18.14
Radiation: Applications
The average temperature of Earth is the subject of much current discussion. Earth is in radiative contact with both the Sun and dark space; it receive…
All bodies with a temperature greater than absolute zero emit heat in the form of electromagnetic radiation, which is absorbed by other bodies at lower temperatures.
A black body is an ideal body that absorbs all the radiation that falls on it and reflects nothing; thus, the surface appears black.
Black holes are very close to an ideal black body; they absorb all the radiation entering the horizon and reflect nothing.
On the contrary, a reflector is that ideal body, which reflects all the radiation and absorbs none of it.
For example, when a car is parked in summer, foldable pads with silver or aluminum coating are used as a reflector to keep the car cool.
Further, out of all the radiation emitted by the Sun, some are absorbed by greenhouse gases like water vapor, carbon dioxide, and methane present in the Earth's atmosphere.
This radiation is then re-emitted in all directions, keeping the Earth warm.
This phenomenon is called the greenhouse effect, which prevents Earth from freezing and making life on Earth possible.
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Q1: What is a black body in physics?
A black body is an ideal body that absorbs all radiation falling on it and reflects nothing, making its surface appear black. Black holes are very close to ideal black bodies, absorbing all radiation entering their event horizon and reflecting nothing. This concept is fundamental to understanding how bodies emit and absorb thermal radiation.
Q2: How do reflectors keep cars cool in summer?
Reflectors are ideal bodies that reflect all radiation and absorb none. Foldable pads with silver or aluminum coating are used as reflectors on parked cars to reflect solar radiation away, preventing heat absorption and keeping the car's interior cool during summer.
Q3: What causes the greenhouse effect on Earth?
Greenhouse gases like water vapor, carbon dioxide, and methane in Earth's atmosphere absorb solar radiation and re-emit it in all directions, trapping heat and keeping the planet warm. This natural phenomenon prevents Earth from freezing and makes life possible, though increased emissions from industry and farming intensify the effect.
Q4: How does Earth maintain its temperature through energy balance?
Earth's average temperature is determined by its energy balance—the point where it radiates heat to space as fast as it receives energy from the Sun. Earth's emissivity, averaging about 0.65, is a key parameter in this calculation, though cloud coverage variations complicate precise determination of this value.
Q5: Why can frost form on clear summer evenings?
Heat transfer from soil and grasses can occur so rapidly on clear nights that frost forms even in warm latitudes. This happens because Earth radiates energy into dark space, which is extremely cold at about 3 K, causing surface temperatures to drop significantly.
Q6: How does the greenhouse effect relate to global warming?
The greenhouse effect varies with wavelength, allowing certain gases to trap heat while letting other radiation escape. Increased carbon dioxide and methane emissions from industry, transportation, and farming intensify this effect, potentially causing intense storms, agricultural disruption, biodiversity loss, and rising sea levels.
Q7: How does radiation transfer heat between bodies at different temperatures?
All bodies with temperature above absolute zero emit heat as electromagnetic radiation, which is absorbed by cooler bodies. This radiative heat transfer occurs without requiring a medium and is governed by the absorption and emission properties of surfaces, which can be analyzed using conduction convection and radiation problem solving.