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Energy balance of the earth IB DP Physics Study Notes

Radiation from real bodies IB DP Physics Study Notes - 2025 Syllabus

Energy balance of the earth IB DP Physics Study Notes

Energy balance of the earth IB DP Physics Study Notes at  IITian Academy  focus on  specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand

  • that methane CH4, water vapour H2O, carbon dioxide CO2, and nitrous oxide N2O, are the main greenhouse gases and each of these has origins that are both natural and created by human activity  

  • the absorption of infrared radiation by the main greenhouse gases in terms of the molecular energy levels and the subsequent emission of radiation in all directions

  •  that the greenhouse effect can be explained in terms of both a resonance model and molecular energy levels

  •  that the augmentation of the greenhouse effect due to human activities is known as the enhanced greenhouse effect.

Standard level and higher level: 6 hours
Additional higher level: There is no additional higher level content

IB DP Physics 2025 -Study Notes -All Topics

The greenhouse effect

  • When solar radiation strikes a planet that has a gaseous atmosphere, the gases comprising the atmosphere can absorb infrared radiation (heat).
  • The remainder of the incoming radiation then reaches the ground to either be scattered back into the atmosphere, or absorbed.
  • Solids can absorb all frequencies of radiation, and convert them to infrared wavelengths.
  • The heated ground can then emit infrared radiation back into the atmosphere, which then intercepts more of the energy on the way out.
  • The result is that the atmosphere traps heat and causes the temperature of the planet to rise.

The enhanced greenhouse effect

  • The process outlined on the Upper side is the so-called greenhouse effect.
  • The main gases that are particularly effective in absorbing infrared radiation are methane (CH4), water vapor (H2O), carbon dioxide (CO2), and nitrous oxide (N2O).
  • Note that there are both natural and man-made (anthropogenic) contributions and changes.
  • This graph shows the correlation between increasing atmospheric concentration of CO2 and our fossil fuel emissions of CO2.
  • Anthropogenic means “human.”

Consequences of enhanced greenhouse effect

  • The increased amount of greenhouse gasses in the atmosphere will lead to changes that humans will have to adjust to:
  • higher temperatures in the oceans and air more frequent extreme weather
  • changes to local climates
  • rising sea levels
  • oceans becoming more acidic

Prevention of enhanced greenhouse effect

  • In order to minimize the production of excess greenhouse gasses some preventative steps can be taken:
  • support governments in taking action to address the issue
  • support initiatives that minimize fossil fuel use
  • waste less
  • support and use public transportation
  • enhance reforestation efforts
  • minimize excess consumption
  • alter lifestyle expectations

Absorption of infrared radiation by greenhouse gases

  • Recall that solar radiation strikes the earth at a rate of 1380 W m-2 or less, the farther from the equator you are.
  • That energy is carried in the form of photons, which are quanta of light.
  • The atmosphere is made up of gases, which are the first layer of matter that the sun’s rays interact with
  • If a photon is at the precise energy for an electron to jump to a different energy level in an atom, it will be absorbed.
  • The excited gases will eventually de-excite and release photons.
  • The absorption and release of photons by the gases is called scattering.
  • Scattering does not produce a net increase in heat energy in the atmosphere.
  • However, some of the scattered photons may be in the infrared region – able to be absorbed as internal energy and kinetic energy via the mechanisms:
  • Greenhouse gases are all molecular in nature, (meaning comprised of more than one atom)
  • For our purposes we will consider the triatomic molecule $\rm {CO}_2$.

  • A simple model of $\rm {CO}_2$ has springs connecting the two oxygen atoms to the carbon atom:

  • Recall that heat energy can be stored in molecules as internal energy in the form of potential (the springs) and kinetic (the vibrations).

  • The difference between these two $\rm {CO}_2$ molecules is in their internal potential energies:

  • The above $\rm {E}_P$ storage does not contribute to the increase in temperature of the $\rm {CO}_2$. Rather, it is the kinetic energy $\rm {E}_K$ of the molecule that determines its temperature.

  • There are three ways an extended molecule such as $\rm {CO}_2$ can store kinetic energy: Via vibration, via translation, and via rotation.

  • Vibration refers to the molecules oscillating in conjunction with the springs:

  • The natural (or resonance) frequency of greenhouse gases is in the infrared region of the spectrum and thus greenhouse gases are prone to absorb such frequencies.

  • Translation refers to the molecule moving as a unit in a straight line:

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  • Rotation refers to the molecule spinning about its center of mass.

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  • All three $\rm{E}_K$ modes can absorb and hold infrared radiation simultaneously.

  • Different gases absorb different wavelengths:

PRACTICE: Consider the absorption graphs below:


(a) Which portion of the electromagnetic spectrum is represented?
(b) Which greenhouse gas contributes the most to the greenhouse effect?
(c) Which gas is the least significant contributor?

▶️Answer/Explanation

SOLUTION:
(a) Infrared (heat).
(b) Water vapor!
(c) Oxygen and ozone.

Energy balance in Earth surface / atmosphere system

  • Although we calculated the intensity of solar radiation to be 1380 W m-2 by the time it reaches Earth, the earth is spherical, and so not all surfaces will receive this intensity.
  • We will take the average for our models to be

EXAMPLE:

(a)Draw a Sankey diagram for a model of the earth without greenhouse gases.

(b) Show that at the space/atmosphere interface and at the ground/atmosphere interface the net power flow is zero.

▶️Answer/Explanation

SOLUTION:

(a ) First draw Earth/atmosphere blocks:

  • Draw incoming solar energy ( $340 \mathrm{~W} \mathrm{~m}^{-2}$ ):
  • Draw energy reflected by atmosphere (clouds):
  • Draw energy reflected by ground (snow, etc.):
  • Draw energy absorbed by ground $(Q=m c \Delta T)$ :
  • Draw energy radiated by ground $\left(P=\sigma A T^4\right)$ :
  •  

(b)
Space/atmosphere interface:

$$
I_{i n}=340\quad \mathrm{IN}
$$

$$
I_{\text {out }}=235+75+30=340\quad \text { OUT. }
$$

Ground/atmosphere interface: 235 IN and 235 OUT.
FYI
 Since $I_{\text {IN }}=I_{\text {OUT }}$, the temperature in each region is constant.

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