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Questions and Answers

Here is a history of questions and answers processed by "Ask the Physicist!". If you like my answer, please consider making a donation to help support this service.

If there is a link to a previously answered question, be patient. Since the files containing the older answers are rather large, it takes some time (maybe as much as 15 seconds or so) to find the appropriate bookmark.


QUESTION:
The units of the Hall coefficient is m^3/C that's, cubic meter per coulomb. How so we get this?

ANSWER:
You just need to know the definition of the Hall coefficient RH, RH
VHt/(IB) where VH is the Hall voltage, t is the thickness of the sample, I is the current, and B is the magnetic field. The Hall voltage may be shown to be VH=-IB/(nte) where n is the charge carrier density (units [m-3]) and e is the charge (units [C]]) of a charge carrier. Therefore RH=-1/(ne) so the units of RH are m3/C.



QUESTION:
What is the true shape of the Sun?

ANSWER:
I was surprised to learn that the sun is almost a perfect sphere.


QUESTION:
Why do the planets revolve only around the sun? There are stars which are bigger than the sun, and apply more gravitational force, but other planets do not attracts towards the stars. Please tell why?

ANSWER:
I guess you have not been following news articles over the past dozen years! In fact hundreds of exoplanets (planets revolving around stars other than the sun) have been observed.


QUESTION:
I have something I have been wondering about.maybe you can answer. I recently drank a glass bottle of rootbeer and then set the empty bottle on the hood of my car that was slanted slightly (1980's model). Within seconds the empty bottle started to vibrate slightly and then"walked" down the hood of my car. The engine was turned off. I was trying to tell my girlfriend that it was from heat convection, but she disagreed. How can this happen? I did it a second time and the empty bottle did it again. Not my imagination.

ANSWER:
Here is what I think: The bottle probably had condensation (water) which was fairly cool and the hood of your car was warm or even hot. The viscosity of water depends quite sensitively on its temperature. If the water is heated from 250C to 500C, for example, the viscosity decreases from about 890 to 547
μPa·s which will result in a much slipperier contact. (Think of a very heavy oil and a very light oil to get an intuitive feeling for how viscosity would affect the frictional force. Or, think of the old expression "…as slow as molasses in January…".)


QUESTION:
I don't know if you get this question alot, but a man standing on the ground is subjected to 2 forces. Gravity and normal reaction force. Do they form an action-reaction pair of forces?

ANSWER:
This is one of the most misunderstood aspects of Newton's laws. Let me state Newton's third law for your man in a couple of ways:

  • The earth exerts a downward force on the man (his weight) and therefore the man exerts an upward force on the earth of the same magnitude as his weight.

  • The ground exerts an upward force on the man (the normal force) and therefore the man exerts a downward force on the ground of the same magnitude as his weight.

Both of these are correct statements of Newton's third law. Notice that the pairs of forces ("action-reaction") are never on the same object. The two forces you give are both on the same object (the man) and can therefore not be an "action-reaction" pair. So why are they equal and opposite? Because Newton's first law states that if the man is in equilibrium the sum of all forces on him must be zero; therefore the ground must exert an upward force to balance the weight. Your example has nothing to do with Newton's third law.


QUESTION:
Why in the michelson and morley experiment that air does not have any effect on the experiment. To me air would be the current medium that the light is using to propagate and would yield a null result. The main key to this line of thought is that light travels slower through air then in a vacuum. The same as the speed of sound through different medium.

ANSWER:
The Michelson-Morley experiment does not attempt to measure the absolute speed of light. Rather, the idea is to find the difference in speeds between two different directions of travel. That measurement would not depend on the presence of air.


QUESTION:
Does a submarine have to work harder to travel at the same speed in deeper water?

ANSWER:
The drag force in water is approximately proportional to the speed; it also depends on the shape and on the viscosity of the water. As you go deeper into the water, the pressure increases and the temperature decreases. Viscosity only very weakly depends on pressure but increases significantly with temperature. After remaining constant around 200C until a depth of about 200 m, temperature rapidly decreases to about 40C at a depth of about 1000 m; the viscosity, and therefore the drag, nearly doubles at that depth.


QUESTION:
Do permanent magnets have an electric current surrounding them? Permanent magnets seems to have different properties to electromagnets, such that electromagnets can be used for induction and energy transfer if a conductor is placed within their changing magnetic field. So I understand that an electromagnet will have a changing magnetic field, which in turn generates a changing electrical current in a conductor placed within this magnetic field. My question is, do permamant magnets generate this same electric current if a conductor is placed within their magnetic field? I.e. if I had a strong permanent magnet, would I be able to generate a current in a coil if it was brought into the permananet magnet's magnetic field?

ANSWER:
A permanent magnet may be thought of as having "bound" currents. These are not currents you could actually connect an ammeter to and measure. If you are interested, there are, in general, two types of bound current, bound volume current density Jb and bound surface current density Kb. If the magnetization of the material is M, you can write Jb=curlM and Kb=Mxn where n is a unit vector normal to the surface and pointing out of the volume. I do not understand any of your rambling about induction. I will simply say that any changing magnetic field can induce currents and a permanent magnet is a source of a magnetic field; moving that magnet, through a wire ring, for example, will induce a current in the ring.


QUESTION:
Can the equations for Compton Scattering be applied to a photon colliding with a neutron? If no, what equations can be used?

ANSWER:
Yes, you just have to replace the electron mass with the neutron mass. However, the effect for this much larger mass would be very difficult to see because the change in wavelength will be much smaller than for electrons. To have any hope of observing the effect you would need to use extremely high-energy gamma rays.


QUESTION:
If matter above absolute zero emits thermal radiation—and thermal radiation is actually electromagnetic radiation—what temperature is required for matter to emit visible light?

ANSWER:
Technically, visible light is emitted by all objects because the spectrum of emitted radiation is continuous and includes all wavelengths. However, you can say at what temperature the visible part of the spectrum is more intense than any other emitted radiation, about 5000 K. Interestingly, that is about the temperature of the surface of the sun.


QUESTION:
if an object ( like a bullet ) is fired vertically at constant speed. Where does it land?

ANSWER:
This is a standard intermediate-level classical mechanics problem for accelerated reference frames. All the mathematical background is much too complicated to include here but the answer is that it lands a distance d=4
ωv03cosλ/(3g2) west of where it was shot; here ω=7.27x10-5 s-1 is the angular velocity of the earth, v0 is the initial velocity of the bullet, λ is the latitude where the gun is located, and g=9.8 m/s2 is the acceleration due to gravity. This, of course, neglects any air drag or wind corrections. For example, for λ=450 and v0=390 m/s (typical 9 mm muzzle speed), d≈60 m. At the poles (λ=900), it goes up and comes back down perfectly vertically.


QUESTION:
what is the cause of buoyancy.? we faced many confusions about then we worked on it and did some experiments and found an another reason which is satisfying all the conditions and aspects...but we wanted to consult about it......

ANSWER:
The reason for buoyancy is simply that the force due to the fluid up on the bottom of something is greater than the force of the fluid down on the top because of the pressure difference.


QUESTION:
Why does stirring a liquid makes it cool faster? Shouldn't the reverse happen, as the kinetic energy imparted to liquid in stirring should also change eventually to heat?

ANSWER:
The liquid in the cup loses energy to its environment (cools) by conduction and radiation from its surfaces. The energy inside the surfaces is transmitted to the surfaces (to continue cooling) by convection. Convection is a relatively slow process for transmitting heat. Stirring more effectively moves hot liquid to the surface. Energy you add by stirring is trivially small compared to the increased cooling rate.


QUESTION:
Suppose light is travelling in a straight line parallel to Y-axis takes time t to reach from y1 to y2 in a frame of reference S. Let there be another frame S' which is travelling parallel to X-axis with velocity v relative to frame S. Then a seen from S' the light ( which was travelling parallel to Y-axis in frame S ) now takes a longer path which is greater than y2-y1 (as now it will be travelling obliquely in frame S' ). Now due to time dilation time taken in frame S' is lower than t as seen by frame S due to time dilation. Then speed of light in frame S' as seen from S will be greater than c as length is higher and time is lower. then how to explain the constancy of speed of light in this case ?

ANSWER:
You are making this too hard! The fact that c is a universal constant is where you should start, not end. When you move with your S' frame with speed v in the positive x direction, the light beam acquires an x-component of cx=-v. If the speed of the light is to remain constant, this must mean that the y-component must therefore decrease such that c2=cx2+c
y2. So, cy=c√[1-(v2/c2)] which means that the angle θ which the light makes with the y-axis is θ=tan-1[β/√(12)] where β=v/c. See an earlier question for an example of this phenomenon.


QUESTION:
If I have two hoses a varying diameters such as a garden hose and a firehose that are vertical and approximately 20 feet long that are filled with the same amount of water and have the same size opening at the bottom, using just gravity, will water flow through each hose in the same amount of time? This question came up during a recent visit with a customer. I sell feeding supplies for neonatal patients. The current product being used is a large bore tube compared to the smaller bore that my company sells. However the opening at the distal tip of the feeding tube is the same size for both. Again, using just gravity, shouldn't the speed at which formula flows through the distal end be the same since it bottlenecks there?

ANSWER:
Your first question ("
will water flow through each hose in the same amount of time?") is ambiguous, so let me answer the question by finding how the speed of the delivered formula (labelled Vbottom in the figure) depends on the geometry. The operative physics principal is Bernoulli's equation, P+½ρV2+ρgy=constant where P is pressure, ρ is fluid density, y is the height above some chosen reference level, and g=9.8 m/s2 is the acceleration due to gravity. In your case, P is atmospheric pressure both at the top and at the bottom, I will choose y=0 at the bottom so y=h at the top. Therefore Bernoulli's equation becomes Vtop2+2gh=Vbottom2 or Vbottom=√(2gh+Vtop2). There are two ways that Vbottom
will be independent of the geometry: (1) if h is held constant by replenishing the formula at the top or (2) if the area of the bore is much larger than the area of the distal tip A>>a. Both of these result in Vtop≈0 so Vbottom≈√(2gh). If neither of these is true, it is a much more complicated problem.



To see questions and answers from longer ago, link here.