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21-11-2006, 02:07 PM
فيز 101 عن درجة الحرارة
أتمنى إنه يلقى إعجابكم
وهذا هو
المقدمة:
كم درجة الحرارة اليوم ؟ للإجابة بهذا السؤال بدقة يلزمك ترمومتر-أي ميزان حرارة لقياس ذلك. جميع الترمومترات مدرجة بمقاييس تستخدم نقطتين ثابتتين هما: درجة حرارة انصهار الجليد، ودرجة حرارة غليان الماء على ضغط جوي عياري. هنالك ثلاثة مقاييس مهمة لدرجة الحرارة هي: مقياس سليسيوس ومقياس فرنهايت والمقياس المطلق أو مقياس كلفن. فدرت انصهار الجليد على مقياس سليسيوس هي صفر5س، ودرجة غليان الماء 5100س. على مقياس فرنهايت درجة انصهار الجليد هي 532ف ودرجة غليان الماء 5212 ف. أما بمقياس كلفن فيبدأ من أدنى درجة حرارة ممكنة نظرياً، وهي درجة الصفر المطلق؛ والدرجة فيه مساوية فيه قدراً للدرجة في مقياس سليسيوس.
العرض:
أنواع الترمومترات:
لقد ذكرنا أننا نقيس درجة الحرارة بالترمومترات، والترمومترات تختلف باختلاف درجة الحرارة المراد قياسها، فهناك ترمومترات لقياس درجات الحرارة الشديدة البرودة، والتي قد تقترب من درجة الصفر المطلق، وهناك ترمومترات لقياس درجات الحرارة العالية، والتي قد تحاكي درجة حرارة الشمس، وسنناقش هنا بعضاً منها:
أ - الترمومتر الزئبقي:
وهو يتكون من أنبوبة زجاجية رفيعة, في نهايتها إنتفاخ به زئبق, وتوضع علامة عند النقطة التي يتجمد فيها الماء, وهي درجة الصفر المئوي, وعلامة أخرى عند النقطة التي يغلي فيها الماء وهي (C 5100), ثم تقسم المسافة بين النقطتين إلى مائة تدريج متساوية، ويمكن أن يقيس هذا الترمومتر درجات الحرارة ما بين (C520 ) و (C5220) ء ولكن إذا أردنا قياس درجات حرارة شديدة البرودة فان الكحول يستخدم بدلا من الزئبق, حيث أن درجة الحرارة التي يتجمد فيها الكحول( (C5220
ب – الترمومتر البلاتيني:وهو ترمومتر تعتمد نظريته على أن مقاومة المواد تتغير بتغير درجة الحرارة، وهذا الترمومتر يمكن أن يقيس درجات الحرارة بين (C540ــ) و) C5(1200بدقة كبيرة, وهو يتكون من سلك بلاتيني رقيق ملفوف حول اسطوانة مصنوعة من الميكا محتواة على أنبوب من الزجاج الصلب.
ج – ترمومترالإزدواج الحراري:
يتكون ترمومتر الإزداوج الحراري من سلكين معدنيين مختلفين موصلين على التوازي. فإذا وضعت وصلة عند درجة حرارة معروفة مثل ماء وثلج في توازن حراري أي عند الصفر المئوي, والوصلة الأخرى توضع عند درجة الحرارة التي يراد قياسها, فإن فرقأ في الجهد الكهربائي ينشأ بين الوصلتين, وهذا الفرق في الجهد مقياس للفرق في درجتي الحرارة بين الوصلتين.
د-ـ ترمومتر الكبس:
تترتب جزيئات البلورات السائلة في صفوف منتظمة كما في البلورات الجامدة لكنها تنساب كالسائل. بعض هذه البلورات يتغير لونه تبعاً لدرجة الحرارة, فيستخدم في ترمومترات شريطية لأخذ درجة حرارة الأطفال, فالحرارة تعيد ترتيب الجزيئات ميسرة بذلك مرور الضوء عبر السائل فتتوهج بلون مختلف تبعاً لدرجة حرارة الطفل.
البيرومتر:
يستخدم البيرومتر في قياس درجات الحرارة العالية كدرجة الحرارة داخل فرن صناعة الزجاج. تتوهج الأشياء بألوان مختلفة حسب درجة حرارتها. ويحتوي البيرومتر فتيلة كهربائية يسخنها تيار كهربائي حتى يتساوى لونها مع لون الجسم المتوهج. ثم تقاس درجة الحرارة بمقياس هذا التيار.
تستمد الأرض الطاقة من الشمس على شكل حرارة تصلها مباشرة وتؤثر طاقة جذب الشمس في الأرض فتبقيها في مدار حولها تحدث خلاله اختلافات في درجة الحرارة على المناطق المختلفة من سطح الأرض، كذلك تؤثر الأرض في القمر الذي يدور في مدار حولها ويستمد جزءا من طاقته الضوئية والحرارية منها. تظهر هذه الصورة التي التقطتها مركبة الفضاء أبوللو 8 كوكب الأرض كما ظهر لها من على سطح القمر، وقد أضاءت الشمس جزءا منه وغرق الجزء الآخر في الليل.
طرق انتقال الحرارة:
انتقال الحرارة بالتوصيل
إذا عرضنا طرف قضيب فلزي للهب فترة من الزمن , في حين نمسك بيدنا الطرف الآخر فإننا نلاحظ إن درجة حرارة الطرف غير المعرض للهب تبدأ بالارتفاع شيئاً فشيئاً، مع أنه غير متعرض للهب مباشرة ، وهذه الظاهرة تسمى (ظاهرة التوصيل الحراري) وتفسيرها أن جزيئات الطرف الساخن والتي تهتز أصلاً , والتي تتحرك حركة توافقية بسيطة ، تزداد سرعتها عند التسخين ، وبالتالي يزداد اتساع اهتزازاتها , نظراً لأنها اكتسبت طاقة . ونظراً لاصطدامها مع الجزيئات المجاورة, فإنها تنقل لها الطاقة شيئاً فشيئاً مع استمرار التسخين, وبالتالي تنتقل الحرارة عبر المادة من طرف إلى طرف آخر.
والمعروف عن الفلزات قاطبة أنها جيدة التوصيل للكهرباء، وكذلك جيدة التوصيل للحرارة، أما توصيلها الجيد للكهرباء، فهو بسبب وجود إلكترونات حرة فيها؛ أما توصيلها الجيد للحرارة, فيعود إلى الجزيئات والى الإلكترونات الحرة معاً.
وقد مر معنا أن الجسم الذي درجة حرارته أعلى يفقد حرارة، وأن الجسم الذي درجة حرارته اقل يكسب حرارة عند حدوث اتصال بين الجسمين ء أي أن اتجاه سريان الحرارة هو دائماً من النقطة التي درجة حرارتها أعلى إلى النقطة التي درجة حرارتها أقل. ولكن معدل سريان الحرارة عبر المواد يختلف باختلاف نوع المادة، وقد ذكرنا أن الفلزات جيدة التوصيل، ولكن لو أخذنا مواد أخرى كالخشب والزجاج والفخار، فإننا نجدها رديئة التوصيل للحرارة
انتقال الحرارة بالحمل
إذا وضعنا إناء به ماء على لهب مصدر حراري , فإن قعر الإناء يسخن , وبالتالي يسخن الماء الملامس له , فيتمدد وتقل كثافته , وبذلك يرتفع إلى أعلى ويحل محله ماء بارد يهبط من المناطق العليا إلى أسفل , فيسخن هذا الماء, ويرتفع ايضاً إلى أعلى وهكذا.
ومن خلال حركة جزيئات الماء التي ترتفع إلى أعلى, يتم نقل الحرارة من المناطق السفلى للإناء إلى المناطق العليا له.
وتسمى الطريقة التي تنتقل الحرارة بها هنا (الحمل )؛ وهي كما نلاحظ تقتضي أن تغادر الجزيئات الساخنة أماكنها ناقلة معها الحرارة إلى الجزيئات الباردة. وهي تختلف عن انتقال الحرارة بطريقة التوصيل؛ إذ أن الجزيئات في الطريقة السابقة (التوصيل) لا تغادر أماكنها.
انتقال الحرارة بالإشعاع:
لا بد أنك جلست يومأ أمام مدفأة, وأنك شعرت المدفأة, لسؤال الذي يتبادر للذهن هو كيف وصلتك حرارة المدفأة ؟
إن حرارة المدفأة لا يمكن أن تكون قد وصلتك بالتوصيل ء إذ أنك لا تلمس المدفأة, كما أنها لا تكون قل وصلتك بالحمل, إذ أنك والمدفأة على مستوى أفقي واحد. إن الطاقة الحرارية وصلتك بطريقة أخرى تسمى, الإشعاع.
والتعبير الذي استعملناه (الإشعاع) يدل على فقد مستمر للطاقة من سطح الجسم.
وهالإشعاعية.سمى الطاقة المشعة, أ.و الإشعاعية. ويفقدها الجسم أو يشعها بسرعة الضوء وإذا سقطت على جسم غير شفاف, فإنه سيمتصها, ويحولها إلى حرارة.
ومقدار الطاقة التي يشعها سطح معين من وحدة المساحة خلال وحدة الزمن, تعتمد على طبيعة السطح, وعلى درجة حرارته.
فإذا كانت درجة حرارة السطح منخفضة, فإن معدل إشعاع الطاقة يكون متدنيا؛ وكلما ارتفعت درجة الحرارة زادت كمية الحرارة التي يفقدها الجسم بالإشعاع.
ولكن التناسب ليس خطياً بين الطاقة المفقودة بالإشعاع, وبين درجة الحرارة؛ إذ أن الطاقة تتناسب مع درجة الحرارة المطلقة مرفوعة للقوة (4).
أما طبيعة الإشعاع, فحتى درجة حرارة 300 س, فإن الإشعاع يكون على شكل موجه تحت حمراء في معظمه, ولكن إذا ارتفعت درجة حرارة الجسم أكثر فأكثر يبدأ بإشعاع موجات أخرى مرئية.
مصادر الحرارة:
يمكن الحصول على الطاقة الحرارية كما ذكرنا سابقا من أي نوع من أنواع الطاقة الأخرى، غير أن هناك مصادر يكون التحويل منها إلى حرارة ضعيفا كالطاقة الضوئية والصوتية، وهناك مصادر قوية تعطي كمية كبيرة من الطاقة الحرارية التي يستفاد منها … ومن المصادر الضعيفة يمكننا الحصول على الطاقة الحرارية لأغراض القياس والتصوير والدارسة فقط كما في تصوير الكون بالأشعة تحت الحمراء، والمنظار الليلي، وقياس درجة حرارة جسم المريض، ودرجة حرارة مياه البحر، ودرجة حرارة الجو …. الخ. أما المصادر القوية فهي التي تعطينا كمية من الحرارة لاستخدامها في حياتنا العملية للأغراض المختلفة، كالطهي والتدفئة في المنازل والصناعات المختلفة من صهر وتشكيل المعادن إلى الصناعات البلاستيكية إلى تكرير البترول والصناعات الغذائية … الخ. وفيما يلي بعض المصادر القوية للطاقة الحرارية.
بعض مصادر الحرارة:
1_ الشمس: تعد الشمس من أكبر مصادر الضوء والحرارة التي سخرها الله سبحانه وتعالى لاستمرار الحياة على سطح كوكبنا الأرض "وهو الذي جعل القمر نورا والشمس ضياء " وتمد الشمس أرضنا والكواكب الأخرى بالحرارة حسب بعدها عنها حيث يتلقى كل متر مربع من سطح الأرض في الثانية الواحدة ما معدله 1400 جول من الطاقة الشمسية، وتتوزع هذه الطاقة على أجزاء الأرض حسب قربها من خط الاستواء الذي يحظى بأكبر نصيب منها والطاقة الشمسية ضرورية لمعظم الكائنات الحية على سطح الأرض لاستمرار حياتها، وهي تمتص هذه الطاقة بطرق مختلفة خاصة الغطاء النباتي، وهي ضرورية للحفاظ على درجة حرارة سطح الأرض والغلاف الجوي والمائي لملائمة الكائنات التي تعيش في كل منها، ويمكننا الاستفادة من الطاقة الشمسية بطرق مختلفة فضلا عن ضرورتها للحياة بشكلها الطبيعي. فالسخانات الشمسية فوق أسطح المنازل تقوم على تسخين المياه بتعرضها المباشر للشمس، والخلايا الشمسية التي تولد الكهرباء تعتمد على تحويل الإشعاع الشمس إلى كهرباء تستخدم في المنازل وفي المشاريع الواقعة في المناطق النائية. وينظر إلى الطاقة الشمسية كمصدر نظيف للطاقة لا يلوث البيئة ولا توجد له مخاطر تذكر على الحياة على سطح الأرض وتتولد الطاقة الشمسية نتيجة تفاعلات الاندماج النووي، التي تحدث داخل الشمس وتجعلها بالتالي مصدرا مستعرا للحرارة إذ تبلغ درجة حرارة سطحها حوالي 6000 درجة مئوية بينما ترتفع في باطنها إلى ملايين الدرجات وترسل الشمس الأشعة الكهرومغناطيسية بكل أنواعها، ومن ضمنها الأشعة تحت الحمراء الحرارية وأشعة الضوء المرئي والأشعة فوق البنفسجية التي تقوم طبقة الأوزون بعكس جزء كبير منها فتمنع عنا ضررها. فهي تصيب جلد الإنسان الذي يتعرض لها لفترة طويلة بالحروق والأذى لكنها ضرورية لبعض الكائنات.
2_ الوقود العضوي/البترول: يشكل البترول عصب الحياة الصناعية والحضارة الحديثة، فهو يزودنا بالطاقة بشكل فعال في كل المجالات، وعلية وعلى الفحم قامت أسس الحضارة التي نشهدها الآن. يعطي البترول لدى احتراقه كمية كبيرة من الحرارة، وتمكن طبيعته وإمكانية تحليله إلى مشتقاته المختلفة من استخدامه في الكثير من المجالات من الصناعات الثقيلة إلى المتوسطة والخفيفة، وفي المواصلات والدفاع والتدفئة والأغراض المنزلية. ويتكون البترول من مركبات عضوية من الكربون والنيتروجين والهيدروجين وعند احتراق هذه المركبات فان الطاقة الموجودة في الروابط بين ذراتها تتحرر لنستفيد منها كطاقة حرارية. وقد كان البترول هو العنصر الرئيسي في الصناعات والنقل خلال القرن الحالي بعد أن كان استخدام الفحم هو المسيطر في مجال الطاقة. ولعل بعض الدول لازالت تنتج وتستعمل الفحم الحجري إلى يومنا هذا. أما في جسم الإنسان فان الغذاء هو الوقود العضوي الذي يوفر للجسم حاجته من الطاقة. فالكربوهيدرات هي مواد عضوية يتم حرقها داخل خلايا الجسم لتمدها بالطاقة الحرارية اللازمة للقيام بالعمليات الحيوية التي تعتمد عليها العضلات والدماغ والأعصاب في عملها. غير أن لهذا المصدر من مصادر الطاقة سلبية كبرى هي مخلفات الاحتراق التي تؤدي إلي تلوث البيئة . فمهما كانت عمليات الاحتراق نظيفة فلا بد لها من إنتاج كميات من ثاني أكسيد الكربون الذي بات يشكل تركيزه في الغلاف الجوي تهديدا لاستقرار المناخ فيما يعرف بظاهرة البيت الزجاجي حيث يمنع ثاني أكسيد الكربون خروج الحرارة الزائدة من الأرض إلى الفضاء على شكل أشعة تحت حمراء الأمر الذي يؤدي إلى ارتفاع درجات الحرارة على سطح الأرض ويهدد بانصهار كميات من جليد القطبين لترفع منسوب مياه البحار والمحيطات مما يهدد بإغراق مدن ساحلية كثيرة في العالم. كما يؤدي ارتفاع درجات الحرارة إلى تصحر وجفاف مزيد من الأراضي الزراعية وبالتالي القضاء على العديد من الكائنات الحية. مما يستوجب الحد من إطلاق ثاني أكسيد الكربون في الجو والعمل على خفض تركيزه الحالي.
3_ الطاقة النووية: بحلول بداية القرن الحالي كانت العلاقة بين المادة والطاقة قد أصبحت في مركز اهتمام علماء الفيزياء، ووضعت نظريات عدة أدت إلى التنبؤ النظري بإمكانية تحويل المادة إلى طاقة وتحويل الطاقة إلى مادة. وبدأ العمل التجريبي لتحقيق ذلك، فتحقق التفاعل المتسلسل الذي يقوم على انشطار نواة ذرة اليورانيوم لتعطي كمية كبيرة جدا من الطاقة ولتنتج نواتين صغيرتين مجموع كتلتيهما أقل من كتلة نواة اليورانيوم مما يعني أن فرق الكتلة قد تحول إلى طاقة، هذه هي الطاقة النووية. وكان من نتائج هذه التجارب خلال الحرب العالمية الثانية أن تكثفت الجهود عند الدول العظمى لصنع القنبلة النووية والتي كان أول من استخدمها الولايات المتحدة الأمريكية ضد اليابان حيث ألقيت قنبلتان على كل من مدينتي هيروشيما وناغازاكي وكان أثر تدميرهما هائلا. واستمر البحث بعد ذلك حتى صنعت القنبلة الهيدروجينية التي تقوم على اندماج أربع نوى هيدروجين لإنتاج نواة هيليوم وإطلاق طاقة هائلة تفوق كثيرا طاقة انشطار اليورانيوم.وهذا في الحقيقة هو التفاعل الذي يحدث داخل الشمس. وبعد ذلك اتجهت جهود العلماء إلى الاستخدامات السلمية للطاقة النووية، فأنشئت المفاعلات النووية التي تزودنا بكميات كبيرة من الحرارة لكنها مصممة بحيث يتم التحكم بها بحيث لا تؤدي إلى انفجار. وتستخدم هذه الحرارة في توليد البخار الذي يدير المولدات الكهربائية التي تزود الكثير من الدول كمعظم الولايات المتحدة وكندا مثلا بالكهرباء. كما تستخدم لتحلية مياه البحر ولتطبيقات أخرى مفيدة. غير أن لهذا النوع من مصادر الحرارة أيضا سلبية كبرى هي النشاط الإشعاعي الناتج من المخلفات النووية المشعة التي تنتجها المفاعلات النووية والتي يصعب التخلص منها والتي تشكل خطرا كبيرا على الكائنات الحية لا ينتهي قبل مرور آلاف وربما ملايين السنين. فضلا عن الحوادث التي قد تقع نتيجة التقصير والإهمال كما حدث في كارثة انفجار مفاعل تشيرنوبل في أوكرانيا عام 1986.
4_ الطاقة الجوفية: عندما يثور بركان نلاحظ أنة يقذف في الجو حمما ساخنة جدا قوامها الصخر المنصهر ودرجة حرارتها عالية جدا وتسيل هذه الحمم على جوانب الفوهة البركانية لتسبب احتراقا وموتا لكل الكائنات المحيطة بها. وهذا دليل على أن باطن الأرض يحوي من الحمم والصهير الكثير مما يمكن أن يكون مصدرا جيدا للحرارة لو أمكن تطويعه واستخدامه لمنفعة البشر. ومن الظواهر التي تساعد في ذلك ما يسمى الفوارات الساخنة، وهي ينابيع مياه ساخنة تقع في مناطق الصدوع الأرضية حيث تتسرب المياه الجوفية عبر الصدوع الأرضية والشقوق إلى أعماق كبيرة بحيث تلامس مناطق شديدة السخونة فتسخن وتصعد إلى أعلى فوارة وبقوة، نافثة بخار الماء. وبعض هذه الينابيع يثور ويهمد عدة مرات في الساعة وبعضها يتدفق باستمرار وبشكل انسيابي حاملا معه المعادن المذابة من طبقات الصخور العميقة. ويقصد الناس هذا النوع من الينابيع للاستشفاء والاستمتاع بحمامات ساخنة من المياه الطبيعية. غير أن هناك مشاريع تقوم على استغلال حرارة المياه المنطلقة من الأرض في توليد الكهرباء.
الخاتمة: نستنتج أن: درجة الحرارة هي قياس لمدى لسرعة تحرك جزيئات الجسم, أما الحرارة فهي طاقة الجسم المكتسبة, وأن طرق انتقال الحرارة ثلاث, الحمل والتوصيل والإشعاع, وتقاس الحرارة بعدة مقاييس أهمها: المقياس السيليزي, المقياس الفهرنهايتي, ومقياس كلفن.
رفـــــاعي
21-11-2006, 02:25 PM
Internet
The Internet, or simply the Net, is the publicly accessible worldwide system of interconnected computer networks that transmit data by packet switching using a standardized Internet Protocol (IP). It is made up of thousands of smaller commercial, academic, domestic, and government networks. It carries various information and services, such as electronic mail, online chat, and the interlinked Web pages and other documents of the World Wide Web.
Contrary to some common usage, the Internet and the World Wide Web are not synonymous: the Internet is a collection of interconnected computer networks, linked by copper wires, fiber-optic cables, etc.; the Web is a collection of interconnected documents, linked by hyperlinks and URLs, and is accessible using the Internet.
Creation of the Internet
Main article: History of the Internet
The USSR's launch of Sputnik spurred the U.S. to create the Defense Advanced Research Projects Agency (DARPA) in February 1958 to regain a technological lead. DARPA created the Information Processing Technology Office to further the research of the Semi Automatic Ground Environment program, which had networked country-wide radar systems together for the first time. J. C. R. Licklider was selected to head the IPTO, and saw universal networking as a potential unifying human revolution. Licklider recruited Lawrence Roberts to head a project to implement a network, and Roberts based the technology on the work of Paul Baran who had written an exhaustive study for the U.S. Air Force that recommended packet switching to make a network highly robust and survivable. After much work, the first node went live at UCLA on October 29, 1969 on what would be called the ARPANET, the "eve" network of today's Internet. In December of 1970, Charles A. Petrik contacted the U.S. Navy and suggested that a special communications network, that the Department of Defense had built for use in the possiblitity of a nuclear attack, could also be used during peace time. Petrik convinced the military to connect the computers of the U.S. National Laboratories for scientific research purposes, and to allow these labs to get data to other labs faster, and safer. The vast majority of today's Internet uses version four of the IP protocol (i.e. IPv4), and although IPv6 is standardised, it exists only as "islands" of connectivity, and there are many ISPs who don't have any IPv6 connectivity at all. [1]
The first TCP/IP wide area network was operational by January 1, 1983 (this is technically the birth of the Internet), when the United States' National Science Foundation (NSF) constructed a university network backbone that would later become the NSFNet. It was then followed by the opening of the network to commercial interests in 1995. Important separate networks that offered gateways into, then later merged into the Internet include Usenet, Bitnet and the various commercial and educational X.25 networks such as Compuserve and JANET. The ability of TCP/IP to work over these pre-existing communication networks allowed for a great ease of growth. Use of Internet as a phrase to describe a single global TCP/IP network originated around this time.
The network gained a public face in the 1990s. In August 1991 CERN in Switzerland publicized the new World Wide Web project, two years after Tim Berners-Lee had begun creating HTML, HTTP and the first few web pages at CERN in Switzerland. In 1993 the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign released the Mosaic web browser version 1.0, and by late 1994 there was growing public interest in the previously academic/technical Internet. By 1996 the word "Internet" was common public currency, but it referred almost entirely to the World Wide Web.
Meanwhile, over the course of the decade, the Internet successfully accommodated the majority of previously existing public computer networks (although some networks such as FidoNet have remained separate). This growth is often attributed to the lack of central administration, which allows organic growth of the network, as well as the non-proprietary open nature of the Internet protocols, which encourages vendor interoperability and prevents any one company from exerting too much control over the network.
Today's Internet
Aside from the complex physical connections that make up its infrastructure, the Internet is held together by bi- or multi-lateral commercial contracts (for example peering agreements) and by technical specifications or protocols that describe how to exchange data over the network. Indeed, the Internet is essentially defined by its interconnections and routing policies.
As of January 2006, over 1 billion people use the Internet according to Internet World Stats.
Internet protocols
In this context, there are three layers of protocols:
at the lowest level is IP - the datagram which carries a block of data from one node to another
next comes TCP & UDP - the protocols by which one host exchanges data with another - the former making a virtual circuit giving some level of guarantee of reliability, the latter being a best-effort connection-less transport
on top comes the application protocol - the specific messages or data stream used by the application running on the hosts to talk to each other.
Unlike older communications systems, the Internet protocol suite was deliberately designed to be independent of the underlying physical medium. Any communications network, wired or wireless, that can carry two-way digital data can carry Internet traffic. Thus, Internet packets flow through wired networks like copper wire, coaxial cable, and fibre optic; and through wireless networks like Wi-Fi. Together, all these networks, sharing the same high-level protocols, form the Internet.
The Internet protocols originate from discussions within the Internet Engineering Task Force (IETF) and its working groups, which are open to public participation and review. These committees produce documents that are known as Request for Comments documents (RFCs). Some RFCs are raised to the status of Internet Standard by the IETF process.
Some of the most used application protocols in the Internet protocol suite are IP, TCP, UDP, DNS, PPP, SLIP, ICMP, POP3, IMAP, SMTP, HTTP, HTTPS, SSH, Telnet, FTP, LDAP, SSL, and TLS.
Some of the popular services on the Internet that make use of these protocols are e-mail, Usenet newsgroups, file sharing, Instant Messenger, the World Wide Web, Gopher, session access, WAIS, finger, IRC, MUDs, and MUSHs. Of these, e-mail and the World Wide Web are clearly the most used, and many other services are built upon them, such as mailing lists and blogs. The Internet makes it possible to provide real-time services such as Internet radio and webcasts that can be accessed from anywhere in the world.
Some other popular services of the Internet were not created this way, but were originally based on proprietary systems. These include IRC, ICQ, AIM, and Gnutella, although all of those mentioned now have Free implementations, which in some cases are the most commonly used.
Internet structure
There have been many analyses of the Internet and its structure. For example, it has been determined that the Internet IP routing structure and hypertext links of the World Wide Web are examples of scale-free networks.
Similar to how the commercial Internet providers connect via Internet exchange points, research networks tend to interconnect into large subnetworks such as:
GEANT
GLORIAD
Internet2
JANET (the UK's Joint Academic Network aka UKERNA)
These in turn are built around relatively smaller networks. See also the list of academic computer network organizations
In network schematic diagrams, the Internet is often represented by a cloud symbol, into and out of which network communications can pass.
ICANN
Main article: ICANN
The Internet Corporation for Assigned Names and Numbers (ICANN) is the authority that coordinates the assignment of unique identifiers on the Internet, including domain names, Internet protocol addresses, and protocol port and parameter numbers. A globally unified namespace (i.e., a system of names in which there is one and only one holder of each name) is essential for the Internet to function. ICANN is headquartered in Marina del Rey, California, but is overseen by an international board of directors drawn from across the Internet technical, business, academic, and non-commercial communities. The US government continues to have a privileged role in approving changes to the root zone file that lies at the heart of the domain name system. Because the Internet is a distributed network comprising many voluntarily interconnected networks, the Internet, as such, has no governing body. ICANN's role in coordinating the assignment of unique identifiers distinguishes it as perhaps the only central coordinating body on the global Internet, but the scope of its authority extends only to the Internet's systems of domain names, Internet protocol addresses, and protocol port and parameter numbers.
Internet culture
The Internet is also having a profound impact on work, leisure, knowledge and worldviews.
Graphic representation of a very small part of the WWW, representing some of the hyperlinks[edit]
The World Wide Web
Main article: World Wide Web
Through keyword-driven Internet research using search engines like Google, millions worldwide have easy, instant access to a vast and diverse amount of online information. Compared to encyclopedias and traditional libraries, the World Wide Web has enabled a sudden and extreme decentralization of information and data.
Some companies and individuals have adopted the use of 'weblogs' or blogs, which are largely used as easily-updatable online diaries. Some commercial organizations encourage staff to fill them with advice on their areas of specialization in the hope that visitors will be impressed by the expert knowledge and free information, and be attracted to the corporation as a result. One example of this practice is Microsoft, whose product developers publish their personal blogs in order to pique the public's interest in their work.
For more information on the distinction between the World Wide Web and the Internet itself — as in everyday use the two are sometimes confused — see Dark internet where this is discussed in more detail.
Remote access
The Internet allows computer users to connect to other computers and information stores easily, wherever they may be across the world. They may do this with or without the use of security, authentication and encryption technologies, depending on the requirements.
This is encouraging new ways of working from home, collaboration and information sharing in many industries. An accountant sitting at home can audit the books of a company based in another country, on a server situated in a third country that is remotely maintained by IT specialists in a fourth. These accounts could have been created by home-working book-keepers, in other remote locations, based on information e-mailed to them from offices all over the world. Some of these things were possible before the widespread use of the Internet, but the cost of private, leased lines would have made many of them infeasible in practice.
An office worker away from his desk, perhaps the other side of the world on a business trip or a holiday, can open a remote desktop session into his normal office PC using a secure Virtual Private Network (VPN) connection via the Internet. This gives him complete access to all his normal files and data, including e-mail and other applications, while he is away.
This concept is also referred to by some network security people as the Virtual Private Nightmare, because it extends the secure perimeter of a corporate network into its employees' homes; this has been the source of some notable security breaches.
Collaboration
This low-cost and nearly instantaneous sharing of ideas, knowledge and skills has revolutionized some, and given rise to whole new, areas of human activity. One example of this is the collaborative development and distribution of Free/Libre/Open-Source Software (FLOSS) such as Linux, Mozilla and OpenOffice.org. See Collaborative software.
File-sharing
Main article: File sharing
A computer file can be e-mailed to customers, colleagues and friends as an attachment. It can be uploaded to a website or FTP server for easy download by others. It can be put into a "shared location" or onto a file server for instant use by colleagues. The load of bulk downloads to many users can be eased by the use of "mirror" servers or peer-to-peer networking.
In any of these cases, access to the file may be controlled by user authentication; the transit of the file over the Internet may be obscured by encryption and money may change hands before or after access to the file is given. The price can be paid by the remote charging of funds from, for example a credit card whose details are also passed - hopefully fully encrypted - across the Internet. The origin and authenticity of the file received may be checked by digital signatures or by MD5 or other message digests.
These simple features of the Internet, over a world-wide basis, are changing the basis for the production, sale and distribution of many types of product, wherever they can be reduced to a computer file for transmission. This includes all manner of office documents, publications, software products, music, photography, video, animations, graphics and the other arts. This in turn is causing seismic shifts in each of the existing industry associations, such as the RIAA and MPAA in the USA, that previously controlled the production and distribution of these products in that country.
Streaming media and VoIP
Many existing radio and television broadcasters have provided Internet 'feeds' of their live audio and video streams (for example, the BBC). They have been joined by a range of pure Internet 'broadcasters' who never had on-air licences. This means that an Internet-connected device, such as a computer or something more specific, can be used to access on-line media in much the same way as was previously possible only with a TV or radio receiver. The range of material is much wider, from pornography to highly specialised technical web-casts. The simplest equipment can allow anybody, with little censorship or licensing control, to broadcast on a worldwide basis. Time-shift viewing or listening is not a problem as the BBC have shown with their Preview, Classic Clips and Listen Again features.
Web-cams can be seen as an even lower-budget extension of this phenomenon. In this case the picture may update only slowly - perhaps once every few seconds or slower, but Internet users can watch animals around an African waterhole, ships in the Panama Canal or the traffic at a local roundabout live and in real time. Video chat rooms, video conferencing, and remote controllable webcams have become popular. Some people install webcams in their bedrooms that can be accessed by other voyeurs, often with two-way sound.
VoIP stands for Voice over IP, where IP refers to the Internet Protocol that underlies all Internet communication. This phenomenon began as an optional two-way voice extension to some of the Instant Messaging systems that took off around the turn of the millennium. In recent years many people and organizations have made VoIP systems as easy to use and as convenient as a normal telephone. The benefit is that, as the Internet carries the actual voice traffic, VoIP is free or costs much less than an actual telephone call, especially over long distances and especially for those with always-on ADSL or DSL Internet connections anyway. The disadvantages are that it is still difficult to initiate a call with someone, unless they also have a VoIP phone or are at their computer and that there are still several competing standards that are mitigating against universal acceptance.
In all of these cases, existing large organizations, that have grown accustomed to regular incomes for their services, are finding increased competition in their service areas, coming directly from the Internet. While newcomers strive to make these inroads, the traditional industries have to adapt, adopt, complain or suffer. Meanwhile the consumer in each case most probably benefits from the increased range of services and possible price reductions. Some worry about censorship and control while others see a continuing globalisation of culture and norms.
Language
Main article: English on the Internet
The most prevalent language for communication on the Internet is English. This may be due to the Internet's origins, as well as English's role as the lingua franca. It may also be related to the poor capability of early computers to handle characters other than those in the basic Latin alphabet.
Further information: Unicode
After English (32% of web visitors) the most-requested languages on the world wide web are Chinese 13%, Japanese 8%, Spanish 7%, German 6% and French 4% (from Internet World Stats, updated November 30, 2005).
By continent, 34% of the world's Internet users are based in Asia, 29% in Europe, and 23% in North America ([2] updated November 21, 2005).
The Internet's technologies have developed enough in recent years that good facilities are available for development and communication in most widely used languages. However, some glitches such as mojibake still remain.
Cultural awareness
From a cultural awareness perspective, the Internet has been both an advantage and a liability. For people who are interested in other cultures it provides a significant amount of information and an interactivity that would be unavailable otherwise. However, for people who are not interested in other cultures there is some evidence indicating that the Internet enables them to avoid contact to a greater degree than ever before.
Internet and the workplace
With the emergence of the internet and recent high speed connections becoming available to the public, the internet has altered the way many people work in significant ways. Contrary to the traditional 9-5 workday where employees commute to and from work, the internet has allowed greater flexibility both in terms of working hours and work location. Today, many employees work from home by "telecommuting".
The internet and the advent of blogs has given employees a forum from which to voice their opinions about their jobs, employers and co-workers, creating a massive amount of information and data on work that is currently being collected by the Worklifewizard.org project run by Harvard Law School's Labor & Worklife Program.
Censorship
Main article: Censorship in cyberspace
Some countries, such as Iran and the People's Republic of China, restrict what people in their countries can see on the Internet, especially unwanted political and religious content. Censorship is sometimes done through government controlled censoring filters, or by means of law or culture, making the propagation of targeted materials extremely hard. However, many internet users are able to bypass these filters, meaning that most Internet content is available regardless of where one is in the world, so long as one has the technical skill and means of connecting to it.
In the Western world, it is Germany that has the highest rate of censorship, especially of Nazis. However, most countries in the Western world do not force Internet Service Providers to block sites.
There are a large number of programs available that will block what are deemed to be offensive sites (such as pornographic or violent) on individual computers or networks.
Some believe that US educational institutions are especially known for censoring content[citation needed]. One controversial Social networking website, MySpace.com, has been blocked by many American school districts because of its use by child abusers and other illegal activity. [citation needed]
Internet access
Main article: Internet access
Internet public access point.Wikibooks has more about this subject:
Online linux connectCommon methods of home access include dial-up, landline broadband (over coaxial cable, fibre optic or copper wires), Wi-Fi, satellite and cell phones.
Public places to use the Internet include libraries and Internet cafes, where computers with Internet connections are available. There are also Internet access points in many public places like airport halls, in some cases just for brief use while standing. Various terms are used, such as "public Internet kiosk", "public access terminal", and "Web payphone". Many hotels now also have public terminals, though these are usually fee based.
Wi-Fi provides wireless access to computer networks, and therefore can do so to the Internet itself. Hotspots providing such access include Wi-Fi-cafes, where a would-be user needs to bring their own wireless-enabled devices such as a laptop or PDA. These services may be free to all, free to customers only, or fee-based. A hotspot need not be limited to a confined location. The whole campus or park, or even the entire city can be enabled. Grassroots efforts have led to wireless community networks. Commercial WiFi services covering large city areas are in place in London, San Francisco, Philadelphia and other cities, including Toronto by the end of 2006. The Internet can then be accessed from such places as a park bench.[1]
Apart from Wi-Fi, there have been experiments with proprietary mobile wireless networks like Ricochet, various high-speed data services over cellular or mobile phone networks, and fixed wireless services.
Capitalisation conventions
In formal usage, Internet is traditionally written with a capital first letter. The Internet Society, the Internet Engineering Task Force, the Internet Corporation for Assigned Names and Numbers, the World Wide Web Consortium, and several other Internet-related organizations all use this convention in their publications. In English grammar, proper nouns are capitalised.
Most newspapers, newswires, periodicals, and technical journals also capitalize the term. Examples include the New York Times, the Associated Press, Time, The Times of India, Hindustan Times and Communications of the ACM.
In other cases, the first letter is often written small (internet), as it is argued that this is the correct form. However, the term internet with a small i refers in network technologies to any interconnected local area networks, whereas the Internet with a capital i is the specific name of the largest internet on Earth; but there are others.
Since 2000, a significant number of publications have switched to using internet. Among them are The Economist, the Financial Times, the London Times, and the Sydney Morning Herald. As of 2005, most publications using internet appear to be located outside of North America although one American news source, Wired News, has adopted the lowercase spelling.
Leisure
The Internet has been a major source of leisure since before the World Wide Web, with entertaining social experiments such as MOOs being conducted on university servers, and humor-related Usenet groups receiving much of the main traffic. Today, many Internet forums have sections devoted to neta; short cartoons in the form of Flash movies are also popular.
The pornography and gambling industries have both taken full advantage of the World Wide Web, and often provide a significant source of advertising revenue for other Web sites. Although many governments have attempted to put restrictions on both industries' use of the Internet, this has generally failed to stop their widespread popularity.
One main area of leisure on the Internet is multiplayer gaming. This form of leisure creates communities, bringing people of all ages and origins to enjoy the fast-paced world of multiplayer games. These range from MMORPG to first-person shooters, from role-playing games to online gambling. This has revolutionized the way many people interact and spend their free time on the Internet.
Online gaming began with services such as GameSpy and MPlayer, which players of games would typically subscribe to. Non-subscribers were limited to certain types of gameplay or certain games. With the release of Diablo by Blizzard Entertainment, gamers were treated to a built in online game service that was free of charge over Battle.net. With Blizzard's next game, StarCraft, the gaming world saw an explosion in the numbers of players using the Internet to play multi-player games. StarCraft may have been the first non-MMO game in which most players utilized the online gameplay as opposed to the single-player gameplay.
Many use the Internet to access and download music, movies and other works for their enjoyment and relaxation. As discussed above, there are paid and unpaid sources for all of these, using centralised servers and distributed, peer-to-peer technologies. Discretion is needed as some of these sources take more care over the original artists' rights and over copyright laws than others.
Many use the World Wide Web to access news, weather and sports reports, to plan and book holidays and to find out more about their random ideas and casual interests.
People use chat, messaging and email to make and stay in touch with friends worldwide, sometimes in the same way as some previously had pen pals. Social networking web sites like Friends Reunited and many others like them also put and keep people in contact for their enjoyment.
Cyberslacking has become a serious drain on corporate resources; the average UK employee spends 57 minutes a day surfing the web at work, according to a study by Peninsula Business Services[3].
A complex system
Many computer scientists see the Internet as a "prime example of a large-scale, highly engineered, yet highly complex system" (Willinger, et al). The Internet is extremely heterogeneous. (For instance, data transfer rates and physical characteristics of connections vary widely.) The Internet exhibits "emergent phenomena" that depend on its large-scale organization. For example, data transfer rates exhibit temporal self-similarity.
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Marketing
The Internet has also become a large market for companies; some of the biggest companies today have grown by taking advantage of the efficient nature of low-cost advertising and commerce through the Internet; also known as e-commerce. It is the fastest way to spread information to a vast amount of people simultaneously. The Internet has also subsequently revolutionized shopping—for example; a person can order a CD online and receive it in the mail within a couple of days, or download it directly in some cases. The Internet has also greatly facilitated personalized marketing which allows a company to market a product to a specific person or a specific group of people moreso than any other advertising medium.
Examples of personalized marketing include online communities such as Myspace, Friendster, and others which thousands of Internet users join to advertise themselves and make friends online. Many of these users are young teens and adolescents ranging from 13 to 25 years old. In turn, when they advertise themselves they advertise intrests and hobbies, which online marketing companies can use as information as to what those users will purchase online, and advertise their own companies' products to those users
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Pyramids
Pyramids are large structures with square bases and four smooth, triangular-shaped sides that come to a point at the top. Several ancient peoples used pyramids as tombs or temples. The most famous pyramids are those built about 4,500 years ago as tombs for Egyptian kings. These Egyptian pyramids are among the Seven Wonders of the Ancient World.
Egyptian pyramids
The ruins of 35 major pyramids still stand near the Nile River in Egypt. Each was built to house the body of an Egyptian king. The Egyptians thought that a person's body had to be preserved and protected so the soul could live forever. The Egyptians mummified (embalmed and dried) their dead and hid the mummies in large tombs. From about 2700 to 1700 B.C., the bodies of Egyptian kings were buried inside or beneath a pyramid in a secret chamber that was filled with treasures of gold and precious objects. Many scholars believe that the pyramid shape had a religious meaning to the Egyptians. The sloping sides may have reminded the Egyptians of the slanting rays of the sun, by which the soul of the king could climb to the sky and join the gods.
Funeral ceremonies were performed in temples that were attached to the pyramids. Most pyramids had two temples connected by a long stone passageway. Sometimes a smaller pyramid for the body of the queen stood next to the king's pyramid. Egypt has at least 40 smaller pyramids that were used for queens or as memorial monuments for kings. The king's relatives and officials were buried in smaller rectangular tombs called mastabas, which had sloping sides and flat roofs.
The first pyramids. Imhotep, a great architect and statesman, built the first known pyramid for King Zoser in about 2650 B.C. Zoser's tomb rose in a series of giant steps, or terraces, and is called the Step Pyramid. This pyramid still stands at the site of the ancient city of Memphis, near Saqqarah.
The first smooth-sided pyramid was built about 2600 B.C. It still stands at Medum. It began as a stepped pyramid, and then the steps were filled in with casing stones to give the building smooth, sloping sides. Other pyramids built during a period of Egyptian history called the Old Kingdom (2686-2181 B.C.) can be seen at Abusir and Dahshur. During the Middle Kingdom (2052-1786 B.C.), pyramids were built at Hawara, Illahun, Lisht, and Dahshur--near what is now Cairo. The remains of these pyramids are still impressive.
The Pyramids of Giza (Al Jizah) stand on the west bank of the Nile River outside Cairo. There are 10 pyramids at Giza, including three of the largest and best preserved of all Egyptian pyramids. They were built for kings in about 2600 to 2500 B.C. The largest was built for King Khufu (called Cheops by the Greeks). The second was built for King Khafre (Chephren), and the third for King Menkaure (Mycerinus). A huge statue of a sphinx, called the Great Sphinx, was probably built for Khafre. It stands near his pyramid.
The pyramid of Khufu, called the Great Pyramid, contains more than 2 million stone blocks that average 2.3 metric tons each. It was originally 147 metres tall, but some of its upper stones are gone now and it stands about 140 metres high. Its base covers an area of about 5 hectares.
A study of the Great Pyramid shows how these gigantic structures were built. The ancient Egyptians had no machinery or iron tools. They cut big limestone blocks with copper chisels and saws. Most of the stones came from quarries nearby. But some came from across the Nile, and others came by boat from distant quarries. Gangs of men dragged the blocks to the pyramid site and pushed the first layer of stones into place. Then they built long ramps of earth and brick, and dragged the stones up the ramps to form the next layer. As they finished each layer, they raised and lengthened the ramps. Finally, they covered the pyramid with an outer coating of white casing stones. They laid these outer stones so exactly that from a distance the pyramid appeared to have been cut out of a single white stone. Most of the casing stones are gone now, but a few are still in place at the bottom of the Great Pyramid.
The burial chamber is inside the Great Pyramid. A corridor leads from an entrance on the north side to several rooms within the pyramid. One of the rooms is called the Queen's Chamber, although the queen is not buried there. The room was planned as the king's burial chamber. But Khufu changed the plan and built another burial chamber, called the King's Chamber. The Grand Gallery, a corridor 47 metres long and 8.5 metres high, leads to Khufu's chamber. The Great Pyramid is considered a marvel of ancient architecture.
No one knows how long it took to build the Great Pyramid. The ancient Greek historian Herodotus said that the work went on in four-month shifts, with 100,000 workers in each shift. Scholars now doubt that account and believe that about 100,000 men worked on the pyramids for three or four months each year. Farm labourers built the pyramids. They worked on the tombs during periods when floodwaters of the Nile covered the fields and made farming impossible.
Thieves broke into most of the pyramids, stole the gold, and sometimes destroyed the bodies. Later Egyptian kings stopped using pyramids, and built secret tombs in cliffs. But some kings of the Kushite kingdom in Nubia, south of Egypt, built pyramids long after they were no longer used in Egypt