Heat transfers through the
window at a rate of 1700 J/s.
What is its thermal conductivity
constant?
k = 0.93 W/m·K
18.19 - Convection
Convection: Heat transfer through a gas or
liquid caused by movement of the fluid.
Gases and liquids usually decrease in density when they are heated (liquid water near
0°C is a notable exception). When part of a body of liquid or gas is heated, the warmed
component rises because of its decreased density, while the cooler part sinks. This
occurs in homes, where heat sources near the floor heat the nearby air, which rises and
moves throughout the room. The warmer air displaces cooler air near the ceiling,
causing it to move near the heat source, where it is heated in turn. This transfer of heat
by the movement of a gas or liquid is called convection.
All kitchen ovens, like the one shown in Concept 1, rely largely on convection for
baking. The heating element at the bottom of the oven warms the air next to it, causing
it to rise. The heated air then reaches the food in the oven to warm it, while the cooler
air sinks to the bottom of the oven. So-called “convection ovens” speed this process
with fans that cause the air to circulate more quickly.
Convection occurs in liquids as well as in gases. If you stir spaghetti sauce as it heats, you are accelerating the process of convection. Again,
your goal is to uniformly distribute the thermal energy.
If you see a hawk soaring upward without flapping its wings, it may be riding what is called a “thermal.” As the Sun warms the ground, the
nearby air also becomes warmer. In the process, it becomes less dense, and is forced upward by air that is cooler and denser. A bird can ride
this upward draft.
Convection
Heat transfer due to movement in gases
and liquids