy, heat was
measured in terms of its ability to raise the temperature of water. Thus, the
calorie (cal) was defined as the amount of heat that would raise the temperature of 1 g of water from 14.5ЊC to 15.5ЊC. In the British system, the corresponding unit of heat was the British thermal unit (Btu), defined as the amount of
heat that would raise the temperature of 1 lb of water from 63ЊF to 64ЊF.
In 1948, the scientific community decided that since heat (like work) is
transferred energy, the SI unit for heat should be the one we use for energy —
namely, the joule. The calorie is now defined to be 4.1868 J (exactly), with no reference to the heating of water. (The “calorie” used in nutrition, sometimes called the
Calorie (Cal), is really a kilocalorie.) The relations among the various heat units are
1 cal ϭ 3.968 ϫ 10
Ϫ3
Btu ϭ 4.1868 J.
(18-12)
The Absorption of Heat by Solids and Liquids
Heat Capacity
The heat capacity C of an object is the proportionality constant between the heat
Q that the object absorbs or loses and the resulting temperature change ⌬T of
the object; that is,
Q ϭ C ⌬T ϭ C(T f Ϫ T i ),
(18-13)
in which T i and T f are the initial and final temperatures of the object. Heat
capacity C has the unit of energy per degree or energy per kelvin. The heat
capacity C of, say, a marble slab used in a bun warmer might be 179 cal/CЊ, which
we can also write as 179 cal/K or as 749 J/K.
The word “capacity” in this context is really misleading in that it suggests analogy
with the capacity of a bucket to hold water. That analogy is false, and you should not
think of the object as “containing” heat or being limited in its ability to absorb heat.
Heat transfer can proceed without limit as long as the necessary temperature difference is maintained.The object may, of course, melt or vaporize during the process.
Specific Heat
Two objects made of the same material — say, marble — will have heat capacities
proportional to their masses. It is therefore convenient to define a “heat capacity
per unit mass” or specific heat c that refers not to an object but to a unit mass of
the material of which the object is made. Equation 18-13 then becomes
Q ϭ cm ⌬T ϭ cm(T f Ϫ T i ).
(18-14)
Through experiment we would find that although the heat capacity of a particular marble slab might be 179 cal/CЊ (or 749 J/K), the specific heat of marble itself
(in that slab or in any other marble object) is 0.21 cal/g иCЊ (or 880 J/kg иK).
524
CHAPTE R 18 TE M PE RATU R E, H EAT, AN D TH E FI RST L AW OF TH E R M ODYNAM ICS
We are led then to this definition of heat:
Heat is the energy transferred between a system and its environment because of
a temperature difference that exists between them.