Embedded Files
General Resources:
General Resources:
Kognity Textbook: Chapter 8 (Use your ACS Login)
IB Physics Site: Topic 8 - Comprehensive notes
IB Physics Site: Topic 8 - More notes
Topic 8 Flashcards - Vocab Devo.
White Balance and Photography
White Balance and Photography
Temperature of a Lamp
Temperature of a Lamp
Using the data below and Wien's Law Eq determine the temperature at each voltage.
Compare the blackbody spectrum of the sun to visible light.
Describe the blackbody spectrum of a light bulb. Where is the peak of the spectrum? Why do light bulbs get hot? Do they seem efficient?
Describe what happens to the shape and peak value of the spectral radiance curve as you change the temperature.
Imagine that you see two stars in the sky, one is glowing orange and the other is glowing blue. Which one is hotter?
Determine the relationship between the peak wavelength and temperature of the blackbody
The surface temperature of a black body can be found if the wavelength of the maximum intensity of radiation on a black body curve is known. The effect of temperature on the black body curve can be investigated using the simulation to the right and above (PhET)
The surface temperature of a black body could also be found if the wavelength of the maximum intensity of radiation on a black body curve is known. The effect of temperature on the black body curve can be investigated using the simulation above (HERE).
Determine the Temperature of the Sun.
Determine the Temperature of the Sun.
Power of the Sun
Use the Vernier Spectrometer to record the spectrum of the Sun. The spectrum can be used to identify elements but also to determine the surface temperature of the Sun.
Plug in the USB cable to your laptop and open LoggerPro.
Click on the Experiment tab and select Change Units => Spectrometer:1 => Select Spectrometer Mode...
In the window that appears select Intensity and then close the window.
The graph should look similar to the one below, measured on the 27 January 2024. For the calculations that follow use EITHER the data from your own spectrum or the one below.
The wavelength of the peak intensity is 497 nm.
The surface temperature of the Sun can be calculated using Wien's law.
Use the wavelength of maximum intensity to show that the surface temperature of the Sun is 5800 K.
The power of the Sun P can be calculated using the Stefan-Boltzmann law.
Use the surface temperature of 5800 K and the radius of the Sun 6.9 x 108 m to show that the power of the Sun is 3.85 x 1026 W. Assume that the Sun is a perfect black body and has an emissivity of 1.
The power of the Sun fluctuates but is considered to be about 3.85 x 1026 W. This means that it radiates 3.85 x 1026 J of energy per second.
The radiation of the Sun spreads out in an ever increasing sphere.
The intensity I of the radiation at any point on the surface of this increasing sphere can be found from
Intensity = Power / Area (I=P/A)
where A is the surface area of the sphere.
The average distance between the Sun and the Earth is 1.5 x 1011 m. Use the power of the Sun 3.85 x 1026 W to show that the intensity of the radiation reaching Earth is 1360 W m-2.
This value is known as the solar constant S. (Not necessarily important but a common question on the IB Exams).
The readings at ACS had a maximum of 904 W m-2. What could account for the loss of power from the sun? (Why didn't we measure the intensity to be 1360 W m-2?)
The radiation reaching the Earth will either be absorbed or reflected. The Earth absorbs radiation only on the side facing the Sun, as shown in the diagram below.
e - emissivity (Black Body = 1) (unitless)
𝜎 - Stefan-Boltzmann Constant (W m-2 K-4)
A - Surface area of radiating body (m2)
T - Temperature (K)
The Earth absorbs like a DISC (πr2) and reflects/radiates like a SPHERE (4πr2)
The area that absorbs the radiation is a disc with area πr2 and not half of the surface area of the Earth.
Since: power = IA
Then the total power absorbed by the earth is 1400A. (or 1360 W m-2A)
Show that the average intensity over the whole earth is 350 W m-2.
The atmosphere and Energy Storage
The atmosphere and Energy Storage
The model above (340 did not include the earth's atmosphere. The atmosphere contains mostly molecular gases; mainly nitrogen (N2) and oxygen (O2) along with noble gases and ozone (O3). There are also a number of molecular gases with variable composition known as the greenhouse gases. The greenhouse gases to be considered are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and water vapour (H2O).
The fraction of radiation reflected (or scattered) depends on the albedo of the surface (this is related to the colour).
The Albedo (α) of a planet is defined as the ratio between the total scattered (reflected) radiation and the total incident radiation of that planet.
The albedo of a planet is affected by the following:
Season (cloud formations)
Latitude
Terrain (ocean has low albedo because it mainly absorbs and snow has high albedo because it mainly reflects)
The average albedo of the earth is assumed to be 0.3. Show that the average power reflected per m2 by the earth's surface is 105 W m-2.
In molecules there are multiple ways in which energy can be stored. Energy can be absorbed by the transfer of energy when an electron moves to a higher energy level (as we studied in Topic 7). This is known as electronic energy. Energy can also be stored as rotational energy or vibrational energy.
1. Electronic energy - energy stored as potential energy when electrons are excited to higher energy levels.
2. Rotational energy - energy transferred to kinetic energy as a molecule rotates.
3. Vibrational energy - energy transferred to kinetic and potential energy as a molecule oscillates due to its bonding.
The way that electromagnetic radiation interacts with the gas molecules found in the earth's atmosphere can be investigated using the PhET simulation below.
Start with carbon monoxide gas and radiation from the microwave part of the electromagnetic spectrum.
Move the slider on the microwave emitter to the right so that microwave radiation passes through the carbon monoxide molecule.
Note if the carbon monoxide molecule absorbs any energy and try to identify whether the energy is stored as rotational or vibrational energy.
Then investigate if the molecule absorbs Infrared, Visible or Ultraviolet radiation in turn.
Once again note whether the energy is stored as rotational or vibrational energy.
Now replace the carbon monoxide molecule with nitrogen and expose it to microwave, infrared, visible and ultraviolet radiation as before.
Absorption graphs
The graphs below show the % absorption of radiation by three of the atmospheric gases: oxygen/ozone, carbon dioxide and water vapour. Use the results from the simulation to explain them.
Thermal Energy Problem Solving
Thermal Energy Problem Solving
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