Heat transfer Notes

How does heat move from one place to another?

1. Heat transfer is the movement of thermal energy from a region of higher temperature to a region of lower temperature.
2. This direction is not a choice, it is a consequence of particles in the hotter region having, on average, more kinetic energy.
3. When energy spreads out, temperatures tend to equalize.
4. There are three main mechanisms of heat transfer:
   1. Conduction (direct contact)
   2. Convection (bulk movement of fluids)
   3. Radiation (electromagnetic waves).

1. Conduction Transfers Energy Through Collisions In Contact

1. Conduction is heat transfer through direct contact.
2. Energy is transferred by collisions between particles.
3. It occurs mainly in solids.
   1. When two objects touch, particles in the hotter object have higher average kinetic energy.
   2. Collisions at the boundary transfer energy to particles in the cooler object.
   3. Conduction also moves energy through an object (for example, along a metal rod).

Why Metals Conduct Heat Well

1. Different materials conduct heat at different rates.
   1. Good thermal conductors transfer heat quickly (many metals).
   2. Insulators transfer heat slowly (for example wood, plastic, and trapped air).
2. In metals, mobile electrons can move through the material and carry energy efficiently.
3. This is why strong electrical conductors like silver, copper, and gold also tend to be strong thermal conductors.

What Affects The Rate Of Conduction

1. For a flat piece of material (a "slab"), the rate of thermal energy transfer by conduction increases when:
   1. temperature difference across the slab is larger
   2. cross-sectional area is larger
   3. slab is thinner
2. This is often summarized by the proportionality: $$\text{conduction rate} \propto \frac{A\,\Delta T}{L}$$
3. where $A$ is area, $\Delta T$ is the temperature difference, and $L$ is thickness.

2. Convection Transfers Energy By Moving Warm Fluid

1. Convection occurs in liquids and gases.
2. When a fluid is heated, particles move faster and tend to spread out.
3. The fluid becomes less dense and rises, while cooler, denser fluid sinks.
4. This circulation transfers thermal energy from one region to another.

How Convection Currents Form

1. When a fluid is heated, its particles move faster.
2. In many situations, this causes the fluid to expand, so its density decreases.
3. Lower-density fluid rises through higher-density fluid, creating a convection current:
   1. warm, less dense fluid rises
   2. cool, denser fluid sinks
4. This circulation transfers thermal energy through the fluid.

Atmospheric Convection And Hadley Cells

1. On Earth, solar heating is stronger near the equator than at the poles.
2. Air near the equator warms, rises, and then later cools and sinks at higher latitudes, forming large-scale convection patterns.
3. Earth's rotation breaks these patterns into circulating cells called Hadley cells.

3. Radiation Transfers Energy Without Any Medium

1. Radiation does not require particles or contact.
2. This is why the Sun can warm Earth through space.
3. All objects emit thermal radiation, and hotter objects emit more.

Temperature Determines The Radiation Emitted

1. All objects emit thermal radiation.
2. As temperature increases, the emitted radiation shifts toward shorter wavelengths.
3. Most everyday objects mainly emit infrared.
4. Very hot objects (around $1000\ \text{K}$ and above) emit some visible light, so they can glow dull red, then orange, then yellow as the temperature rises.

Heat Transfer Depends On Environment

1. In still air, convection can be slow, and air's low conductivity makes it a good insulator.
2. If air is forced to move (wind, fans, or flapping ears), convection increases, keeping the air near a surface from warming up too much and maintaining a larger temperature difference.
3. In a vacuum, convection and conduction (through air) are greatly reduced, so radiation becomes the main way to transfer heat.

How Insulation Works

1. Insulation reduces unwanted heat transfer.
2. Trapped air is a good insulator because air is a poor conductor.
3. Vacuum prevents both conduction and convection.
4. Reflective surfaces reduce radiation.