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 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 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


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)


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


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


Second Law of Thermodynamics– States that thermal energy can flow from colder objects to hotter objects only if work is done on the system


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


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


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


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Mechanical Waves & SoundTransverse Wave



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.


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


Longitudinal Waves– Waves in springs– P-waves (primary waves caused by seismic activity)

– See Figure 3 on pg. 502

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https://share.ehs.uen.org/node/9430


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)


Frequency and Period– A wave’s frequency equals the frequency of the vibrating source producing the wave

QuickTime™ and a decompressor


www.airynothing.com/high_energy_tutorial/basics/basics02.html


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


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


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


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



http://dictionary.reference.com/illus/illustration.html/ahsd/Doppler%20effect/dopple

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



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


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




http://hyperphysics.phy-astr.gsu.edu/hbase/emwav.html


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


EM waves vary in wavelength and frequency– Not all EM waves are the same


Wave or Particle?– EM radiation behaves sometimes like a wave and sometimes like a stream of particles


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


Intensity– Can think of it as brightness– The intensity of light decreases as photons travel farther from the source

Electromagnetic Spectrum



http://zebu.uoregon.edu/~imamura/122/images/electromagnetic-spectrum.jpg

Electromagnetic Spectrum



http://www.dnr.sc.gov/ael/personals/pjpb/lecture/lecture.html


Electromagnetic Spectrum- the full range of frequencies of EM radiation – Includes:

•Radio waves•Infrared rays•Visible light•Ultraviolet rays•X-rays•Gamma rays


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

physical science: physics

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