Now we use the fact that the heat transfers sum to zero as the two masses of water reach thermal equilibrium to calculate the final temperature
of the total mass of water.
Step Reason
5. QL + QS = 0 equation above
6. cmL(T – TLf) + cmS(T – TSf) = 0 specific heat equation
7. solve for T
8. substitute values
9. T = 9.42°C evaluate
18.17 - Conduction
Conduction: The flow of thermal energy
directly through a material without motion of
the material itself.
When a frying pan is placed on a burner, heat flows from the burner to the pan. The
heat then spreads through the pan, soon reaching the handle even though the handle is
not in direct contact with the burner. This process illustrates the flow of thermal energy
via conduction.
Conduction is the direct flow of thermal energy without a net motion of the materials
involved. The heat flows from the bottom of the pan to its handle. Heat also flows by
conduction where the burner and the bottom of the pan are in direct contact.
Conduction results from interactions at the atomic level of particles in an object:
molecules, atoms and, in metals, electrons. Particles with greater average energy
collide with nearby lower-average-energy particles, increasing their energy. Heat spreads as particles collide with their neighbors, and those
neighbors collide with their neighbors, and so on. In this way, heat flows throughout the object. Although motion is involved at this level, the pan
itself does not move, and this is part of the definition of conduction: The flow of thermal energy within an object that does not involve motion.
Different materials conduct heat at different rates. This is the topic of the next section.
Conduction
Heat flow within object
·Does not involve “bulk” motion18.18 - Thermal conduction quantified
For appliances that heat or chill (such as ovens or
refrigerators), and indeed for an entire house or
apartment, thermal conduction is the dominant form
ofunintended heat transfer. Since this transfer of
heat is costly and wasteful, a good amount of effort is
spent reducing it. In this section, we focus on building
materials that are designed to minimize this heat
transfer, keeping the interior of a house cool during
the summer and warm during the winter.
The thermal conductivity of a material specifies how
quickly heat transfers through it. Above, you see a
table of thermal conductivity values for materials such
as wood, insulating foam, glass, and some metals.
The letter k represents the thermal conductivity of a material. Materials with small values for k are used as insulators to reduce the transfer of
heat.
The units for thermal conductivity are joules per second-meter-Kelvin. Since watts are joules per second, this is the same as watts per meter-
Kelvin, and these are the units used in the table above.
Pc represents the rate of heat transfer through conduction. As shown in Equation 1, Pc is defined as the heat transferred divided by the
elapsed time. It is calculated for a slab of material like a glass window pane using the equation shown in Equation 2. The rate of heat transfer
increases with the thermal conductivity kof the material that makes up the slab, the area of the slab, and the temperature difference between
its two sides. It decreases with the thickness of the material. The equation also can be used to explain why the units for k have meters in the
denominator, not “meters squared.” Because there is a length term (thickness) in the denominator of the heat-transfer equation, a length term
cancels out of the units.
Values for k and R.