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Transcript of Bernoulli’s Principle - Astronomyjfielder/Phys101week8slides.pdf3/13/15 1 Bernoulli’s Principle...
3/13/15
1
Bernoulli’s Principle
“ Where the speed of a fluid increases, internal pressure in the fluid decreases.” • Due to continuous flow of a fluid: what
goes in must come out! • Fluid flows faster through a narrower pipe
Demo: Blowing on a sheet of paper
Bernoulli’s Principle
• If speed of a fluid increases, the pressure in the fluid decreases.
• This phenomenon is due to energy conservation; when fluid’s KE increases (velocity increases) its internal P (pressure) decreases.
Application: Lift • Airplane wings at an angle
produce more crowded streamlines along the top of the wing than along the bottom
• Avg. pressure difference x surface area of wing = net upward force
• This works even when the plane flies upside down, as long as the angle is similar!
Water is flowing continuously in the pipe from point A to point C. Rank
the three points in terms of the internal pressure from biggest to
smallest.
Main Points
• Chapter 12: – Density
• Chapter 13: – Pressure – Pressure in liquids – Buoyancy – Archimedes’ Principle – Pascal’s Principle
• Chapter 14: – Pressure in gases – Boyle’s Law – Buoyancy in air – Bernoulli’s Principle
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Part 3: Heat
• Chapter 15: Temperature, Heat, & Expansion
• Chapter 16: Heat Transfer • Chapter 17: Change of Phase • Chapter 18: Thermodynamics
Temperature
• Temperature (T) is a measure of how “hot” or “cold” something is
• Temperature measures the random KE of each particle in an object. – The greater the motion/vibration the greater
the T – The smaller the motion/vibration the lower the
T • SI Unit: kelvin (K)
– Room temperature is about 295K
Other Temperature Scales
• Celsius – Water freezes at 0ºC, boils at 100ºC
• Fahrenheit – Water freezes at 32ºF, boils at 212ºF
Kelvin Temp. Scale
• The Kelvin scale has the same step size (size of one degree) as the Celsius scale, but the Kelvin scale has its zero at absolute zero.
• Conversion between a Celsius temperature and a Kelvin temperature:
Heat (Q)
Definition of heat: • Heat is the energy transferred between
objects because of a temperature difference.
• Objects are in thermal contact if heat can flow between them.
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Thermal Equilibrium
• When the transfer of heat between objects in thermal contact stops, they are in thermal equilibrium.
• The objects will then be at the same temperature.
Units of Heat • Since heat is just a flow of energy, the SI unit
is the energy unit, the joule (J). • Other heat units
– calorie (cal): Heat needed to raise temperature of 1 gram of water by 1°C (or 1 K)
– Calorie (Cal or kcal): Heat needed to raise temperature of 1 kg of water by 1°C (or 1 K)
– Calorie also used to measure energy content of food
Conversions: 1 cal = 4.186 J 1 kcal = 1 Cal (food Cal.) = 4.186 kJ
Thermometers
• Thermometers are instruments designed to measure temperature. In order to do this, they take advantage of some property of matter that changes with temperature. – Length of a solid or liquid column – Volume of a solid, liquid, or gas – Electromagnetic waves (infrared light) given off
by hot objects
Specific Heat Capacity
• Specific heat capacity is the amount of heat energy required to raise the temperature of one unit mass of a material by one degree.
• SI Unit: J/(kg•K),or J/(kg•°C)
Thermometers
• Thermometers are instruments designed to measure temperature. In order to do this, they take advantage of some property of matter that changes with temperature. – Length of a solid or liquid column – Volume of a solid, liquid, or gas – Electromagnetic waves (infrared light) given off
by hot objects
Thermal Expansion
• When you heat something up, it expands! • The effect is less dramatic in solids than in
liquids or gases
Demos: bimetallic strip, and metal ball & ring
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Common Thermometers
• Liquid-in-tube
• Bimetallic Strip Chapter 16: Heat Transfer
• Conduction: Thermal kinetic energy passed from particle-to-particle along a length of material.
• Convection: Thermal energy carried by moving fluid.
• Radiation: Thermal energy carried by electromagnetic waves (light)
Conduction • Heat conduction can be visualized as
occurring through molecular collisions. • Thermal kinetic energy is passed along as
“hotter” particles collide with “colder” ones.
Conduction
• Conduction is heat flow by direct contact.
• Some materials are good thermal conductors (like the tile), others are insulators (like the wood).
Convection • Convection is flow of
fluid due to difference in temperatures, such as warm air rising.
• Fluid “carries” heat with it as it moves.
• “Natural” convection: Warm fluid will rise because it is less dense then cold fluid.
Convection
• Heat transfer in a fluid often occurs mostly by convection.
• Buoyancy causes warm air to rise, which carries thermal energy directly by its motion.
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Convection Oven • Convection oven
has a fan to enhance the circulation of the air, increasing the transfer of heat.
Fiberglass Insulation
• Air is a poor thermal conductor but easily transfers heat by convection.
• Fiberglass insulation is mostly air, with the fibers disrupting the convection flow.
Radiation
• How does energy get from the Sun to Earth?
• No atmosphere out in space, so it’s can’t be convection or conduction
• The energy is transferred through radiation; specifically, electromagnetic radiation
Radiation • Radiation has many
different wavelengths, most of which are not visible to the eye.
• All radiation carries energy, and thus transfers heat.
The Electromagnetic Spectrum
• Gamma Rays • X-rays • Ultraviolet Light • Visible Light (ROY G BIV) • Infrared Light • Microwaves & Radio Waves
Properties of Waves
• Wavelength is the distance between two wave peaks
• Frequency is the number of times per second that a wave vibrates up and down
wave speed = wavelength x frequency
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Wavelength and Frequency
wavelength x frequency = speed of light = constant
Frequency vs. Temperature
The Electromagnetic Spectrum
• Gamma Rays • X-rays • Ultraviolet Light • Visible Light (ROY G BIV) • Infrared Light • Microwaves & Radio Waves
How do light and matter interact?
• Emission • Absorption • Transmission
– Transparent objects let light through – Opaque objects block or absorb light
• Reflection or Scattering
Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
The Greenhouse Effect • Glass is transparent to sunlight (short-wavelength). • Glass is opaque to infrared radiation (long-
wavelength) produced by objects inside greenhouse, trapping the heat.
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Earth’s Greenhouse Effect • Earth’s atmosphere
acts as a greenhouse, trapping solar energy.
• Most of the trapping is due to carbon dioxide and water vapor, which is why they’re called “greenhouse gasses.”
Greenhouse Gases: CO2 • Over past 1,000 yrs temperatures were nearly
constant until CO2 emissions increased starting with the industrial revolution.
Global Warming Chapter 17: Phase Changes Sequence of increasing molecule motion (and kinetic energy): How do we get from
one phase to another?
Solid Liquid Gas
What are the phases of matter?
• Familiar phases: – Solid (ice) – Liquid (water) – Gas (water vapor)
• Perhaps not-so-familiar: – Plasma (atoms stripped of electrons)
Phase Changes
• Substances can exist in any of the phases, but behave differently
• The phase depends on temperature and pressure
• Phase changes almost always require a transfer in energy
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Phase Changes
• Evaporation/Condensation – Liquid ! Gas , Gas ! Liquid
• Boiling – Liquid ! Gas, but not at the surface!
• Sublimation – Solid ! Gas (skips liquid)
• Melting/Freezing – Solid ! Liquid, Liquid ! Solid
Energy and Changes of Phase
Chapter 18: Thermodynamics
• Thermodynamics: the study of heat moving from one body to another
• Recall: Temperature is a measure of the average kinetic energy of molecules in an object
• Recall: “Absolute zero” or 0 K, where there would (theoretically) be no more kinetic energy in the molecules of a substance
0th Law of Thermodynamics
• Imagine three systems: A, B, and C • If A and B are each in thermal equilibrium
with C, then A and B must also be in thermal equilibrium with each other.
A
B
C
1st Law of Thermodynamics
When heat flows to (or from) a system, the system gains (or loses) an amount of energy equal to the amount of heat transferred. Caution: Remember that numerically,
ΔQ ≠ ΔT !
More 1st Law
ΔQ = Δinternal energy + work • This is the thermal version of conservation
of energy! • You can never get more energy out of a
system than you originally put in • In an adiabatic process, ΔQ = 0 so the Δinternal energy is the same as the work done by the system