Chapter 2b Flashcards
Convection and convection cell
convection the transfer of heat through the bulk movement of matter convection cell a cyclical flow of fluid caused by differences in temperature
and hence fluid densities
Density
mass per unit volume; a measure of how closely packed matter is
Fluid
a substance that flows; a liquid or gas
Heat
energy that flows between systems due to a difference in temperature
Medium
a physical substance through which energy (e.g. heat or sound) travels
Thermal conduction
the transfer of heat through direct contact
Thermal contact
two systems are in thermal contact if heat can transfer between them
Thermal radiation
the transfer of heat in the form of electromagnetic radiation
Vacuum
a region that does not contain matter
Heat flow and rate of conduction formula
Q/t (time) open infinity symbol: triangle T (magnitude of difference in temperature
Heat transfer
For two systems in thermal contact, there is a net flow of heat from the hotter (higher temperature) system to the colder (lower temperature) system. This is called heat transfer, and occurs faster when the difference in temperature between the systems is greater.
We feel a burning sensation when we touch a hot pan not because of the temperature of the metal, but because heat transfers from the hot pan to our cool hand.
If heat didn’t flow from hot to cold, we’d be able to pick up the pan without suffering any burns
How are two objects in thermal contact
If he can flow from one object to another
Is heat temperature
No
Conduction
Form of heat transfer between systems due to collision between their particles, which can occur for all phases of matter (solids, liquids, and gases).
How is heat transferred through conduction
Conduction between two systems only occur when:
• they are in physical contact, and
• they are at different temperatures.
For example, an electric stovetop works by using conduction to transfer
heat from the hot cooktop to the base of a pot or pan.
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The heat transfer rate (measured in J s ) between two systems due to conduction is directly proportional to their difference in temperature.
If area is greater what happens to rate of conduction
It is greater
Insulators v conductors
Something that is good at conducting heat is called a good thermal conductor. • Something that is bad at conducting heat is called a good thermal insulator.
• A list of common conductors and insulators is shown in Table 1.
A good thermal conductor will transfer heat faster, both within itself and to other systems. Pure metals are good conductors because the atoms are close together and they have free-moving electrons. This is why metals feel very cold on cold days and very hot on hot days, they transfer heat rapidly to or from our skin.
The opposite is true for thermal insulators: they transfer heat slowly. Air is an example of a good insulator. Its particles are spaced out and do not collide very often, so the rate that heat is transferred is very slow. A vacuum is the ideal insulator since there are no particles available to have collisions.
Insulator and conductor example (thermal)
C:diamonds , steel, copper, gold
I: wood, air, most plastics, wool
Convection
Convection is a form of heat transfer within a fluid that is due to the overall movement of matter between hotter regions and colder regions.
A hotter substance will generally rise above a cooler substance due
to convection. This is a key factor to consider when designing air conditioners, hot air balloons, and more, which all rely on convection to function.
How is heat transferred through convection
According to the kinetic theory of matter, a higher temperature means that particles have greater average translational kinetic energy. This additional kinetic energy means that particles at higher temperatures tend to have more space between them:
this decreases the density of the material. Fluid with higher density sinks and displaces the less dense fluid, pushing the less dense fluid upwards.
How is heat transferred through convection?
• Convection only occurs in fluids (substances that flow). • Convection can be natural or forced.
Figure 5 shows natural convection where a fluid is constantly heated from the bottom.
• The colder fluid is denser, so sinks and pushes up the hotter fluid.
• The hotter fluid cools down as it rises by transferring heat to its cooler surroundings, increasing in density and falling back down to the bottom.
• If the heating persists, particles continue rising and falling in convection cells.
When we heat up the air inside of a hot air balloon, convection causes it to rise above the cooler outside air, due to its lower density. This provides an overall lift force which is strong enough to lift an entire balloon and basket. Once the air cools down, it becomes denser and falls to where it is then heated up again, and the cycle continues.
Figure 5 Water particles in a pot that is heated at its base. The arrows represent convection cells.
Figure 6 A convection cell formed by a heater
Figure 7 Forced convection by a heater with a fan can heat a room from above.
Convection cells are cyclical flows of fluid caused by ongoing convection. The formation of convection cells explains why indoor heaters are often located close to the floor.
• As shown in Figure 6, the heater draws in cooler air from below and blows out hot air upwards, which cools across the top of the room, falls, and is drawn back to the heater. In this way, thermal energy from the heater is transferred around the room.
Forced convection is any fluid flow that transfers thermal energy where the heating itself does not drive the flow. This implies that another energy source must drive the flow such as a fan blowing air or a spoon mixing a cup of tea. This can transfer heat from anywhere in the fluid, not just from the bottom. For example, if a heater was placed at the top of a room, a built-in fan could convert heat away by blowing hot air downwards
