Physics HL
5
Chapters
329
Notes
Theme A - Space, Time & Motion
Theme B - The Particulate Nature Of Matter
Theme C - Wave Behaviour
Theme D - Fields
Theme E - Nuclear & Quantum Physics
Physics HL
Theme B - The Particulate Nature Of Matter
Unlocking Emissivity: Grey Bodies Vs. Black Bodies Explained
682 words
4 mins read
Last edited on 5th Nov 2024
Table of content
Introduction 🌌
- Black bodies: Ideal objects that absorb all radiation and have a spectrum with a peak at a specific wavelength (λmax) for its temperature (T).
- Grey bodies: Real-world objects that come close to black bodies but not 100%.
Key concepts & laws 🔑
- Wien’s Displacement Law
- Formula: λmax × T = 2.9 × 10–3 m K
- Meaning: The peak wavelength at which a body emits radiation shifts depending on its temperature.
- Stefan–Boltzmann Law
- Original Formula for Black Bodies: P = σAT4
- Modified for Grey Bodies with Emissivity (e): P = eAσT4
- Meaning: The total power radiated by a body is linked to its surface area (A) and absolute temperature (T).
- Emissivity (e)
- Formula: e = (Power emitted by object) / (Power emitted by a black body of the same size and temperature)
- Value Range
- Perfect Black Body: 1
- Object that reflects everything: 0
- All real objects: Between 0 and 1 (e.g., surprisingly, snow has high emissivity at infrared wavelengths, even though it looks white!)
Real-world examples 🌍
- Metal Sample
- Found λmax = 1250nm. Using Wien's law, T = 2300K.
- Emissivity at 1250nm was calculated as 0.33.
- Human Body
- Considered a grey body with emissivity 0.97.
- A person radiates around 3.3MJ of energy in an hour at a body temp of 37°C.
- If the surroundings are 25°C, the net energy radiated is 480kJ.
- Questions & Problem Solving
- For a body with 1.4m2 area and 0.90 emissivity that radiates at 1.1 kW, find its absolute temperature.
- Find the emissivity of a sphere (r=0.12m) at 55°C emitting at 100W.
- For a cube with 0.15m side at 0°C in a 50°C environment:
- Calculate the net power exchange due to radiation.
- Find the initial rate of temperature change (considering mass and specific heat capacity).
- The actual temperature rise might be faster than calculations because the cube might absorb more radiation or there may be other heat sources.
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Physics HL
Theme B - The Particulate Nature Of Matter
Unlocking Emissivity: Grey Bodies Vs. Black Bodies Explained
682 words
4 mins read
Last edited on 5th Nov 2024
Table of content
Introduction 🌌
- Black bodies: Ideal objects that absorb all radiation and have a spectrum with a peak at a specific wavelength (λmax) for its temperature (T).
- Grey bodies: Real-world objects that come close to black bodies but not 100%.
Key concepts & laws 🔑
- Wien’s Displacement Law
- Formula: λmax × T = 2.9 × 10–3 m K
- Meaning: The peak wavelength at which a body emits radiation shifts depending on its temperature.
- Stefan–Boltzmann Law
- Original Formula for Black Bodies: P = σAT4
- Modified for Grey Bodies with Emissivity (e): P = eAσT4
- Meaning: The total power radiated by a body is linked to its surface area (A) and absolute temperature (T).
- Emissivity (e)
- Formula: e = (Power emitted by object) / (Power emitted by a black body of the same size and temperature)
- Value Range
- Perfect Black Body: 1
- Object that reflects everything: 0
- All real objects: Between 0 and 1 (e.g., surprisingly, snow has high emissivity at infrared wavelengths, even though it looks white!)
Real-world examples 🌍
- Metal Sample
- Found λmax = 1250nm. Using Wien's law, T = 2300K.
- Emissivity at 1250nm was calculated as 0.33.
- Human Body
- Considered a grey body with emissivity 0.97.
- A person radiates around 3.3MJ of energy in an hour at a body temp of 37°C.
- If the surroundings are 25°C, the net energy radiated is 480kJ.
- Questions & Problem Solving
- For a body with 1.4m2 area and 0.90 emissivity that radiates at 1.1 kW, find its absolute temperature.
- Find the emissivity of a sphere (r=0.12m) at 55°C emitting at 100W.
- For a cube with 0.15m side at 0°C in a 50°C environment:
- Calculate the net power exchange due to radiation.
- Find the initial rate of temperature change (considering mass and specific heat capacity).
- The actual temperature rise might be faster than calculations because the cube might absorb more radiation or there may be other heat sources.
Unlock the Full Content!
Dive deeper and gain exclusive access to premium files of Physics HL. Subscribe now and get closer to that 45 🌟
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