৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি [২৮১] · তারিখ অনির্ধারিত · ৪৭ প্রশ্ন
The SI unit of electric charge is the Coulomb (C). It is defined as the amount of charge transferred by a current of one ampere in one second.
Example:
If a current of 1 ampere flows through a conductor for 1 second, the amount of charge transferred is 1 coulomb.
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett
A proton carries a positive charge, whereas an electron carries a negative charge. A neutron has no charge, and a photon is a neutral particle associated with electromagnetic radiation.
Example:
The proton in the nucleus of an atom interacts with electrons (which are negatively charged) to form an atom.
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett
Like charges repel each other, as described by Coulomb’s Law. If both charges are positive or negative, they will experience a repulsive force.
Example:
If you place two positively charged spheres near each other, they will push away from each other because of the repulsive force.
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett
The mathematical form of Coulomb’s Law is given by
F=Kq1q2/r2
Where:
F is the electrostatic force between two point charges,
q1 and q2 are the charges,
r is the distance between them,
K is Coulomb’s constant and K = 8.99×109 Nm2/C2
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett.
The electric field due to a positive charge radiates outward, meaning the field lines point away from the charge. The field lines represent the direction of the force that a positive test charge would experience if placed at that point.
Example:
For a positive point charge located at the origin, the electric field at a point to the right (along the x-axis) would point away from the charge, in the positive x-direction.
Source: Introduction to Electrodynamics by David J. Griffiths
The electric field is defined as the force per unit charge exerted on a small positive test charge. Its SI unit is Newton per Coulomb (N/C), which represents the amount of force (in Newtons) experienced by a unit charge (in Coulombs).
Example:
If a charge of +1C experiences a force of +5N in an electric field, the field strength is 5N/C.
Source: Introduction to Electrodynamics by David J. Griffiths
In a vacuum, the speed of electromagnetic (EM) waves is the speed of light and is approximately:
c=3×108 m/s
This is the speed at which all electromagnetic waves, including light, radio waves, microwaves, X-rays, and others, propagate through a vacuum.
The speed of EM waves in a vacuum is a fundamental constant in physics and is denoted by c.
This value is the same for all types of electromagnetic waves, regardless of their frequency or wavelength.
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett.
Source: Physics for Scientists and Engineers by Raymond A. Serway and John W. Jewett
In an electromagnetic wave, energy is carried by both the electric field and the magnetic field. The energy in an electromagnetic wave is distributed between the electric and magnetic components, with the energy density in each field being proportional to the square of its field strength.
Example:
In a radio wave, both the oscillating electric and magnetic fields carry energy that propagates through space.
Source: Introduction to Electrodynamics by David J. Griffiths
Among the given types of electromagnetic waves, radio waves have the longest wavelength. The wavelength of electromagnetic waves inversely correlates with frequency: waves with lower frequencies (like radio waves) have longer wavelengths, while higher frequency waves (like gamma rays) have shorter wavelengths.
Example:
Radio waves can have wavelengths ranging from a few millimeters to thousands of kilometers, whereas gamma rays have wavelengths on the order of picometers (pm), which are much shorter.
Summary of wavelength:
X-rays: 0.01nm to 10nm
Radio waves: 1mm to 100km
Visible light: 400nm to 700nm
Gamma rays: <0.01nm
Source: Introduction to Electrodynamics by David J. Griffiths.
The 5G network uses a combination of microwaves and millimeter waves to provide higher data speeds, low latency, and improved connectivity. These types of electromagnetic waves are particularly useful in 5G because they can support large bandwidths and are capable of transmitting large amounts of data quickly over shorter distances.
Microwaves and Millimeter Waves in 5G:
Microwaves are used in communication systems for a range of frequencies, particularly in sub-6 GHz bands.
Millimeter waves (with wavelengths between 1 mm and 10 mm, corresponding to frequencies from 30 GHz to 300 GHz) are used in the higher-frequency 5G bands. These provide ultra-fast data transfer rates and are key to achieving the high-speed performance of 5G, though their range is typically shorter than lower-frequency waves.
Sources: Introduction to Wireless Communications and Networks by William Stallings
According to special relativity, as an object’s speed approaches the speed of light, its relativistic mass increases without bound. This means that the object requires more and more energy to continue accelerating as it approaches the speed of light. The relativistic mass (m) is given by the equation:
Example:
If a spaceship moves at 99% of the speed of light, its mass will be significantly higher than when it is at rest.
Source: Special Relativity by Albert Einstein.
In special relativity, the speed of light in vacuum (c) is the maximum speed at which information or matter can travel. No object with mass can reach the speed of light. As an object's velocity approaches the speed of light, its relativistic mass increases, requiring more and more energy to continue accelerating.
Example:
Particles in particle accelerators approach but never reach the speed of light, no matter how much energy is added.
Source: Special Relativity by Albert Einstein
we use Einstein's famous equation:
E=mc2
Where:
E is the energy (in joules),
m is the mass (in kilograms),
c is the speed of light in a vacuum (c=3.0×108m/s).
Given that,
Mass,m=1 gram = 0.001 kg
Speed of light, c=3.0×108 m/s.
Now,Substitute the values into the equation:
E=mc2
E=(0.001)×(3.0×108)2
E=9×1013 Joules
Source: Special Relativity by Albert Einstein.
The unit Hertz (Hz) is the SI (International System of Units) unit of frequency. It measures the number of cycles or oscillations of a wave that occur in one second.
1 Hz means that one full cycle (e.g., one complete oscillation, wave, or rotation) occurs every second.
Example:
if a light wave or sound wave has a frequency of 1 Hz, it completes one cycle every second.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Sound waves are mechanical waves that require a medium (solid, liquid, or gas) to propagate. They transfer energy through the vibration of particles in the medium. Electromagnetic waves (like light, radio waves, and microwaves) do not require a medium and can travel through a vacuum.
Example:
Sound cannot travel through the vacuum of space because there is no medium to transmit the sound waves. However, sound waves can travel through air or water.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Light waves are a type of electromagnetic wave, so the correct answer is Light waves.
Electromagnetic waves are waves that consist of oscillating electric and magnetic fields, which propagate through space at the speed of light. They do not require a medium to travel, meaning they can propagate through a vacuum. Light waves (visible light) are a type of electromagnetic wave, along with other types like radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays, and gamma rays.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Gamma rays are a type of electromagnetic wave that has the highest energy and the shortest wavelengths in the electromagnetic spectrum. Due to their high energy, gamma rays are used in radiation therapy for cancer treatment.
In radiotherapy, gamma rays are directed at cancer cells to damage their DNA and stop them from growing and dividing, which helps to kill the cancer cells.
Used in Cancer Treatment:
Gamma radiation is used in external beam radiation therapy or internal radiation therapy (also called brachytherapy) for the treatment of various types of cancer.
Cobalt-60, a radioactive isotope that emits gamma rays, is commonly used in radiotherapy machines for cancer treatment.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The principle of superposition states that when two or more waves overlap in space, the resulting displacement at any point is the sum of the displacements of the individual waves. This applies to all types of waves, including transverse, longitudinal, and electromagnetic waves.
Example:
If two sound waves meet at the same point in space, the sound heard by a listener is the sum of the displacements caused by each wave at that point.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
A transverse wave is one in which the oscillations are perpendicular to the direction of wave propagation. Water waves are an example of transverse waves because the water particles move up and down while the wave moves horizontally.
Example:
If you throw a stone into a pond, you see ripples propagating outward, while the water itself moves up and down.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
In electromagnetic theory, Gauss's Law for Electricity states that the electric flux through a closed surface is proportional to the charge enclosed within the surface. However, when considering electric current, we need to understand the relationship between current, charge, and the area through which the current flows.
Current and Charge:
The electric current through a closed surface is related to the amount of charge passing through the surface per unit time.
Current (I) is defined as:
I = Q/t
Where Q is the charge passing through a conductor in time t.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
A diode is a semiconductor device that allows current to flow in one direction (forward) and blocks it in the opposite direction (reverse).
Breakdown in a Diode:
The breakdown in a diode occurs when the reverse voltage exceeds a certain critical value known as the reverse breakdown voltage or Zener voltage. This typically happens under reverse bias conditions.
In reverse bias, the diode is designed to block current flow, but when the reverse voltage becomes large enough, the electric field inside the diode becomes strong enough to break the bonds in the semiconductor, causing a sudden increase in current. This is known as reverse breakdown.
Forward Bias vs. Reverse Bias:
In forward bias, current flows easily through the diode as long as the forward voltage is above a certain threshold (typically around 0.7V for silicon diodes).
In reverse bias, the diode ideally does not allow current to flow, but if the reverse voltage becomes too high, it can lead to breakdown.
Source: Introduction to Electrodynamics by David J. Griffiths
Maxwell’s correction to Ampère’s Law adds the term which accounts for the displacement current in situations where the electric field changes with time. This term allows Ampère’s Law to apply to all situations, including those involving time-varying electric fields.
The corrected form is:
Where,
Example:
In a capacitor with a time-varying electric field between the plates, the displacement current is responsible for generating a magnetic field even if no physical current flows between the plates.
Source: Introduction to Electrodynamics by David J. Griffiths
Faraday's Law applies to situations where there is a change in the magnetic flux through a surface, such as when a magnet is moving near a coil, or when the magnet's strength changes with time. This changing magnetic flux induces an electric field and EMF.
Example:
If you move a magnet near a coil of wire, it will induce a current in the coil due to the changing magnetic flux.
Source: Introduction to Electrodynamics by David J. Griffiths
Induction motors are based on Faraday’s Law of electromagnetic induction. They work by using a time-varying magnetic field to induce an electric current in a coil of wire, which in turn produces motion.
Example:
In an induction motor, the rotating magnetic field induces a current in the rotor, causing it to spin.
Source: Introduction to Electrodynamics by David J. Griffiths
The energy of the emitted electrons in the photoelectric effect depends on the frequency of the incident light, not its intensity. According to Einstein’s equation for the photoelectric effect:
Where,
Example:
If light of a higher frequency (like ultraviolet) strikes a metal, it can emit electrons with higher kinetic energy compared to light of lower frequency (like red light).
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker.
For the photoelectric effect to occur, the frequency of the incident light must be higher than a threshold frequency, which is determined by the work function of the material. If the frequency is too low, no electrons are emitted, regardless of the intensity of the light.
Example:
For zinc, the minimum frequency of light needed to emit electrons is in the ultraviolet range, since its work function is higher than the frequency of visible light.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Increasing the intensity of the incident light increases the number of photons hitting the material, which can cause more electrons to be emitted. However, the energy of the emitted electrons depends on the frequency of the light, not its intensity.
Example:
If you increase the intensity of ultraviolet light on a metal surface, you will get more electrons emitted, but their energy will remain the same as long as the frequency of the light is unchanged.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker.
The photoelectric effect showed that light has both wave and particle properties, leading to the wave-particle duality.
The photoelectric effect provided evidence for the particle nature of light. According to Einstein, light consists of photons, which are discrete packets of energy. When light hits a metal surface, individual photons transfer their energy to electrons, which is consistent with the particle model of light.
Example:
The photoelectric effect cannot be explained by the classical wave theory of light, which predicted that the energy of electrons would depend on the intensity of light, not its frequency.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker.
The quantum theory of light proposes that light is not just a wave but also behaves as particles called photons. Each photon carries a discrete amount of energy determined by its frequency, and this particle-like nature of light explains phenomena like the photoelectric effect.
Einstein explained that light can be thought of as discrete packets of energy (photons) that interact with matter, causing phenomena such as electron emission.
Example:
When ultraviolet light hits a metal surface, it can eject electrons. The energy required to release electrons is not related to the intensity of the light but to the frequency of the light. This is consistent with the photon concept.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The energy of a photon is given by the equation:
E = hf
Where,
E is the energy of the photon,
h is Planck's constant (6.626×10−34J\cdotps),
f is the frequency of the photon.
This shows that the energy of a photon is directly proportional to its frequency. Higher frequency photons (like blue light) have higher energy than lower frequency photons (like red light).
Example:
A blue photon (with a higher frequency) has more energy than a red photon (with a lower frequency).
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker.
The photoelectric effect demonstrated that light behaves as a particle (photon) because only light with a frequency above a certain threshold could emit electrons from a metal surface, regardless of the intensity of the light. This could not be explained by the classical wave theory, which predicted that light of any frequency would eventually release electrons with enough intensity.
Einstein's explanation of the photoelectric effect showed that the energy of the photons is directly related to the frequency of the light, not its intensity.
Example:
When ultraviolet light shines on a metal surface, electrons are ejected if the frequency is high enough. Red light, even with high intensity, does not cause electron emission because its frequency is below the threshold.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The quantum theory of light incorporates the idea of wave-particle duality, where light exhibits properties of both waves and particles. This dual nature is necessary to explain various phenomena such as interference and diffraction (wave properties) as well as the photoelectric effect (particle properties).
Example:
Light can exhibit wave-like behaviors, such as interference patterns, in some experiments, but it can also behave as discrete particles (photons), as shown in the photoelectric effect.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
According to the equation E=hf and the relationship f=c/λ, the energy of a photon is inversely proportional to its wavelength. As the wavelength increases, the energy of the photon decreases.
Example:
Photons of red light (longer wavelength) have less energy than photons of blue light (shorter wavelength).
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Albert Einstein proposed the concept of the photon in 1905 to explain the photoelectric effect. He suggested that light could be thought of as discrete particles (photons), each carrying energy proportional to its frequency. This was a pivotal moment in the development of quantum theory.
Example:
Einstein showed that when light strikes a metal, it transfers its energy to electrons in discrete packets, or photons, leading to their ejection from the metal.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
X-rays are produced when high-energy electrons (typically accelerated in a cathode ray tube) strike a metal target. The sudden deceleration of the electrons upon hitting the target causes the emission of X-rays. These X-rays can then be used for imaging and analysis.
Example:
In an X-ray tube, electrons are accelerated and directed towards a tungsten target. When the electrons collide with the tungsten atoms, X-rays are emitted.
Source: Introduction to X-ray Crystallography by Michael M. Woolfson
X-rays are primarily used in medical diagnostics for creating images of the internal structures of the body, such as bones, organs, and tissues. These images help in detecting fractures, tumors, infections, and other abnormalities.
Example:
A chest X-ray is commonly used to detect lung diseases, such as pneumonia or tuberculosis.
Source: Introduction to X-ray Crystallography by Michael M. Woolfson
X-ray diffraction (XRD) is a technique used to study the crystal structure of materials. When X-rays are directed at a crystalline material, they are diffracted in specific patterns depending on the arrangement of atoms in the crystal. By analyzing these patterns, the atomic structure of the material can be determined.
Example:
X-ray diffraction is used extensively in materials science, chemistry, and biology for analyzing the crystalline structure of substances like metals, minerals, and proteins.
Source: Introduction to X-ray Crystallography by Michael M. Woolfson
X-rays have wavelengths in the range of 0.01–10 nm, which places them in the short wavelength end of the electromagnetic spectrum. This is much smaller than the wavelength of visible light, which ranges from about 400 nm to 700 nm.
Example:
X-ray machines used in medical imaging and crystallography use X-rays with wavelengths typically around 0.1 nm to 10 nm.
Source: Introduction to X-ray Crystallography by Michael M. Woolfson.
In the Compton effect, the energy of the incident photon is partially transferred to the electron during scattering. As a result, the photon loses energy, which leads to an increase in the wavelength of the scattered photon. This is consistent with the idea that photons have momentum and can transfer part of their energy to the electron.
Example:
When X-rays scatter off an electron at an angle, the energy of the scattered X-ray is less than the energy of the incident X-ray, leading to a longer wavelength (lower energy) of the scattered X-ray.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The Compton effect confirmed the dual nature of light that light behaves as both a wave and a particle (photon). It was a critical experiment in the development of quantum mechanics, showing that light exhibits particle like behavior, especially in its interaction with matter, such as electrons.
Example:
The Compton effect demonstrated that light can be understood not only as a continuous wave (as described by classical physics) but also as discrete packets of energy (photons), consistent with the quantum theory of light.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The De Broglie wavelength (λ) is related to the momentum (p) of a particle by the equation:
λ=h/p
Where,
h is Planck’s constant,
p is the momentum of the particle.
Since momentum (p=mv, where m is the mass and v is the velocity) is directly related to the particle’s mass and velocity, the De Broglie wavelength is inversely proportional to momentum.
Example:
An electron traveling with higher momentum (higher velocity or mass) will have a smaller De Broglie wavelength compared to a slower electron.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
The Davisson-Germer experiment (1927) confirmed the wave nature of electrons. Electrons were directed at a crystal, and the resulting diffraction pattern was consistent with the predictions made by De Broglie’s hypothesis, confirming that electrons behave as matter waves.
Example:
In the Davisson-Germer experiment, electrons were scattered by a crystal, and a diffraction pattern was observed, just like in X-ray diffraction, showing that electrons have wave-like properties.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
In a dispersive medium, the phase velocity and group velocity are typically different. This occurs because the wave components of different frequencies travel at different speeds. In such a medium, the wave packet (group velocity) and the individual wave components (phase velocity) propagate at different speeds.
Example:
In optical fibers, different wavelengths of light propagate at different speeds, meaning that the group velocity is different from the phase velocity of individual frequency components.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
In a non-dispersive medium, all frequency components of the wave travel at the same speed. As a result, the phase velocity and the group velocity are the same, because the wave components move together as a single entity. This is typically true for ideal cases, such as in light propagation in a vacuum where the speed of light is constant.
Example:
In a vacuum, the speed of light is constant, so both the phase velocity and group velocity of light waves are the same.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
Planck's constant (h) is a fundamental constant in quantum mechanics and is used to describe the quantization of energy levels.
Its accepted SI value is 6.626×10-34 J\cdotps.
Planck's constant is critical in equations such as the energy of a photon, which is given by:
E=h⋅f
Where,
E is the energy of the photon,
h is Planck's constant,
f is the frequency of the electromagnetic wave.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker
In the case of water waves, the phase velocity is influenced by the depth of the water. In deep water, the phase velocity is primarily determined by the frequency of the wave, but in shallow water, it depends on both the frequency and the depth of the water. The equation for phase velocity in shallow water waves is given below,
Where,
g is the acceleration due to gravity,
d is the depth of the water.
Example:
In a shallow lake, the phase velocity of waves will be slower than in the ocean, where the water is deeper.
Source: Fundamentals of Physics by David Halliday, Robert Resnick, and Jearl Walker