Home » Physics (Page 14)
Category Archives: Physics
Gravitational waves
Astrophysicists may finally have discovered gravitational waves
In TechInsider, Dave Mosher writes:
Gravitational waves may have been detected for the first time, but we won’t know for sure until February 11, 2016 — when scientists will either confirm or dispel the rumors, sources close to the matter tell Tech Insider.
Detection of gravitational waves would be unprecedented. Whoever finds them is also likely to pick up a Nobel prize, since the phenomenon would confirm one of the last pieces of Albert Einstein’s famous 1915 theory of general relativity.
Confirming they exist would tell us we’re still on the right track to understanding how the universe works. Failing to find them after all these years might suggest we need to revisit our best explanation for gravity or rethink our most sensitive experiments, or that we simply haven’t looked long enough.
“Gravitational waves are ripples in the fabric of space-time, predicted by Einstein 100 years ago,” Szabi Marka, a physicist at Columbia University, told Tech Insider. “They can be created during the birth and collision of black holes, and can reach us from distant galaxies.”
Black holes are the densest, most gravitationally powerful objects in existence — so a rare yet violent collision of two should trigger a burst of gravitational waves that we could detect here on Earth.
Colliding neutron stars and huge exploding stars, called supernovas, are thought to generate detectable gravitational waves, too.
However, any sort of signal has eluded the planet’s brightest minds and the most advanced experiments for decades. Until now — maybe.
Columbia University in New York City is hosting a “major” event the morning of Thursday February 11, 2016, a source who is close to the matter, but asked not to be named, told Tech Insider.
Another source also confirmed the event but downplayed the significance of the event as anything “major.”
Regardless, several physicists and astronomers with expertise in gravitational wave science are scheduled to attend.
The topic? The latest data from the Laser Interferometer Gravitational-Wave Observatory (LIGO), a $1 billion experiment that has searched for signs of the phenomenon since 2002.
LIGO has two L-shaped detectors that are run and monitored by a collaboration of more than 1,000 researchers from 15 nations, and Marka is one of them.
Marka said that he and his colleagues have worked in the field for more than 15 years and that “these are very exciting and busy times for all of us.”
He also said that Advanced LIGO, an upgrade that went online in September 2015, finished a period of hunting for gravitational waves on January 12, 2016. (That was one day after we saw the first alluring rumors of detection.)
Both LIGO instruments are L-shaped arrays of lasers and mirrors that should be able to detect gravitational waves. Szabi Marka compared them to a pair of giant ears that can “hear” the spacetime ripples that result from black hole mergers, or some other catastrophic event in space. The closer a collision is to Earth, the “louder” the signal should be.
LIGO’s hearing is sensitive enough to detect mind-blowingly small disturbances of space, “much smaller than the size of the atoms the detector is built of,” he said. PhD Comics says LIGO’s level of sensitivity is “like being able to tell that a stick 1,000,000,000,000,000,000,000 meters long has shrunk by 5mm.”
Put another way, detecting a gravitational wave is like noticing the Milky Way — which is about 100,000 light-years wide — has stretched or shrunk by the width of a pencil eraser.
It would be no wonder why it has taken researchers so long to find gravitational waves; it’s terribly difficult work. (Even a truck driving on a nearby road can disturb LIGO, despite the instruments having state-of-the-art vibration-dampening equipment.)
It would also be no wonder why scientists might try to stay tight-lipped about the discovery yet “suck at keeping secrets just like everyone else,” as Jennifer Ouellette wrote at Gizmodo.
But at this point, there’s only one way to know for sure if the latest rumors are true: Wait until Thursday.
Einstein’s wildest prediction could be confirmed within days: Tech Insider
________________________________________
When Einstein’s General Relativity was first proposed, it was incredibly different from the concept of space and time that came before. Rather than being fixed, unchanging quantities that matter and energy traveled through, they are dependent quantities: dependent on one another, dependent on the matter and energy within them, and changeable over time. If all you have is a single mass, stationary in spacetime (or moving without any acceleration), your spacetime doesn’t change. But if you add a second mass, those two masses will move relative to one another, will accelerate one another, and will change the structure of your spacetime. In particular, because you have a massive particle moving through a gravitational field, the properties of General Relativity mean that your mass will get accelerated, and will emit a new type of radiation: gravitational radiation.
This gravitational radiation is unlike any other type of radiation we know. Sure, it travels through space at the speed of light, but it itself is a ripple in the fabric of space. It carries energy away from the accelerating masses, meaning that if the two masses orbit one another, that orbit will decay over time. And it’s that gravitational radiation — the waves that cause ripples through space — that carries the energy away. For a system like the Earth orbiting the Sun, the masses are so (relatively) small and the distances so large that the system will take more than 10^150 years to decay, or many, many times the current age of the Universe. (And many times the lifetime of even the longest-lived stars that are theoretically possible!) But for black holes or neutron stars that orbit each other, those orbital decays have already been observed.
We suspect there are even stronger systems out there that we simply haven’t been able to detect, like black holes that spiral into and merge with one another. These should exhibit characteristic signals, like an inspiral phase, a merger phase, and then a ringdown phase, all of which result in the emission of gravitational waves that Advanced LIGO should be able to detect. The way the Advanced LIGO system works is nothing short of brilliant, and it takes advantage of the unique radiation of these gravitational waves. In particular, it takes advantage of how they cause spacetime to respond.
What Will It Mean If LIGO Detects Gravitational Waves? – Forbes
________________________________________
Detection range of AdvLIGO
LIGO and Gravitational Waves: A Graphic Explanation
Tesla and wireless power transmission
Nikola Tesla is one of the great scientists of the 20th century. He patented close to 300 inventions in electrical and mechanical engineering.
Many of Nikola Tesla’s inventions actually work. However, there are many urban legends surrounding his work, some of which have become the basis of conspiracy theories. Perhaps the most widely known is related to Tesla’s discovery that electrical power can be transmitted wirelessly, through the air, from one device to another.
Tesla demonstrated that some power from a Tesla coil could effectively be used to power light bulbs tens or hundreds of feet away. He then envisioned extending the power and range of these devices: he wanted to build a remote power station which could wirelessly power entire cities and towns. However, Tesla never actually worked through the math to prove that this would be efficient or possible, nor did he even demonstrate this level of usefulness.
There is a belief that Tesla “proved” that these towers could wirelessly power cities, and that either the government, or power companies, conspired to keep the details of how this works secret. Electrical engineers and physicists, however, hold that not only is there no conspiracy, but that basic laws of physics show that Tesla’s proposal was unworkable in practice. Below you will find details on why it does not work for large geographical areas.
The information below has been excerpted & adapted from https://en.wikipedia.org/wiki/Wireless_power (1/29/16)
Also see “The Cult of Nikola Tesla”
Inventor Nikola Tesla performed the first experiments in wireless power transmission at the turn of the 20th century. He has done more to popularize the idea than any other individual. From 1891 to 1904 he experimented with transmitting power by inductive and capacitive coupling, using spark-excited radio frequency resonant transformers, now called Tesla coils, which generated high AC voltages. With these he was able to transmit power for short distances without wires.
He found he could increase the distance by using a receiving LC circuit tuned to resonance with the transmitter’s LC circuit, using resonant inductive coupling.
At his Colorado Springs laboratory during 1899–1900, by using voltages of the order of 10 mega-volts generated by an enormous coil, he was able to light three incandescent lamps at a distance of about one hundred feet.
The resonant inductive coupling which Tesla pioneered is now a familiar technology used throughout electronics and is currently being widely applied to short-range wireless power systems.
The inductive and capacitive coupling used in Tesla’s experiments is a “near-field” effect, so it is not able to transmit power long distances. However, Tesla was obsessed with developing a wireless power distribution system that could transmit power directly into homes and factories, as proposed in his visionary 1900 article in Century magazine.
He claimed to be able to transmit power on a worldwide scale, using a method that involved conduction through the Earth and atmosphere. Tesla believed that the entire Earth could act as an electrical resonator, and that by driving current pulses into the Earth at its resonant frequency from a grounded Tesla coil working against an elevated capacitance, the potential of the Earth could be made to oscillate, and this alternating current could be received with a similar capacitive antenna tuned to resonance with it at any point on Earth.
Another of his ideas was to use balloons to suspend transmitting and receiving electrodes in the air above 30,000 feet (9,100 m) in altitude, where the pressure is lower. At this altitude, Tesla claimed, an ionized layer would allow electricity to be sent at high voltages (millions of volts) over long distances.
In 1901, Tesla began construction of a large high-voltage wireless power station, now called the Wardenclyffe Tower, at Shoreham, New York. Although he promoted it to investors as a transatlantic radiotelegraphy station, he also intended it to transmit electric power as a prototype transmitter for a “World Wireless System” that was to broadcast both information and power worldwide.
By 1904 his investors had pulled out, and the facility was never completed. Although Tesla claimed his ideas were proven, he had a history of failing to confirm his ideas by experiment, and there seems to be no evidence that he ever transmitted significant power beyond the short-range demonstrations above.
The only report of long-distance transmission by Tesla is a claim – not found in reliable sources – that in 1899 he wirelessly lit 200 light bulbs at a distance of 26 miles (42 km). There is no independent confirmation of this putative demonstration; Tesla did not mention it, and it does not appear in his meticulous laboratory notes. It originated in 1944 from Tesla’s first biographer, John J. O’Neill, who said he pieced it together from “fragmentary material… in a number of publications”.
In the 110 years since Tesla’s experiments, efforts using similar equipment have failed to achieve long distance power transmission, and the scientific consensus is his World Wireless system would not have worked. Tesla’s world power transmission scheme remains today what it was in Tesla’s time, a fascinating dream.
Tesla’s Big Mistake. Amasci.com – William Beaty
The real science of non-Hertzian waves, By Paul Nicholson
Wireless Energy Transfer, By Yue Ma
Advanced materials
Wireless Power Transmission: From Far-Field to Near-Field
How Relativity Connects Electric and Magnetic Fields
By Professor Michael Fowler, University of Virginia
A Magnetic Puzzle…
Suppose we have an infinitely long straight wire, having a charge density of electrons of –λ coulombs per meter, all moving at speed v to the right (recall typical speeds are centimeters per minute) and a neutralizing fixed background of positive charge, also of course λ coulombs per meter.
The current in the wire has magnitude I = λv (and actually is flowing to the left, since the moving electrons carry negative charge).
Suppose also that a positive charge q is outside the wire, a distance r from the axis, and this outside charge is moving at the same exact velocity as the electrons in the wire.
So how does this lead to the effect we know as magnetism? Read on . . .
ow Relativity Connects Electric and Magnetic Fields
AP Physics Learning Objectives
Essential Knowledge 1.D.3: Properties of space and time cannot always be treated as absolute.
a. Relativistic mass–energy equivalence is a reconceptualization of matter and energy as two manifestations of the same underlying entity, fully interconvertible, thereby rendering invalid the classically separate laws of conservation of mass and conservation of energy. Students will not be expected to know apparent mass or rest mass.
b. Measurements of length and time depend on speed. (Qualitative treatment only.) physics
Learning Objective 1.D.3.1: The student is able to articulate the reasons that classical mechanics must be replaced by special relativity to describe the experimental results and theoretical predictions that show that the properties of space and time are not absolute.
[Students will be expected to recognize situations in which non-relativistic classical physics breaks down and to explain how relativity addresses that breakdown, but students will not be expected to know in which of two reference frames a given series of events corresponds to a greater or lesser time interval, or a greater or lesser spatial distance; they will just need to know that observers in the two reference frames can “disagree” about some time and distance intervals.]
Rainbow reflections: Rainbows are not Vampires
Physics Forums. January 5, 2016, written by anorlunda
For several years, I have been contemplating this beautiful picture by photographer Brian McPhee. I have a personal interest in the photograph because that boat is my year round home. I also have a scientific interest in the photograph because of what it teaches me about rainbow physics. The simplest explanation of rainbow physics is based on internal reflections in the near-spherical shape of a raindrop….
credit: Brian McPhee
Look carefully at the photo with the boat. You will see that the sky inside the arc is much brighter than the sky outside the arc. Some scientists claim that no such effect exists, but it’s pretty plain in the picture. The explanation is that raindrops inside the arc reflect sunlight toward me, while drops outside the arc reflect sunlight away from me. The colors appear in the transition region where only certain colors are reflected towards my eye.
More challenging physics comes from the image of the rainbow seen on the surface of the water. At first, I assumed that it was a reflection of the rainbow in the sky, just like reflections of blue sky and white clouds one sees on a calm day in a reflecting pool.
But then I came across Can Rainbows Cast Reflections? on the web site of noted astronomer Bob Berman. Paraphrasing Berman. “No, they do not. Rainbows are not 3D objects and they do not cast reflections. In the water you see a different rainbow, not a reflection.”
I spent a lot of time puzzling over that, because I didn’t understand Berman’s explanation. I also doubted its truth because I’m sure that I have seen rainbows in the rear view mirror as I drive. It sounds like the Hollywood version of vampires that don’t make reflections in mirrors.
At first, I thought that Berman meant that the image in the water was sunlight hitting the surface and creating a rainbow effect as it was refracted back to my eye. But no, that won’t work because water in the lake is not in the form of spherical droplets.
After much thinking, I think I’ve got it. No vampire magic is required. The colored light you see from a rainbow is not omnidirectional, it is a unidirectional beam aimed at your eye.
By analogy, imagine a man at the far end of a hall of mirrors holding a laser pointer pointed at your eye (assume a laser suitably attenuated for safety). The mirrors on the walls, ceiling and floor of this hall will show many images of the man, but they will not show the red dot of the laser because the laser beam doesn’t hit those mirrors.
However, if you turn your back, step to the side, and hold up a rear view mirror, you’ll see both the man and the red dot. That is because the rear view mirror is inside the cone of light from the laser pointer.
So, to say that the red dot (or the rainbow) does reflect, and that it does not reflect are both true statements depending on which mirror it refers to. Yet, the image of the man appears in all the mirrors. The man is a 3D object, but the red dot is not….
Read the rest of the article here: Rainbow reflections: rainbows are not vampires
____________________________________
A related article with a good (and short) explanation:
Reflected Rainbows: Atmospheric Optics
And the original post which inspired this:
Can rainbows cast reflections? By SkyManBob
…Bob Berman
{regarding a reflection of a rainbow seen in a mirror} How can we tell if it’s the same rainbow?
First: Every rainbow is a set of refractions and reflections precisely beamed in one direction — the eye of the observer. The person next to you is at the apex of a different set of light rays emanating from different droplets, making it a separate rainbow on two counts.
Second, the rainbow seen in a mirror is coming from a different part of the sky where the raindrops may be smaller or bigger or incomplete, changing the appearance (larger drops makes the rainbow more vivid, while robbing it of blue.)
Third, try it with a nearby rainbow like from a lawn sprinkler a few feet away. Now have someone hold a mirror. You’ll see the spray but no rainbow at all in this reflection….
…A traffic light sends photons in all directions. But a rainbow sends its light only to your retina, and nowhere else.
A person next to you is receiving the photons from an entirely separate rainbow (meaning a different set of raindrops, which may have different properties from the ones you are seeing.)
Pulleys
WHAT DOES A PULLEY DO?
A pulley changes the direction of the force, making it easier to lift things.
Interactive lecture demonstrations
from Interactive Lecture Demonstrations:
Created by Dorothy Merritts, Robert Walter (Franklin & Marshall College), Bob MacKay (Clark College). Enhanced by Mark Maier with assistance from Rochelle Ruffer, Sue Stockly and Ronald Thornton
What is an Interactive Lecture Demonstration?
Interactive Lecture Demonstrations introduce a carefully scripted activity, creating a “time for telling” in a traditional lecture format. Because the activity causes students to confront their prior understanding of a core concept, students are ready to learn in a follow-up lecture. Interactive Lecture Demonstrations use three steps in which students:
- Predict the outcome of the demonstration. Individually, and then with a partner, students explain to each other which of a set of possible outcomes is most likely to occur.
- Experience the demonstration. Working in small groups, students conduct an experiment, take a survey, or work with data to determine whether their initial beliefs were confirmed (or not).
- Reflect on the outcome. Students think about why they held their initial belief and in what ways the demonstration confirmed or contradicted this belief. After comparing these thoughts with other students, students individually prepare a written product on what was learned.
Why Use Interactive Lecture Demonstrations
Research shows that students acquire significantly greater understanding of course material when traditional lectures are combined with interactive demonstrations. Each step in Interactive Demonstrations – Predict, Experience, Reflect – contributes to student learning.
Prediction links new learning to prior understanding. The experience engages the student with compelling evidence. Reflection helps students identify and consolidate that they have learned.
More on why use interactive demonstrations
How to Use Interactive Lecture Demonstrations in Class
Effective interactive lecture demonstrations require that instructors:
- Identify a core concept that students will learn.
- Chose a demonstration that will illustrate the core concept, ideally with an outcome different from student expectations.
- Prepare written materials so that students can easily follow the prediction, experience and reflection steps.
More on how to use Interactive Demonstrations in class
__________________________________________
Using PhET interactive labs with interactive lecture demonstrations
Using PhET as an (Interactive) Lecture Demonstration
How to solve any physics problem
Baffled as to where to begin with a physics problem?
There is a logical process to solving any physics problem.
How to solve any physics problem
How to draw free body diagrams
How to solve kinematic equation problems
Half Atwood machine: cart on a frictionless track
When we do use conservation of momentum to solve a problem? When do we use Newton’s laws of motions?
Ferris wheel physics
Fun, imaginative physics discussion questions!
The Physics of Hollywood Movies
Don’t Try This At Home! The Physics of Hollywood Movies is a fresh look at the basics of physics through the filmmaker’s lens. It will deconstruct, demystify, and debunk popular Hollywood films through the scientific explanations of the action genre’s most dynamic and unforgettable scenes. Sample movie sequence and related physics concepts: In “Speed,” a city bus going over 50 mph jumps over a 50-foot chasm–successfully. An examination of force, acceleration, Newton’s Laws, impulse, momentum, and projectile motion follows.
Adam Weiner has been a teacher of physics and AP physics at the Bishop’s School, a highly academic college preparatory school in La Jolla, CA for the last 11 years. Prior to that he worked as a physics instructor at Green River Community College in Auburn, WA in a department very active in physics education research
To order the book at Amazon Don’t Try This At Home!: The Physics of Hollywood Movies
The author’s website Hollywood Movie Physics
His blog on Popular Science Pop Sci – Adam Weiner
How to Solve a Physics Problem Undergrads Usually Get Wrong
By Rhett Allain , 07.09.15
This is a classic introductory physics problem. Basically, you have a cart on a frictionless track (call this m1) with a string that runs over a pulley to another mass hanging below (call this m2). Here’s a diagram.
Now suppose I want to find the acceleration of the cart, after it is let go.
The string that attaches the two carts does two things.
First, the string makes the magnitude of the acceleration for both carts is the same.
Second, the magnitude of the tension on cart 1 and cart 2 has the same value (since it’s the same string).
This means I can draw the following two force diagrams for the two masses.
So, how do you find the acceleration of cart 1? It seems clear, right?
You just need to find the tension in the string since that’s the only force in the horizontal direction. You could write:
If I know the tension, then I can calculate the acceleration. Simple, right?
Even simpler, the tension would just be equal to the gravitational force on the hanging mass (m2).
WRONG! This is not the correct way to solve this problem — I actually remember making this exact mistake when I was an undergraduate student. But why is it wrong?
Here’s the link to the full article:
How to Solve a Physics Problem Undergrads Usually Get Wrong
Why is the tension not the same as the weight of mass 2? The answer is simple — mass 2 is not in equilibrium but instead it is accelerating downward.
Since it’s accelerating, the net force is not equal to zero (vector). This means that the tension should be smaller than the weight of mass 2 — which it is.
Solution to the Half-Atwood Machine
The tension in the string depends on the weight of mass 2 as well as the acceleration of mass 2. However, the acceleration of mass 2 is the same as mass 1 — but the acceleration of mass 1 depends on the tension. Does this mean you can’t solve the problem? Of course not, it just means that it’s slightly more complicated.
Let’s say mass 2 is accelerating in the negative y-direction. This means that I can write the following force equation (in the y-direction).
Now I can do a similar thing for mass 1 with its acceleration in the x-direction. Since the magnitudes of these two accelerations are the same, I will use the same variable.
With two equations and two variables (a and T), I can solve for both variables. If I substitute the expression for T for mass 1 into the equation for mass 2, I get:
Instead of completely solving for the acceleration, let me leave it in the form above. Think of the problem like this: suppose you consider the system that consists of both mass 1 and mass 2 and it’s accelerating.
What force causes this whole system to accelerate? It’s just the weight of mass 2. So, that is exactly what this equation shows — there is only one force (m2g) and it accelerates the total mass (m1 + m2).
From this I can solve for the acceleration.
Using the values of mass 1 = 1.207 kg and mass 2 = 0.145 kg, I get an acceleration of 1.05 m/s2. This is pretty close to the experimental value (seen above) at 1.109 m/s2. I’m happy.
With the value of the acceleration, I can plug back into the original equation to solve for the tension. With this, I get a tension of 1.267 N. This is fairly close to the experimental value of 1.285 N. Again, I’m happy. It seems physics still works.
KaiserScience 