Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Heat and Thermodynamics → Heat transfer ↓


heat radiation


Heat transfer is a fundamental concept in the study of physics and thermodynamics. When an object is at a different temperature than its surrounding environment, heat transfer occurs, moving energy from a higher temperature object to a lower temperature environment or object. There are three modes of heat transfer: conduction, convection, and radiation. In this detailed explanation, we will dive deep into the intricacies of heat radiation, explain what it is, how it works, and provide examples to help illustrate its many aspects.

What is radiation of heat?

Radiation is the process by which heat is transferred via electromagnetic waves. Unlike conduction and convection, which depend on the presence of particles and mediums, radiation requires no medium; it can occur in a vacuum. This characteristic enables the Sun to warm the Earth, since there is no direct physical medium between them over the vast distances of space.

Understanding the process

Radiation involves the emission of energy in the form of electromagnetic waves. All objects emit thermal radiation at temperatures above absolute zero. The energy emitted depends on the temperature of the object; as the temperature increases, the energy emitted as radiation also increases.

Planck's Law

Planck's Law describes how much electromagnetic radiation is emitted by a black body in thermal equilibrium at a given temperature. The formula can be written as:

B(λ, T) = (2hc^2 / λ^5) * (1 / (e^(hc / λkT) - 1))

Where:

  • B(λ, T) is the spectral irradiance.
  • λ is the wavelength.
  • h is the Planck constant.
  • c is the speed of light in vacuum.
  • k is the Boltzmann constant.
  • T is the absolute temperature of the body.

Features and characteristics of thermal radiation

Some important characteristics of thermal radiation are as follows:

  • Wavelength range: Thermal radiation falls primarily in the infrared region of the electromagnetic spectrum, although it also extends into the visible light and ultraviolet regions at higher temperatures. As the temperature increases, the maximum wavelength of the emitted radiation shifts to shorter wavelengths.
  • Surface properties: The emissivity of a surface determines how effectively it emits thermal radiation. A real object does not emit radiation as efficiently as a black body, but emissivity determines how perfect the emissivity is relative to a perfect black body.
  • Temperature dependence: The Stefan–Boltzmann law states that the total energy emitted per unit surface area of a black body is proportional to the fourth power of the absolute temperature of the body.
Stefan-Boltzmann Law

The Stefan-Boltzmann law can be represented by the formula:

E = σT^4

Where:

  • E is the energy radiated per unit area.
  • σ is the Stefan–Boltzmann constant (about 5.67 × 10^-8 W/m^2K^4).
  • T is the absolute temperature of the black body.

Examples of radiation

The concept of radiation can be better understood through practical and visual examples:

Sun

This diagram shows the Sun radiating heat to Earth, despite the vacuum of space. The Sun, being at a very high temperature, emits a great deal of infrared radiation, which travels through the vacuum of space to warm the Earth.

Another common example is a campfire. When you sit near a campfire, you feel warmth even without touching it. This warmth is due to the heat coming directly from the fire to your skin.

Practical example: calculation of radiated heat

Let's calculate the energy radiated by a tungsten filament in a light bulb with 0.35 emission efficiency, 0.01 m² surface area, and 3000 K temperature using the Stefan-Boltzmann law:

E = εσT^4

Where:

  • The ε of tungsten is (0.35).
  • σ is the Stefan–Boltzmann constant (5.67 × 10^-8 W/m²K^4).
  • T is the temperature in Kelvin (3000 K).

Substitute the known values into the equation:

E = 0.35 × 5.67 × 10^-8 W/m²K^4 × (3000 K)^4 E = 0.35 × 5.67 × 10^-8 × 8.1 × 10^13 E = 0.35 × 4.5927 × 10^6 E = 1.6074 × 10^6 W/m²

Thus, the energy radiated by the filament is about 1.6074 × 10^6 W/m².

Radiation emission visualization

objectRadiated heat

In this diagram, we see an object emitting radiation in different directions. This emitted radiation travels away from the object in straight lines, symbolizing heat being radiated into the surrounding space.

Practical applications of radiation

Heat radiation has many practical applications in everyday life and science:

  • Thermal imaging: Devices such as night-vision cameras detect infrared radiation to "see" objects in complete darkness, and operate on the principle of radiation emitted by hot objects.
  • Solar panels: Solar panels collect electromagnetic radiation from the sun and convert it into electricity, allowing practical uses of solar thermal energy.
  • Cooking: Microwave ovens and infrared grills use electromagnetic waves to heat food quickly and efficiently through radiation.
  • Climatology: Understanding the Earth's radiation balance is important for studying global warming and weather patterns.

Factors affecting radiation

Several factors affect the rate and efficiency of heat radiation:

  • Surface temperature: Higher the surface temperature, greater is the heat radiation.
  • Surface area: Larger surfaces emit more heat.
  • Emissivity: Materials with high emissivity are efficient radiators of heat.

Example: Role of colour in radiation

The colour of an object plays an important role in its absorption and emission of radiation. Darker objects absorb and emit more radiation than lighter coloured objects, which is why black clothes feel hotter when exposed to sunlight than white clothes.

Conclusion

Understanding heat radiation is essential in many branches of physics and practical applications. From the heat of the Sun to the efficiency of solar panels, radiation plays a key role in how we understand and use energy. Understanding how objects emit and absorb thermal radiation can provide insight into many phenomena, from everyday events to advanced technology and environmental science. With this broader understanding of heat radiation, we can better understand its role in the physical world and its interactions.


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heat radiation

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