Bernoulli’s Principle - Astronomyjfielder/Phys101week8slides.pdf3/13/15 1 Bernoulli’s Principle “ Where the speed of a fluid increases, internal pressure in the fluid decreases.”

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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