Emitting & absorbing EM radiation (Edexcel GCSE Physics Combined Science): Revision Notes
📚 Revision Notes
Emitting & absorbing EM radiation
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Intensity: The amount of power per unit area. In terms of EM radiation, intensity refers to how much energy is transferred in a given area over a certain time.
Every object is constantly emitting and absorbing EM radiation across a range of wavelengths.
The distribution and intensity of the emitted wavelengths depend on the object's temperature:
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- As the temperature increases, the intensity of every wavelength emitted increases.
- However, the intensity increases more rapidly for shorter wavelengths, causing the peak wavelength (the wavelength with the highest intensity) to shift to shorter wavelengths as the temperature rises.
Absorbing & Radiating EM Radiation and Temperature
The rate at which an object absorbs and radiates EM radiation affects its temperature.
1. Average Power and Heating:
- If the object absorbs more energy than it radiates, its temperature increases.
2. Cooling:
- If the object radiates more energy than it absorbs, its temperature decreases.
3. Constant Temperature:
- An object maintains a constant temperature if it radiates and absorbs the same average power.
How Radiation Affects Earth's Temperature
The Earth's temperature is determined by the balance between the radiation it reflects, absorbs, and emits.
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1. Daytime:
- During the day, the Earth absorbs a lot of radiation from the Sun.
- Some of this radiation is reflected, but most is absorbed by:
- The atmosphere.
- Clouds.
- The Earth's surface.
- This absorption causes a local increase in temperature.
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2. Nighttime:
- At night, radiation is emitted by the atmosphere, clouds, and Earth's surface, causing a local decrease in temperature.
3. Overall Temperature:
- The Earth's temperature stays fairly constant if the amount of absorbed radiation is roughly equal to the amount emitted.
4. Global Warming:
- Changes in the atmosphere can upset this balance. If the atmosphere absorbs more radiation without emitting an equal amount, the Earth's overall temperature will rise, contributing to global warming.
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Investigating How Well Different Surfaces Emit Radiation
You can investigate how effectively different surfaces emit radiation with a simple experiment:
Steps:
- Wrap four identical test tubes with different surface materials or colours (e.g., shiny, matte, black, white).
- Boil water in a kettle and fill each test tube with the same volume of water.
- Use a thermometer to measure the temperature of the water in each test tube at regular intervals (e.g., every minute).
- Seal the test tubes with bungs to prevent heat loss through evaporation.
Results:
- The temperature of the water will decrease faster in test tubes with materials that are better emitters of radiation.
- Matte or dull surfaces are better emitters than shiny surfaces.
- Black surfaces are better emitters than white surfaces.