Name: radiation pressure in light and comets
Uploaded: 2023-04-30T12:46:50.000Z
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K12 Standards

Pennsylvania State Academic Standards

9-12

Earth and Space Science

Earth's Systems

3.3.9-12.H

Momentum And Radiation Pressure

Le onde elettromagnetiche trasportano la quantità di moto mentre viaggiano nello spazio. La corrispondente densità di quantità di moto è espressa in termini di magnitudine del vettore di Poynting e velocità della luce.
Sostituendo il volume occupato dall'onda che attraversa un'area in un breve intervallo di tempo, si può ottenere la portata di quantità di moto per unità di area.
La velocità media di trasferimento della quantità di moto per unità di area si ottiene sostituendo il valore medio del vettore di Poynting con l'intensità.
A causa del trasferimento di quantità di moto, le onde elettromagnetiche esercitano una pressione sulla superficie, nota come pressione di radiazione.
Ora, il tasso medio di variazione della quantità di moto è uguale alla forza media e la forza per unità di area è la pressione di radiazione.
Pertanto, la pressione di radiazione è direttamente proporzionale all'intensità dell'onda.
Se la superficie assorbe perfettamente l'onda elettromagnetica, anche la quantità di moto corrispondente viene trasferita completamente alla superficie.
Per una superficie perfettamente riflettente, la variazione di slancio sarà doppia. Pertanto, anche la pressione di radiazione raddoppia.

<ol>
	<li>OpenStax. (2019). Fisica universitaria Vol. 2. [Versione web]. Estratto da <a href="https://openstax.org/books/university-physics-volume-2/pages/16-4-momentum-and-radiation-pressure">16.4 Quantità di moto e pressione di radiazione - Fisica universitaria Volume 2 | OpenStax</a></li>
</ol>

momento e pressione di radiazione

Un oggetto che assorbe un'onda elettromagnetica subirebbe una forza nella direzione di propagazione dell'onda. Questa forza si verifica perché le onde elettromagnetiche contengono e trasportano impulso. La forza tiene conto della pressione radiativa dell'onda esercitata sull'oggetto. La previsione di Maxwell è stata confermata nel 1903 da Nichols e Hull mediante la misurazione precisa delle pressioni radiative con un bilanciere a torsione. Lo strumento di misurazione aveva specchi sospesi da una fibra mantenuta all'interno di un contenitore di vetro. Nichols e Hull ottennero una leggera deflessione misurabile dei specchi dal far luce su uno di essi. Dalla deflessione misurata, calcolarono la forza sbilanciata sullo specchio e ottennero un accordo con il valore previsto della forza. La pressione radiativa applicata da un'onda elettromagnetica su una superficie perfettamente assorbente si rivela essere uguale alla densità di energia dell'onda. Supponiamo che il materiale sia perfettamente riflettente e che le onde elettromagnetiche siano incidenti normalmente alla superficie. In tal caso, la pressione esercitata è il doppio perché la direzione dell'impulso si inverte al momento della riflessione.

La pressione radiativa gioca un ruolo nell'esplicare molti fenomeni astronomici osservati, inclusa l'apparizione delle comete. Quando una cometa si avvicina al Sole, si riscalda e la sua superficie, composta da gas congelati e particelle di roccia e polvere, evapora. La coma della cometa è l'area nebulosa attorno ad essa composta dai gas e dalla polvere. Alcuni dei gas e della polvere formano code quando lasciano la cometa. La coda ionica è composta principalmente da gas ionizzati. Questi ioni interagiscono elettromagneticamente con il vento solare, un flusso continuo di particelle cariche emesse dal Sole. La forza del vento solare sui gas ionizzati è abbastanza forte da far sì che la coda ionica punti quasi sempre direttamente lontano dal Sole. La seconda coda è composta da particelle di polvere. Poiché la coda di polvere è elettricamente neutra, non interagisce con il vento solare. Tuttavia, questa coda è influenzata dalla pressione radiativa prodotta dalla luce del Sole. Anche se relativamente piccola, questa pressione è abbastanza forte da causare la deviazione della coda di polvere dalla traiettoria della cometa.

Momento e pressione di radiazione

Un oggetto che assorbe un'onda elettromagnetica subirebbe una forza nella direzione di propagazione dell'onda. Questa forza si verifica perché...

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Electromagnetic Waves

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KEY TERMS AND CONCEPTS

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radiation pressure in light and comets

Radiation pressure is the force that electromagnetic waves can exert on matter. It appears because light carries momentum, so an object that absorbs a wave feels a push in the direction the wave travels. This same force helps explain why sunlight can affect small particles in space.

Maxwell predicted radiation pressure, and Nichols and Hull confirmed it in 1903. They used a torsion balance with mirrors suspended from a fiber inside a glass container. When they shone light on one mirror, the mirrors shifted slightly. From that tiny deflection, they calculated the unbalanced force and found agreement with the predicted value.

The size of the pressure depends on the surface. For a perfectly absorbing surface, the radiation pressure equals the energy density of the wave. For a perfectly reflecting surface with the wave hitting straight on, the pressure is twice as large because the light reverses momentum when it reflects.

Radiation pressure also helps explain comet tails. When a comet nears the Sun, it warms up and releases frozen gases, rock, and dust. This forms a coma, which is the hazy cloud around the comet, and it can also create tails.

The ion tail is made mostly of ionized gases. These ions interact with the solar wind, a stream of charged particles from the Sun, and the tail usually points away from the Sun. The dust tail is different. It contains neutral dust, so it does not respond to the solar wind. Instead, sunlight’s radiation pressure pushes the dust tail away from the comet’s path.

Radiation Pressure in Light and Comets

electromagnetic waves

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Maxwell’s equations explain how electric and magnetic fields work together to create electromagnetic waves. James Clerk Maxwell brought together the ideas of Oersted, Coulomb, Gauss, and Faraday and built a single theory of electromagnetism. Together with the Lorentz force law, his equations describe the laws of electricity and magnetism. A key part of Maxwell’s work is the symmetry in the equations. Faraday’s law shows that a changing magnetic field produces an electric field. Maxwell added...

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momentum and radiation pressure

Radiation pressure is the force that electromagnetic waves can exert on matter. It appears because light carries momentum, so an object that absorbs a wave feels a push in the direction the wave travels. This same force helps explain why sunlight can affect small particles in space. Maxwell predicted radiation pressure, and Nichols and Hull confirmed it in 1903. They used a torsion balance with mirrors suspended from a fiber inside a glass container. When they shone light on one mirror, the...

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Radiation Pressure in Light and Space

Radiation pressure is the push that light can exert on matter. When an electromagnetic wave is completely absorbed by a surface, its energy density is transferred as momentum to that surface. The rate of momentum transfer on a surface perpendicular to the wave gives the force, and the force per unit area is the radiation pressure. A perfectly absorbing surface receives this pressure directly from the incoming wave. If the surface is perfectly reflecting, such as a metal surface, and the light...

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Explore how light carries momentum, creates radiation pressure, and was measured by Nichols and Hull. See how it helps shape comet tails.

Radiation Pressure in Light and Comets | JoVE K12

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HS-ESS2-4

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3.3.9-12.L

Radiation Pressure in Light and Comets

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