Condenser vs Dynamic Microphones: Transducer Science Explained

Electromagnetic induction is the process where a changing magnetic field induces an electric current in a conductor. When a conductor moves through a magnetic field, it generates an induced EMF that creates a potential difference, driving current through a closed circuit. Faraday's Law states that a changing magnetic flux through a closed loop induces an EMF proportional to the rate of flux change. Magnetic flux (Φ) is calculated as Φ = B × A × cos(α). When a rectangular loop enters a uniform magnetic field, flux increases, inducing current. When the loop is completely inside, flux is constant, so no current flows. When the loop exits, flux decreases, inducing current in the opposite direction. The key condition is flux change, not flux presence.

Magnetic induction, discovered by Michael Faraday in 1831, is the process by which moving a conductor through a magnetic field generates electricity; this principle, described by Faraday's Law (EMF = -N(dΦ/dt)), forms the foundation of most power generation methods including wind, hydroelectric, coal, gas, and nuclear plants, where mechanical energy from various sources spins coils within magnetic fields to transform energy into electrical current.

This comprehensive segment covers the foundational concepts of electromagnetic induction for moving conductors and loops. The instructor introduces the five problem groups: moving straight conductors, loops and coils, AC generators, magnetic energy, and coupled coils. For moving conductors, the key formulas are: induced EMF ε = B × L × v × sin(θ), induced current I = ε / R, magnetic force F = B × I × L × sin(θ), and power P = I² × R. The instructor demonstrates complete problem-solving methodology, including identifying angles from diagrams, calculating EMF, current, force, and power with proper units. The segment also covers Faraday's Law for loops: ε = -N × (ΔΦ / Δt), with key observations about calculating area, converting units, and determining angles. The instructor emphasizes that all exam questions will come from this review material.

Electromagnetic induction is the phenomenon of generating an induced electromotive force (EMF) and induced current in a closed electrical circuit when there is a change in the magnetic flux through that circuit. This phenomenon was discovered by Michael Faraday, who found that a changing magnetic field can produce electricity, which is the reverse of Oersted's discovery that an electric current produces a magnetic field. The key components include: induced EMF (voltage), induced current, closed electrical circuits (loops or coils), and the requirement of changing magnetic flux. Magnetic field lines are imaginary lines that form closed loops from north to south pole, with density indicating field strength. Magnetic flux (Φ) is calculated as Φ = B × A, measured in Weber (Wb), with flux density (B) measured in Tesla (T). For electromagnetic induction to occur, three conditions must be met: a closed electrical circuit must be present, a magnetic field must exist, and there must be a change in the magnetic flux through the circuit. When both the magnet and loop are stationary relative to each other, no induced current is generated even though magnetic flux exists. The key requirement is a change in flux, not the mere presence of flux. Faraday's Law states that an induced EMF is generated in a closed electrical circuit when there is a change in the magnetic flux through that circuit. The induced EMF is directly proportional to the rate of change of magnetic flux. In experimental demonstrations, a magnet is moved relative to a conducting loop connected to a galvanometer. When the magnet approaches the loop, the galvanometer deflects in one direction, indicating induced current. When the magnet moves away, the galvanometer deflects in the opposite direction. The deflection returns to zero when the magnet is stationary, as the flux becomes constant. This demonstrates that induced current requires a changing magnetic environment. The direction of induced current depends on whether the flux is increasing or decreasing, as shown by galvanometer deflection in opposite directions. Hans Christian Oersted discovered that an electric current flowing through a wire produces a magnetic field around it, establishing the connection between electricity and magnetism. When current flows through a wire, magnetic field lines form concentric circles around the wire, with direction determined by the right-hand rule. The strength of the magnetic field is proportional to the current and inversely proportional to the distance from the wire. When the current in a wire changes, the magnetic field around it also changes, which can induce an EMF in a nearby circuit.

This comprehensive segment covers the foundational principles of electromagnetic induction. The instructor explains that 70-80% of final exam questions come from chapter material, making thorough chapter review essential. Multiple choice questions carry 10 points each with no partial credit, making them high-risk questions. The segment covers Faraday's Law, which states that induced EMF equals the negative rate of change of magnetic flux through a closed loop. The instructor demonstrates this using a copper ring moving in a magnetic field, showing how moving from left to right decreases flux while moving from right to left increases flux. The segment emphasizes that an open circuit cannot support induced current, and the direction of induced current depends on whether flux is increasing or decreasing. The instructor also covers the right-hand rule for determining current direction in magnetic fields.

Capacitance depends on three factors: (1) Plate area - capacitance is directly proportional to plate area, (2) Distance between plates - capacitance is inversely proportional to distance, (3) Dielectric material - capacitance depends on the dielectric constant of the material between plates. The relationship is C = εA/d, where ε is the permittivity, A is the plate area, and d is the distance between plates. Capacitance is defined as C = Q/V, where Q is charge and V is voltage. When distance between plates increases, capacitance decreases, which causes charge to decrease if voltage remains constant. When a dielectric is inserted between the plates of an isolated (disconnected) capacitor, the capacitance increases. Since charge remains constant, the voltage decreases. The relationship is V' = V/K, where K is the dielectric constant.

This comprehensive section covers electrostatic fundamentals: Conservation of Charge (charge cannot be created/destroyed, only transferred, discovered by Benjamin Franklin); Electric Field Intensity (force per unit charge, E = F/Q, unit N/C or V/m); Electric Dipole Moment (product of charge and separation distance, p = q × 2l); Dielectric Strength (maximum field a material can withstand); Relative Permittivity (ratio of material permittivity to vacuum permittivity); Quantization of Charge (charge exists in discrete units of e = 1.6 × 10⁻¹⁹ C, Q = ne); Coulomb's Law (force between point charges proportional to product of charges and inversely proportional to square of distance, F = kq₁q₂/r²); Gauss's Law (electric flux through closed surface equals enclosed charge divided by ε₀); Fundamental properties of electric charge (conservation, quantization, additivity); Limitations of Coulomb's Law (only for point charges, valid for distances > 10⁻¹⁵ m); Volume Charge Density (ρ = Q/V, unit C/m³); Electric Field Lines (originate from positive charges, terminate on negative charges, never intersect); Electric Potential (work done per unit charge to bring it from infinity, V = W/Q, unit Volt); Electric Potential Difference (work done moving charge between two points); Relationship between Electric Field and Potential (E = -dV/dx, field is negative gradient of potential); Equipotential Surfaces (constant potential, field lines perpendicular to them); Capacitance (ability to store charge, C = Q/V, unit Farad); Factors affecting parallel plate capacitor capacitance (plate area, distance between plates, dielectric material); Series and Parallel combinations of capacitors (1/C_eq = 1/C₁ + 1/C₂ for series, C_eq = C₁ + C₂ for parallel); Dielectric materials increase capacitance by reducing potential difference for same charge.

This segment explains the relationship between plate distance and capacitance when a capacitor remains connected to a battery. The instructor demonstrates that increasing the distance between plates decreases capacitance, and since the battery maintains constant voltage, the charge on the plates also decreases. The electric field intensity remains constant because E = V/d, and both V and d increase proportionally. This is described as a 'very strong question' that tests understanding of the interdependence of capacitance, charge, and voltage.

A parallel plate capacitor consists of two parallel conductive plates separated by a distance. The electric field between the plates is E = σ/ε₀, where σ is the surface charge density. The capacitance is derived as follows: C = Q/V, where V = Ed and E = σ/ε₀. Since σ = Q/A, we get V = (Qd)/(Aε₀). Therefore, C = Q/V = Aε₀/d. This shows that capacitance depends on the plate area A, the permittivity of the medium ε₀, and the distance between plates d. The formula demonstrates that capacitance is directly proportional to plate area and permittivity, and inversely proportional to the distance between plates.

The capacitance (C) of a parallel plate capacitor is derived as follows: Consider two parallel plates with area A separated by distance d. One plate has charge +q and the other has charge -q. The surface charge density σ = q/A. The electric field between the plates is E = σ/ε₀ = q/(Aε₀). The potential difference V = E × d = qd/(Aε₀). Capacitance C = q/V = q / [qd/(Aε₀)] = Aε₀/d. Thus, C = ε₀A/d, where ε₀ is the permittivity of free space. This shows that capacitance is directly proportional to the plate area and inversely proportional to the separation distance. The capacitance can be increased by: (1) Increasing the area (A) of the plates - since C ∝ A, larger plates provide more surface for charge storage. (2) Decreasing the distance (d) between the plates - since C ∝ 1/d, smaller separation allows more charge to be stored for the same potential difference.

Sound is a physical phenomenon consisting of elastic waves of mechanical vibrations that propagate through solid, liquid, or gaseous media, with key characteristics including amplitude (the maximum sound pressure during compressions and rarefactions, representing sound intensity), frequency (the number of air vibrations per second measured in Hertz, where most humans hear 20-20,000 Hz and frequencies in a 2:1 ratio create the musical octave effect), and phase (the relative temporal alignment of sound waves, where in-phase waves reinforce each other while anti-phase waves can cancel out). Sound intensity is measured in decibels (dB), a logarithmic unit where 10 dB represents a 10-fold change in power and 20 dB represents a 100-fold change, while Sound Pressure Level (SPL) measures sound pressure relative to the threshold of human hearing at 20 micropascals.

The physical characteristics of sound include: frequency (tần số), sound intensity (cường độ âm), sound intensity level (mức cường độ âm), and amplitude of oscillation (biên độ dao động). These characteristics determine the physical properties of sound waves and are measurable quantities.

Sound pressure levels (SPL) measure sound intensity in decibels. Common references: 10 dB for rustling leaves, 50-60 dB for conversation, 70-80 dB for toilets/vacuums, 120-130 dB for live rock bands, 130 dB for jackhammers, and 180 dB for rocket launches. The pain threshold is approximately 120 dB, causing potential eardrum damage. A complete sound wave cycle spans 360 degrees, returning to its starting point. Wavelength represents the physical length of one cycle. Frequency measures cycles per second in hertz (Hz), determining perceived pitch—higher frequencies produce higher pitches.

Sound is produced by vibrating objects, where the frequency of sound equals the frequency of vibration. Sound has the same nature as mechanical waves and can propagate through solid, liquid, and gaseous media but not through a vacuum. The speed of sound depends on the medium's properties: 331 m/s in air at 0°C, 1,280 m/s in hydrogen, 1,500 m/s in water, 5,850 m/s in steel, and 6,260 m/s in aluminum. Sound has three physical characteristics: frequency (most important, determines pitch), intensity (measured in W/m²), and intensity level (measured in decibels using L = 10 × log₁₀(I/I₀)). A sound source produces a fundamental tone with the largest amplitude and harmonics at integer multiples of the fundamental frequency. The waveform represents the unique pattern of air pressure variations, allowing us to distinguish between different sound sources.

Acoustics is the branch of physics that discovers and mathematically formulates the physical laws governing acoustic phenomena, with specialized branches including Physical Acoustics, Physiological Acoustics, Musical Acoustics, Architectural Acoustics, and Electroacoustics. Sound is not particles but rather waves of particles that already exist in the medium. When sound is generated, the source moves existing air molecules (approximately 10^23 to 10^24 molecules in a room) rather than creating new ones. This creates an avalanche effect where molecules strike neighboring molecules, producing alternating compression and expansion waves. These waves propagate through the medium until reaching the ear, where they impact the eardrum with varying pressure. Pressure is defined as force per unit surface area (measured in Pascals), and sound pressure can be mathematically expressed based on molecular mass, velocity, and collision frequency. An oscilloscope visualizes these pressure variations over time, showing moments of high and low molecular density striking the receptor.