Section 2.2: Heat Transfer
Heat transfer is the process by which thermal energy moves from one object or substance to another due to temperature differences. Understanding these mechanisms is crucial for thermodynamics and energy applications.
Key Concepts
- Conduction: Transfer of heat through a material without bulk motion, typically in solids.
- Convection: Transfer of heat by the movement of fluid (liquid or gas).
- Radiation: Transfer of energy through electromagnetic waves, does not require a medium.
- Thermal Insulators and Conductors: Materials that resist or promote heat transfer.
- Heat Flow Equation (Conduction): \( Q = \frac{k A \Delta T t}{L} \)
Example
A metal rod of length 0.5 m and cross-sectional area 0.01 m² has one end at 100°C and the other at 25°C. Thermal conductivity \( k = 200 \, W/m·K \). Find the heat transferred in 10 seconds.
Using conduction formula:
\( Q = \frac{k A \Delta T t}{L} = \frac{200 \cdot 0.01 \cdot (100-25) \cdot 10}{0.5} = 3000 \, \text{J} \)
Practice Problems
- Explain the difference between conduction, convection, and radiation with examples.
- A copper plate has thickness 0.02 m and area 0.5 m². If \( \Delta T = 50°C \) and \( k = 385 \, W/m·K \), find heat transferred in 5 s.
- Why does a metal spoon feel colder than a wooden spoon at the same temperature?
- Explain how convection currents form in a heated room.
- A black body radiates energy. How does changing its temperature affect radiation power?