Physical Science: Physics
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Transcript of Physical Science: Physics
Physical Science: Physics
…last two weeks : )
Work and Power
Work in physics is not the same as the everyday meaning of work…– Work- the product of force and distance
-the transfer of energy•Work is done when a force is exerted on an object and that object moves– Work requires motion– For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done
Work and Power
Work– Work is done when a force is exerted on an object and that object moves• Work requires motion• For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done
– Work depends on Direction• Any part of a force that does not act in the direction of motion does no work on an object– See figure 2-B
Work and Power
Calculating Work– Work = Force x Distance
• Force in newtons (N)• Distance in meters (m)• Work in joules (J)
– Sample Problem: A weight-lifter applies force to a 1600 N barbell, to lift it over his head (a height of 2.0 m). How much work is done by the weight-lifter?• W = F x D
F = 1600 ND = 2.0 m
W = 1600(2.0) = 3200 J
Work and Power
Power- is the rate of doing work– Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a give time, or you can do a given amount of work in less time.
Work and Power
Calculating Power– Power = Work/Time
• Work in joules (J)• Time in seconds (s)• Power in watts (W)
– One watt is equal to one joule per second
– Sample Problem: When you lift a box, work is done (1340 N). It takes you 1.8 seconds to lift the box. How much power is done?• P = W/t
W = 1340 Nt = 1.8 s
P = 1340/1.8 = 744.44444 ~ 740 W
Work and Power
Sample Problem:– You exert a vertical force of 88 N to lift a box to a height of 1.5 m in a time of 2.3 seconds. How much power is used to lift the box?•W = FxD & P = W/t
F = 88 ND = 1.5 mW = ?t = 2.3 s
W = F x D = 88(1.5) = 132 J
P = W/t = 132/2.3 = 57.391304 ~ 57 W
Thermal Energy and Matter
Work and Heat– Friction makes machines inefficient
•Friction causes some of the work done to be converted to thermal energy, rather than be used to do useful work
– Heat- the transfer of thermal energy from one object to another because of a temperature difference•Heat flows spontaneously from hot objects to cold objects
Thermal Energy and Matter
Thermal energy depends on the mass, temperature, and phase (solid, liquid, or gas) of an object.– Temperature- is a measure of how hot or cold an object is in relation to reference point
– The more mass an object has, the more thermal energy it will have
Thermal Energy and Matter
Thermal expansion occurs when particles of matter move farther apart as temperature increases– When objects heat up they expand, and when objects cool down they contract
Thermal Energy and Matter
Specific Heat- the amount of heat needed to raise the temperature of one gram of a material by one degree Celsius– The lower a material’s specific heat, the more its temperature rises when a given amount of energy is absorbed by a given mass• Specific Heat Equation
– Q = m x c x ∆T» Q is energy absorbed/heat needed-unit is joule (J)
» m is mass in grams (g)» c is specific heat in J/g˚C» ∆T is change in temperature (˚C)
Thermal Energy and Matter
Specific Heat-Sample Problem– An iron skillet has a mass of 500.0 g. The specific heat of iron is 0.449 J/g˚C. How much heat must be absorbed to raise the temperature of the skillet by 95.0 ˚C?• Q = mc∆T
m = 500.0 gc = .449 J/g˚C∆T = 95.0 ˚C
Q = (500.0)(.449)(95.0) = 21327.5 ~ 21300 J
Thermal Energy and Matter
First Law of Thermodynamics– States that energy is conserved
•If energy is added to a system, it will either increase the thermal energy of the system or do work on the system– Ex: bike tire, air inside the tire, and air pump are the system…when you use the pump, there is a force exerted on the pump, which does work on the system (adding air to the tire) some of the work is converted to thermal energy as well
Thermal Energy and Matter
Second Law of Thermodynamics– States that thermal energy can flow from colder objects to hotter objects only if work is done on the system
Thermal Energy and Matter
Third Law of Thermodynamics– States that absolute zero cannot be reached
Mechanical Waves & Sound
Mechanical Wave- a disturbance in matter that carries energy from one place to another– Require matter to travel through– Medium- material through which waves travel•Solids, liquids, and gases can act as the medium
Mechanical Waves & Sound
A mechanical wave is created when a source of energy causes a vibration to travel through a mediumTypes of Mechanical Waves– Classified by the way they move through a medium
– The three main types of mechanical waves are transverse, longitudinal, and surface waves
Mechanical Waves & Sound
Transverse Waves- a wave that causes the medium to vibrate at right angles to the direction in which the wave travels– Trough- the lowest point below the rest position of a wave
– Crest- the highest point of the wave above the rest position
Mechanical Waves & Sound
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Mechanical Waves & SoundTransverse Wave
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Figure 2 A transverse wave causes the medium to vibrate in a direction perpendicular to the direction in which the wave travels. In the wave shown here, each point on the rope vibrates up and down between a maximum and minimum height. A The ribbon is at a crest. B The ribbon is at the rest position. C The ribbon is at a trough.
Mechanical Waves & Sound
Longitudinal Waves- a wave in which the vibration of the medium is parallel to the direction the wave travels– Compression- an area where the particles in a medium are spaced close together
– Rarefaction- an area where the particles in a medium are spread out
Mechanical Waves & Sound
Longitudinal Waves– Waves in springs– P-waves (primary waves caused by seismic activity)
– See Figure 3 on pg. 502
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Mechanical Waves & Sound
Surface Waves-a wave that travels along the surface separating two media
Properties of Mechanical Waves
Frequency and Period– Periodic Motion- any motion that repeats at regular
– Period- the time required for one cycle, a complete motion that returns to is starting point
– Frequency- the number of complete cycles in a given time• Any periodic motion has a frequency• Measured in cycles per second, or hertz (Hz)
Properties of Mechanical Waves
Frequency and Period– A wave’s frequency equals the frequency of the vibrating source producing the wave
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Properties of Mechanical Waves
Wavelength- the distance between a point on one wave and the same point on the next cycle of the wave– Transverse waves: wavelength = distance between two adjacent crests or troughs
– Longitudinal waves: wavelength = distance between two adjacent compressions or rarefactions
Properties of Mechanical Waves
Wave Speed– Can be calculated by divided a wave’s wavelength by its period
– Or by multiplying its wavelength by its frequency
– Speed = wavelength x frequency•Wavelength in meters•Frequency in hertz•Speed in m/s
Properties of Mechanical Waves
Sample Problem: A wave on a rope has a wavelength of 3.1 m and frequency of 2.5 Hz. What is the speed of the wave?– Speed - wavelength x frequency
Wavelength = 3.1 mFrequency = 2.5 Hz
Speed = 3.1(2.5) = 7.75 ~7.8 m/s
Properties of Mechanical Waves
Amplitude- the maximum displacement of the medium from its rest positionThe more energy a wave has, the greater its amplitude
Behavior of Waves
Reflection-occurs when a wave bounces off a surface that it cannot pass through– Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down
Behavior of Waves
Refraction- the bending of a wave as it enters a new medium at an angle– When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side
Behavior of Waves
Diffraction- is the bending of a wave as it moves around an obstacle or passes through a narrow opening– A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle
Behavior of Waves
Interference- occurs when two or more waves overlap and combine together– Two types of interference:
•Constructive Interference•Destructive Interference
Behavior of Waves
Interference– Constructive Interference- occurs when two or more waves combine to produce a wave with a larger displacement
– Destructive Interference- occurs when two or more waves combine to produce a wave with a smaller displacements
– See figure 12 on pg. 511
Behavior of WavesStanding Wave- a wave that appears to stay in one place--it does not seem to move through the medium– Node- a point on a standing wave that has no displacement from the rest position• At nodes there is complete destructive interference between the incoming and reflected waves
– Antinode- is a point where a crest or trough occurs midway between two nodes
A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord
Sound and Hearing
Properties of Sound Waves– Sound waves- are longitudinal waves that travel through a medium
– Many behaviors of sound can be explained using a few properties--speed, intensity, and loudness, and frequency and pitch
Sound and Hearing
Properties of Sound Waves– Speed
•In dry air, at 20 ˚C, the speed of sound is 342 m/s
•In general sound travels fastest in solids, slower in liquids, and slowest in gases
•Speed of sound depends on many factors--including the density of the medium and how elastic the medium is
Sound and Hearing
Properties of Sound Waves– Intensity
•Intensity- the rate at which a wave’s energy flows through a given area– Depends on the wave’s amplitude and the distance from the sound source
– Decibel (dB)- a unit that compares the intensity of different sounds» Used to measure sound intensity levels» Based on powers of 10--a 20 dB sound has 100 times more energy/second than a 0 dB sound
Prolonged exposure to sounds with an intensity greater than 90 dB can cause hearing damage
Sound and Hearing
Properties of Sound Waves– Loudness
•Subjective--subject to a person’s interpretation
•Loudness- a physical response to the intensity of sound, modified by physical factors– Depends on sound intensity, health of your ears, how your brain interprets the info in sound waves…
Sound and HearingProperties of Sound Waves– Frequency and Pitch
• Frequency of a sound wave depends on how fast the source of the sound is vibrating– Musical instruments--the longer the tubing the lower the frequency and longer the wavelength of the standing wave
• Pitch- the frequency of a sound as you perceive it– The higher the frequency the higher the pitch and vice versa
– Depends on a sound’s frequency, your age, health of your ears
Sound and Hearing
Ultrasound– Most people hear sounds between 20-20,000 Hz• Infrasound is sound at frequencies lower than most can hear
• Ultrasound is sound at frequencies higher than most can hear
– Ultrasound is used in a variety of applications, including sonar and ultrasound imaging• Sonar- a technique for determining the distance to an object under water (sound navigation and ranging)
Sound and Hearing
The Doppler Effect- a change in sound frequency caused by motion of the sound source, motion of the listener, or both– Discovered by the Austrian scientist Christian Doppler (1803-1853)
– As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency
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Sound and Hearing
Hearing and the Ear– The outer ear gathers and focuses sound into the middle ear, which receives and amplifies the vibrations. The inner ear uses nerve endings to sense vibrations and send signals to the brain
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http://jordansternmd.com/hearing_balance_how_the_ear_works.html
Sound and Hearing
How Sound is Reproduced– Sound is recorded by converting sound waves into electronic signals that can be processed and stored. Sound is reproduced by converting electronic signals back into sound waves.
Sound and Hearing
Music– Most musical instruments vary pitch by changing the frequency of standing waves•Resonance- the response of a standing wave to another wave of the same frequency– Used to amplify sound
Electromagnetic Waves
Electromagnetic Waves- transverse waves consisting of changing electric fields and changing magnetic fields– Produced and travel differently compared to mechanical waves
Electromagnetic Waves
How Produced?– EM waves are produced by constantly changing fields• Electric field- in a region of space exerts electric forces on charged particles– Produced by electric charges and by changing magnetic fields
• Magnetic field- in a region of space produces magnetic forces– Produced by magnets, changing electric fields, and by vibrating charges
– EM waves are produced when an electric charge vibrates or accelerates
Electromagnetic Waves
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http://hyperphysics.phy-astr.gsu.edu/hbase/emwav.html
Electromagnetic Waves
How they Travel– Changing electric fields produce changing magnetic fields and changing magnetic fields produce changing electric fields--> the fields regenerate each other; as this happens their energy travels in the form of waves
– Do not need a medium to travel through– EM waves can travel through a vacuum, or empty space, as well as through matter
– EM radiation- the transfer of energy by EM waves traveling through matter or across space
Electromagnetic WavesThe Speed of EM Waves– Albert Michelson (1852-1931), an American physicist•In 1926 was able to measure the speed of light more accurately than before
– Light and all other EM waves travel at the same speed when in a vacuum, regardless of the observer’s motion•The speed of light in a vacuum, c, is 299,792,458 m/s, often rounded to 3.00 x 108 m/s
Electromagnetic Waves
EM waves vary in wavelength and frequency– Not all EM waves are the same
Electromagnetic Waves
Wave or Particle?– EM radiation behaves sometimes like a wave and sometimes like a stream of particles
Electromagnetic Waves
Evidence for Wave Model•Thomas Young-1801-Passed light through a single slit and then through a double slit, where the light hit a dark screen…there were alternating bright and dark bands caused by constructive and deconstructive interference– Interference occurs only when two or more waves overlap
Electromagnetic WavesEvidence for Particle Model– When blue light hits the surface of a metal like cesium, electrons are emitted (the brighter the light the more e-’s emitted)
– When red light hits the same surface, no electrons are emitted (no matter how bright the light is)
– 1905-Einstein proposed that light, and all EM radiation, consists of packets of energy (now called photons)• Photon- packets of EM energy• Each photon’s energy is proportional to the frequency of light--> the larger the frequency, the more energy of each photon
Electromagnetic Waves
Intensity– Can think of it as brightness– The intensity of light decreases as photons travel farther from the source
Electromagnetic Spectrum
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Electromagnetic Spectrum
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Electromagnetic Waves
Electromagnetic Spectrum- the full range of frequencies of EM radiation – Includes:
•Radio waves•Infrared rays•Visible light•Ultraviolet rays•X-rays•Gamma rays
Electromagnetic Waves
Behavior of Light– Materials can be transparent, translucent, or opaque• Transparent- a material that transmits light/allows most of the light that strikes it to pass through it
• Translucent- a material that scatters light– You can see through it, but the objects do not look clear or distinct
• Opaque- a material that either absorbs or reflects all of the light that strikes it