Physics Past Paper (PDF)

# Phase separation in liquid heterogeneous systems. ## Uniform circular motion, centripetal acceleration and force, centrifugal force. ## Sedimentation and centrifugation. Sedimentation velocity, centrifugation, types of centrifuges. Inertial force effect on human organism, overloading, weightlessn...

# Phase separation in liquid heterogeneous systems. ## Uniform circular motion, centripetal acceleration and force, centrifugal force. ## Sedimentation and centrifugation. Sedimentation velocity, centrifugation, types of centrifuges. Inertial force effect on human organism, overloading, weightlessness. - An object that moves in a circle at constant speed _v_ is said to experience uniform circular motion. The magnitude of the velocity remains constant in this case, but the direction of the velocity continuously changes as the object moves around the circle. - Since the object is moving in a circle, upon measuring acceleration and change of speed for a short period of time, those will be directed towards the center of the circle. Therefore, this acceleration is called centripetal acceleration. - An object that is accelerating must have a net force acting on it. An object moving in a circle, such as a ball on the end of a string, must have a net force applied to keep it moving in a circle. A net force is necessary to give it centripetal acceleration. The magnitude of the required force can be calculated using Newton's second law for the radial component: $ΣF = m (v^2/r)$ ## Centripetal acceleration and force: is what we describe the inward force of a uniform circular motion as. This is calculated by: Centripetal force = mass. velocity²/radius of motion Force = mass. acceleration, therefore acceleration = velocity/radius ## Centrifugal force: - An inertial force directed away from the axis of rotation that appears to act on all objects when viewed in a rotating frame of reference - Also called a "fictitious" or "pseudo" force The concept of the centrifugal force can be applied in rotating devices, such as centrifuges, centrifugal pumps, centrifugal governors, and centrifugal clutches, and in centrifugal railways, planetary orbits, banked curves, etc., when they are analysed in a rotating coordinate system. The centrifugal force is an outward force apparent in a rotating reference frame; it does not exist when measurements are made in an inertial frame of reference. ## Sedimentation and centrifugation: - Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier - This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism - The term is typically used in geology to describe the deposition of sediment which results in the formation of sedimentary rock, but it is also used in various chemical and environmental fields to describe the motion of often-smaller particles and molecules. - This process is also used in the biotech industry to separate cells from the culture media. ## Sedimentation Velocity Method - Centrifugation is a process which involves the application of the centrifugal force for the sedimentation of heterogeneous mixtures with a centrifuge, and is used in industrial and laboratory settings. - This process is used to separate two miscible substances, but also to analyse the hydrodynamic properties of macromolecules. - More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture migrate towards the axis. - There is a correlation between the size and density of a particle and the rate that the particle separates from a heterogeneous mixture, when the only force applied is that of gravity. - The larger the size and the larger the density of the particles, the faster they separate from the mixture. - By applying a larger effective gravitational force to the mixture, like a centrifuge does, the separation of the particles is accelerated. ## Types of centrifuges: - A centrifuge is a piece of equipment that puts an object in rotation around a fixed axis (spins it in a circle), applying a potentially strong force perpendicular to the axis of spin (outward). - The centrifuge works using the sedimentation principle, where the centripetal acceleration causes denser substances and particles to move outward in the radial direction. - At the same time, objects that are less dense are displaced and move to the center. There are 3 types of centrifuge designed for different applications: - **Industrial scale centrifuges** are commonly used in manufacturing and waste processing to sediment suspended solids, or to separate immiscible liquids. - **Very high speed centrifuges and ultracentrifuges** able to provide very high accelerations can separate fine particles down to the nano-scale, and molecules of different masses. - **Large centrifuges** are used to simulate high gravity or acceleration environments (for example, high-G training for test pilots). - **Medium-sized centrifuges** are used in washing machines and at some swimming pools to wring water out of fabrics. - **Gas centrifuges** are used for isotope separation, such as to enrich nuclear fuel for fissile isotopes. ## Inertial force effect on human organism: <start_of_image> Forces: - **F=m.g** - the vector that represents force of gravity acting on the object. - **F=m.a** - the vector that represents the instantaneous inertial force acting on object. - **Vector Fr** - rhe resultant or sum of the gravitational and inertial forces. ## Overloading: The current in a circuit depends on the rating of the appliances connected to it. The choice of wires depends upon the maximum current estimated to pass through them. If the total power rating of the appliances exceeds this permitted limit, they tend to draw a large current. This is known as overloading. ## Weightlessness: - Weightlessness, or an absence of 'weight', is an absence of stress and strain resulting from externally applied mechanical contact-forces, typically normal forces from floors, seats, beds, scales, and the like. - Counterintuitively, a uniform gravitational field does not by itself cause stress or strain, and a body in free fall in such an environment experiences no g-force acceleration and feels weightless. - This is also termed "zero-g" where the term is more correctly understood as meaning "zero g-force". - When bodies are acted upon by non-gravitational forces a sensation of weight is produced, as the contact forces from the moving structure act to overcome the body's inertia. - In such cases, a sensation of weight, in the sense of a state of stress can occur, even if the gravitational field were zero, then g-force are felt and bodies are not weightless. - When the gravitational field is non-uniform, a body in free fall suffers tidal effects and is not stress-free. - Near a black hole, such tidal effects can be very strong. - In the case of the Earth, the effects are minor, especially on objects of relatively small dimension and the overall sensation of weightlessness in these cases is preserved. # Mechanical oscillations and waves. ## Basic parameters - period, frequency, wavelength. Equation of harmonic oscillations. Equation of mechanical wave propagation, transversal and longitudinal waves. - When we throw a stone into a lake or pool of water, circular waves form and move outward. It is an example of a wave motion. - Waves of water can move with a recognizable speed, but without the particles moving, they are simply oscillating. The medium, water, is oscillating about an equilibrium point as a simple harmonic motion. A wave is not matter, but it travels in matter. It consists of oscillations that move without carrying matter with them. - A wave is measured as distance between successive crests, **wavelength**. The number of crests, or completed cycles, that pass through a given point per unit time. The **period** is 1/f and is the time elapsed between two successive crests passing by the same point in space. - **Sound** is a mechanical wave with a frequency from 20 Hz - 20,000 Hz. ## Transversal and longitudinal waves. For example, in the shaking of a rope, the wave travels with the medium in transversal direction. A wave can also travel as a vibration of particles of the medium in the direction of the wave's motion. An example of longitudinal wave is sound wave. - **Mechanical** - physically vibrating objects passing the vibration between one another, need a medium unlike electromagnetic waves which can travel through a vacuum. - **Oscillations** = repeating process (e.g. a pendulum oscillates). - **Waves** = an oscillation accompanied by energy transfer. Waves can be transverse or longitudinal. - **Frequency** = number of period per unit of time (measured in Hz) - **Wavelength** = distance between 2 sequential crests or troughs (m) **Harmonic oscillation equation:** All oscillators have a restoring force; this is a force that tries to get the oscillator back to its equilibrium position. Harmonic oscillators have a restoring force that is proportional to the oscillator's displacement. The restoring force is: *Force = -spring constant. displacement from equilibrium* **Equation of mechanical wave propagation** = Amplitude. Sin (radian frequency. frequency of oscillation + phase of oscillation) # Sound ## Physical characteristics of sound. Diffraction and interference. Extracorporeal shock-wave lithotripsy. Sound refers to the physical sensation that stimulates our ears, longitudinal waves. Sound can travel in waves and other materials. The speed of sounds is different in different materials. In air at 0 degrees and 1 atm the speed is 331 m/s. Sound is defined as mechanical oscillations and waves of frequencies between 16 Hz - 20 kHz that is perceived by the human ear. In liquid and gases waves are only longitudinal but in solids they can also be transverse. ## Basic physical characteristics of sound are: - **Sound frequency** - Sound frequency, f, is cycles per second which is expressed in hertz, Hz. - **Sound velocity** - Sound velocity, v, is dependent on the elastic properties and density of the medium. Sounds spread faster in solids and liquid than in gases. - **Wavelenght** - The wavelength of a sound is the distance of two analogous points of two successive sound waves. The wavelength is an inverse to its frequency. λ = v/f - **Acostic impedance** - is proportional to the density and speed of sound. Its value z = pu - **Sound pressure** - Ap = Additional pressure that occurs in the distribution of a sound wave in the parts of the wave where expansion or compression of the medium is observed. Ap = cz = zpu - **Sound intensity** - is the average energy carried by a sound wave per unit time through unit area perpendicular to the direction of wave propagation. I = E/St (W/m^2) - **Level of intensity** – The lowest level of intensity is set to 10 which is 10^-12 W/m^2. L = Log 1/10 ## Extracorporeal shock-wave lithotripsy: - Non-invasive medical procedure to break up stones in kidney or gallbladder. - Done by applying a focused intense acoustic pulse following location of stones by imaging - Shock waves with impulse of 120 shocks per minute can be used. - Provides a shearing force, process takes about an hour. ## Min requirements for Question as per slides: - Define mechanical wave - Relationship between speed wavelength and frequency: Speed = wavelength x frequency - Sound definition: vibrations that travel through the air or another medium and can be heard when they reach a person's or animal's ear. - Formulas for sound velocity for liquids and solid matter - Solid (longitudinal) - velocity = VYoung's modulus/density - Solid (transverse) - velocity = Vsliding modulus/density - Liquid - Velocity = Vmodulus of bulk deformations/density - Gas - velocity = Vmodulus of bulk deformations. gas constant/absolute temp (in K)/molar mass - Relationship between sound pressure and intensity - Intensity = Pressure x particle velocity # Physical characteristics of sound ## Psychophysical parameters of sound, range of hearing, audiometry. Frequency resolution, binaural effect. Physics of vocal apparatus of humans. Acoustic methods in medical diagnosis. Noise protection. ## Physics of vocal and hearing apparatus of humans: - **Psychophysical parameters of sound** - This looks at how sound is detected, the characteristics are determined by human ear based on frequency and intensity - **Pitch** - subjected determined by human ear based on frequency and intensity - **Timber** - determined by spectral composition - **Loudness level** - evaluation of the degree of sound pressure, - **The range of hearing** is somewhat between 16-20 and 20 kHz. Waves with a frequency above that is called ultrasound and those with a frequency below is called infrasound. - **Psychophysical characteristics** are measurements of sounds perceived by the human ear. - **The hearing threshold** starts at the lowest level of loudness, 10^-12 W/m^2. Measuring the lower limit of hearing is one of the tests in audiology which determines the individual range of hearing. The measuring of hearing threshold is called audiometry. - **Sound pitch** - is measured in the unit octave. It depends mainly of frequency, intensity and spectral composition. - **Timber** - is determined by the spectral composition of the sound depending on the number and intensity of the overtones (harmonics). Two tones can have the same frequency but different perceived sound because of the presence of different amounts of the various harmonics. - **Loudness level** - is used to evaluate the volume as stronger or weaker. If the frequency is f=1kHz then k=1. K is a coefficient of the function of frequency and intensity. The formula for loudness level is: Г = kL = klg 1/10