A close analogy exists between the flow of energy by heat because of a temperature difference (see Section 19.6) and the flow of electric charge because of a potential difference. In a metal, energy dQ and electrical charge dq are both transported by free electrons. Consequently, a good electri- cal conductor is usually a good thermal conductor as well. Consider a thin conducting slab of thickness dx, area A, and electrical conductivity ơ, with a potential difference dV between opposite faces. (a) Show that the current I = dq/dt is given by the equation on the left: Charge conduction Thermal conduction dq = oA dt dT kA dt dQ AP dx dx In the analogous thermal conduction equation on the right (Eq. 19.17), the rate dQ/dt of energy flow by heat (in SI units of joules per second) is due to a temperature gradient dT/dx in a material of thermal conductivity k. (b) State anal- ogous rules relating the direction of the electric current to the change in potential and relating the direction of energy flow to the change in temperature. dT P = kA dx (19.17)

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A close analogy exists between the flow of energy by heat
because of a temperature difference (see Section 19.6) and
the flow of electric charge because of a potential difference.
In a metal, energy dQ and electrical charge dq are both
transported by free electrons. Consequently, a good electri-
cal conductor is usually a good thermal conductor as well.
Consider a thin conducting slab of thickness dx, area A,
and electrical conductivity ơ, with a potential difference dV
between opposite faces. (a) Show that the current I = dq/dt
is given by the equation on the left:
Charge conduction
Thermal conduction
dq
= oA
dt
dT
kA
dt
dQ
AP
dx
dx
In the analogous thermal conduction equation on the right
(Eq. 19.17), the rate dQ/dt of energy flow by heat (in SI units
of joules per second) is due to a temperature gradient
dT/dx in a material of thermal conductivity k. (b) State anal-
ogous rules relating the direction of the electric current to
the change in potential and relating the direction of energy
flow to the change in temperature.
Transcribed Image Text:A close analogy exists between the flow of energy by heat because of a temperature difference (see Section 19.6) and the flow of electric charge because of a potential difference. In a metal, energy dQ and electrical charge dq are both transported by free electrons. Consequently, a good electri- cal conductor is usually a good thermal conductor as well. Consider a thin conducting slab of thickness dx, area A, and electrical conductivity ơ, with a potential difference dV between opposite faces. (a) Show that the current I = dq/dt is given by the equation on the left: Charge conduction Thermal conduction dq = oA dt dT kA dt dQ AP dx dx In the analogous thermal conduction equation on the right (Eq. 19.17), the rate dQ/dt of energy flow by heat (in SI units of joules per second) is due to a temperature gradient dT/dx in a material of thermal conductivity k. (b) State anal- ogous rules relating the direction of the electric current to the change in potential and relating the direction of energy flow to the change in temperature.
dT
P = kA
dx
(19.17)
Transcribed Image Text:dT P = kA dx (19.17)
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