شدة (فيزياء)

في الفيزياء، تعتبر الشدة (بالإنگليزية: Intensity) قياسًا للمتوسط الزمني لتدفق الطاقة.

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الوصف الرياضي

لنفترض وجود منبع نقطي يشع الطاقة في الفضاء ثلاثي الأبعاد، وبفرض عدم وجود ضياع في الطاقة إلى الوسط الذي ينتقل فيه الإشعاع، تتناقص الشدة متناسبة مع مربع المسافة من الجسم. وباعتبار مبدأ حفظ الطاقة، فإن الطاقة الصافية الآتية من المنبع يجب أن تكون ثابتة، ونكتب:

 

حيث P طاقة الإشعاع الصافية الصادرة عن المنبع، I الشدة كتابع للموضع، و dA الشريحة العنصرية من السطح المغلق المحيط بالمنبع. باعتبار P ثابتة. فإذا قمنا بالتكامل على السطح المتعرض لشدة متجانسة I، للحظة معينة، على كرة متمركزة في المصدر النقطي تشع بشكل متساوي في جميع الاتجاهات، تصبح المعادلة بالشكل:

 

حيث I هي الشدة عند سطح الكرة، و r نصف قطر الكرة. (  معادلة مساحة الكرة). بالحل نحصل على:

 

وباعتبار الضياعات في الوسط الذي تنتقل في الأشعة، عندها تقل قيمة الشدة المعطاة في المعادلة السابقة.


أي شيء يمكنه نقل الطاقة يكون له شدة مصاحبة له. بالنسبة للأشعة الكهرومغناطيسية، إذا كانت E سعة الحقل الكهربائي، عندها تكون كثافة طاقة الموجة:

 

ونحصل على الشدة بضرب هذه المعادلة بسرعة الموجة  :

 

حيث n هي قرينة الانكسار،   هي سرعة الضوء في الخلاء، و   هي السماحية الكهربائية في الخلاء.


تعاريف أخرى للشدة

في علم قياس الإشعاع، و في علم قياس الضوء، يكون للشدة معاني مختلفة:

وهي قدرة الإشعاع أو الإضاءة خلال وحدة الزاوية الصلبة. وهذا يسبب اختلاطًا في علم البصريات، حيث يشير مصطلح الشدة إلى شدة الإشعاع، أو شدة الإضاءة، أو التشعيع، وذلك بحسب الخلفية العلمية للشخص المستخدم للمصطلح. وفي بعض الأحيان يعبر عن الإشعاعية بالشدة، وخصوصًا من قبل علماء الفلك وعلماء الفيزياء الفلكية، وفي انتقال الحرارة.

الوحدات الدولية لقياس الإشعاع
الكمية الوحدة الأبعاد ملاحظات
الاسم الرمز[nb 1] الاسم الرمز الرمز
طاقة إشعاعية Qe[nb 2] جول J ML2T−2 طاقة الإشعاع الكهرومغناطيسي.
كثافة الطاقة الإشعاعية we joule per cubic metre J/m3 ML−1T−2 الطاقة الإشعاعية لكل وحدة حجم.
التدفق الإشعاعي Φe[nb 2] واط W = J/s ML2T−3 Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".
تدفق طيفي Φe,ν[nb 3] واط لكل هرتس W/Hz ML2T−2 Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.
Φe,λ[nb 4] واط للمتر W/m MLT−3
الشدة الإشعاعية Ie,Ω[nb 5] watt per steradian W/sr ML2T−3 Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
الشدة الطيفية Ie,Ω,ν[nb 3] watt per steradian per hertz W⋅sr−1⋅Hz−1 ML2T−2 Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity.
Ie,Ω,λ[nb 4] watt per steradian per metre W⋅sr−1⋅m−1 MLT−3
إشعاعية Le,Ω[nb 5] watt per steradian per square metre W⋅sr−1⋅m−2 MT−3 Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
الإشعاعية الطيفية Le,Ω,ν[nb 3] watt per steradian per square metre per hertz W⋅sr−1⋅m−2⋅Hz−1 MT−2 Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".
Le,Ω,λ[nb 4] watt per steradian per square metre, per metre W⋅sr−1⋅m−3 ML−1T−3
التشعيع
كثافة التدفق
Ee[nb 2] واط للمتر المربع W/m2 MT−3 Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".
التشعيع الطيفي
كثافة التدفق الطيفي
Ee,ν[nb 3] watt per square metre per hertz W⋅m−2⋅Hz−1 MT−2 Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = 10−26 W⋅m−2⋅Hz−1) and solar flux unit (1 sfu = 10−22 W⋅m−2⋅Hz−1 = 104 Jy).
Ee,λ[nb 4] watt per square metre, per metre W/m³ ML−1T−3
كثافة التدفق الإشعاعي
Radiosity
Je[nb 2] watt per square metre W/m² MT−3 Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".
كثافة التدفق الطيفي
Spectral Radiosity
Je,ν[nb 3] watt per square metre per hertz W⋅m−2⋅Hz−1 MT−2 Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity".
Je,λ[nb 4] watt per square metre, per metre W/m³ ML−1T−3
Radiant exitance Me[nb 2] watt per square metre W/m2 MT−3 Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance Me,ν[nb 3] watt per square metre per hertz W⋅m−2⋅Hz−1 MT−2 Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
Me,λ[nb 4] watt per square metre, per metre W/m3 ML−1T−3
Radiant exposure He joule per square metre J/m2 MT−2 Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure He,ν[nb 3] joule per square metre per hertz J⋅m−2⋅Hz−1 MT−1 Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence".
He,λ[nb 4] joule per square metre, per metre J/m³ ML−1T−2
Hemispherical emissivity ε N/A 1 Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.
Spectral hemispherical emissivity εν
 or
ελ
N/A 1 Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.
Directional emissivity εΩ N/A 1 Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.
Spectral directional emissivity εΩ,ν
 or
εΩ,λ
N/A 1 Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.
Hemispherical absorptance A N/A 1 Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance".
Spectral hemispherical absorptance Aν
 or
Aλ
N/A 1 Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance".
Directional absorptance AΩ N/A 1 Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance".
Spectral directional absorptance AΩ,ν
 or
AΩ,λ
N/A 1 Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance".
Hemispherical reflectance R N/A 1 Radiant flux reflected by a surface, divided by that received by that surface.
Spectral hemispherical reflectance Rν
 or
Rλ
N/A 1 Spectral flux reflected by a surface, divided by that received by that surface.
Directional reflectance RΩ N/A 1 Radiance reflected by a surface, divided by that received by that surface.
Spectral directional reflectance RΩ,ν
 or
RΩ,λ
N/A 1 Spectral radiance reflected by a surface, divided by that received by that surface.
Hemispherical transmittance T N/A 1 Radiant flux transmitted by a surface, divided by that received by that surface.
Spectral hemispherical transmittance Tν
 or
Tλ
N/A 1 Spectral flux transmitted by a surface, divided by that received by that surface.
Directional transmittance TΩ N/A 1 Radiance transmitted by a surface, divided by that received by that surface.
Spectral directional transmittance TΩ,ν
 or
TΩ,λ
N/A 1 Spectral radiance transmitted by a surface, divided by that received by that surface.
Hemispherical attenuation coefficient μ reciprocal metre m−1 L−1 Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral hemispherical attenuation coefficient μν
 or
μλ
reciprocal metre m−1 L−1 Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Directional attenuation coefficient μΩ reciprocal metre m−1 L−1 Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral directional attenuation coefficient μΩ,ν
 or
μΩ,λ
reciprocal metre m−1 L−1 Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
See also: SI • Radiometry • Photometry
  1. ^ Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
  2. ^ أ ب ت ث ج رموز بديلة تُستخدَم أحياناً: W أو E للطاقة الإشعاعية، P أو F للتدفق الإشعاعي، I للتشعيع، W for radiant exitance.
  3. ^ أ ب ت ث ج ح خ Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek)—not to be confused with suffix "v" (for "visual") indicating a photometric quantity.
  4. ^ أ ب ت ث ج ح خ Spectral quantities given per unit wavelength are denoted with suffix "λ" (Greek).
  5. ^ أ ب Directional quantities are denoted with suffix "Ω" (Greek).

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