Heat, Waves, Sound, and Light
Heat and Energy
Internal energy is the sum of all the kinetic and potential energy of the particles within a body. It depends on:
- The amount of matter
- The type of substance
- Temperature (a measure of the average kinetic energy of the particles)
Heat is energy exchanged when two bodies at different temperatures come into contact or when a change of state occurs. It is energy in transit.
Two bodies in contact with the same temperature are in thermal equilibrium.
Specific heat of a substance is the amount of heat required to raise the temperature of 1g of it by 1K. The SI unit is J/(kg·K).
The relationship between the amount of heat (Q) and the temperature change (ΔT) is: Q = m·c·ΔT
The water equivalent of a calorimeter is the mass of water that absorbs the same amount of heat as the calorimeter. Q1 + Q2 = 0
Changes of State
Boiling Point: The temperature at which a substance changes from a liquid to a gas throughout the liquid. It coincides with the condensation point. Water’s boiling point is 100°C.
Melting Point: The temperature at which a substance changes from a solid to a liquid. It coincides with the freezing point. Water’s melting point is 0°C. Q = m·L
Latent heat (L): The amount of heat required to change the state of 1 unit mass of a substance without changing its temperature.
Heat Transfer
Conduction: The way thermal energy is transmitted in solids. Energy spreads without mass transfer through collisions between particles.
Convection: The way thermal energy is transmitted in fluids. It involves mass transport.
Radiation: The way thermal energy is transmitted between two bodies without any contact. It occurs through electromagnetic waves and is the only way to transmit thermal energy in a vacuum.
Wave Motion
Wave Motion: The propagation of a disturbance through a medium. Energy is transmitted from one point to another without the transport of matter.
Types of Waves:
- According to propagation dimension: One-dimensional, Two-dimensional, Three-dimensional
- According to the medium: Mechanical (need a medium), Electromagnetic (propagate in a vacuum)
- According to particle vibration direction: Longitudinal (vibrate in the same direction as the disturbance), Transverse (vibrate perpendicular to the disturbance)
Wave Characteristics
- Focus: Where the disturbance originates.
- Crest: The highest point of the wave.
- Trough: The lowest point of the wave.
- Amplitude (A): The maximum displacement of a particle from its equilibrium position. SI unit: meters.
- Wavelength (λ): The minimum distance between two points in the same state of vibration. SI unit: meters.
- Period (T): The time it takes for one complete vibration. SI unit: seconds.
- Frequency (f): The number of vibrations per second. SI unit: Hertz (Hz). f = 1/T
- Velocity of propagation (v): The speed at which the wave travels. SI unit: m/s. v = λ/T = λ·f
A wave is more intense when it has more energy, a larger amplitude, and a higher frequency.
Sound
Sound energy is a form of energy caused by vibrations that propagate as mechanical waves.
Sound waves are longitudinal and three-dimensional. Sound requires:
- A sound source (a vibrating body)
- An elastic medium
- A receiver to capture and interpret the sound wave
Sound travels as particles in the medium vibrate and transmit the vibration to their neighbors. The speed of sound depends on the medium and its temperature.
Sound Characteristics
Pitch: Distinguishes high-pitched sounds from low-pitched sounds (higher frequency = higher pitch). The human ear perceives sounds between 20 and 20000 Hz (below 20 Hz: infrasound, above 20000 Hz: ultrasound).
Intensity: Controlled by the volume, it identifies a sound as strong or weak (higher amplitude = stronger sound). The threshold of hearing is called the threshold of audibility, and the maximum intensity before pain is the threshold of pain.
Timbre: Relates to the shape of the sound wave. It allows us to distinguish sounds of the same frequency and amplitude produced by different instruments and voices.
Sound Phenomena
Echo: Occurs when our ears can distinguish between the incident sound and the reflected sound after it hits an obstacle.
Reverberation: If the obstacle is closer than 17m, the reflected sound overlaps with the incident sound.
Uses of Sound Waves
- Sonar: Used in maritime shipping to measure the depth of the seabed or locate schools of fish.
- Ultrasound: Used in medicine for imaging and treatment, and in photography to clean camera sensors.
Sound Pollution
Excessive noise is considered noise pollution. There are two types of noise control:
- Active: Reducing noise at the source.
- Passive: Dampening sound propagation and impact.
Light
Light is a form of energy emitted by bodies that allows us to see. It is a transverse wave that propagates in a vacuum and in various media. Its speed depends on the medium. In a vacuum, it travels at approximately 3 x 108 m/s.
Refraction: The change in direction of light as it passes from one medium to another. Snell’s Law describes this phenomenon.
Dispersion: The separation of light into its different wavelengths due to the refractive index’s dependence on wavelength.
Spectrum Analysis: The study of the different wavelengths that make up a light source, providing information about the energy and intensity of the radiation.
Electromagnetic Spectrum: The range of all possible electromagnetic radiation, including visible light, radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
