Mosquitoes are able to find a person’s vein in three stages; at a range of 50 meters the mosquito detects a CO2 plume due to breath, at 10 meters the mosquito is able to see the person in the visible range, and finally inside 20 cm the mosquito can detect the low energy light emitted from our warm veins. If the average body temperature is 37.5 ◦ C and the mosquitoes eye is sensitive enough to detect 10^16 photons per second in the infrared, estimate the surface area of the mosquitoes eye (in mm^2 ). Hint: consider the mosquito at 20 cm from a person’s arm and approximate the arm as a cylinder with a surface area of 4.5% where all radiation leaves perpendicular to the surface the total. You will need: • the average length of a person’s arm ≈ 64 cm. • the average surface area of a person ≈ 1.75 m^2 .
In: Physics
A plane-parallel monoenergetic beam of 1O^I2 uncharged particles
per second
is incident perpendicularly on a layer of material 0.02 m thick,
having a density
p = 11.3 X lo^3 kg/m3. For values of the mass attenuation
coefficient m/p =
1X lo^-3 3 X lo^-4 and 1 X lo^-4 m2/kg, calculate the number of
primary par-
ticles transmitted in 1 minute. Compare in each case with the
approximation
in Eq. (3.5); give percentage errors.
In: Physics
1) Identical twins Anna and Hannah visit you at the optical clinic. Anna, whose eyes can easily focus on distant objects (her far point), is also able to focus on objects within 20 cm of her eyes (her near point). Assuming the diameter and, hence, the distance between the cornea and retina, of Anna's eye is 20 mm, what is the range (in diopters) of Anna's vision? The limits of this range correspond to the total refractive power of her eyes at their far point and and the refractive power at their near point.
a) from 50 to 50.5 diopters
b) from 50 to 55 diopters
c) from 50 to 60 diopters
d) from 0 to 5 diopters
2) Hannah's eyes have the same range as her sister's, with the same focal power for her cornea (50 diopters) and for her variable lens (5 diopters), but Hannah suffers from myopia. She cannot focus on any object that lies more than 0.7 meters from her eyes since they are slightly longer -- the cornea to retina distance is larger -- than her sister's eyes. Considering this new far point, what is the diameter of Hannah's eyes (in millimeters, to the nearest tenth of a millimeter) assuming Anna's eye diameter was ideally, again, 20.0 mm? Hint: the focal power of the cornea remains the same for Hannah as for Anna for focusing distant objects, but the farthest Hannah can see (object distance) changes from infinity to 0.7 meters.
3) Assuming, instead, that the diameters of Hannah's myopic eyes were 20.4 mm, but, again, that Hannah's eyes share the same focal powers for her cornea and lens as Anna's, what would be Hannah's near point (to the nearest tenth of a cm, in cm) if Anna's, again, is 20 cm?
4) Now assuming Anna's far point was found to be 0.8 m (i.e., her eyes can't focus on any object more than 0.8 m away), what power corrective lenses would you prescribe to Hannah so that, when wearing these lenses, her visual range was the same as Anna's (from a near point of 20 cm to a far point of infinity? Give your answer in units of diopters, to the nearest tenth of a diopter, with the correct sign.
5) One treatment of cataracts is to surgically remove the variable lens of the eye. If we assume that the cornea's refractive power focuses objects at infinite distances onto the retina of a person who has had this surgery, what power correcting lenses would they need to be able to read text at a 21-cm near-point distance? Again, give your answer in units of diopters, to the nearest tenth of a diopter and with the correct sign.
In: Physics
When violet light and yellow light shine on a metal surface, only one color produces an electric current. Write 3 - 4 sentences describing the effect of each light on the metal. Would increasing the intensity of either type of light have an effect on the current produced? How does quantization help to explain these observations? (6 points) Please don't write too long :)
In: Physics
You have constructed a 5-kg toy iceboat which you set on a frictionless frozen pond when a sudden wind exerts a constant force on the boat of 100 N due west. Due to how you have set the sails, the wind causes the boat to slide 10.0 m in a straight line at 30◦ north of west. What is the speed of the boat at the end of those 10.0 m?
In: Physics
explain the following:
a. what is lattice, basis, and crystal? explain the difference of them
b. bravais lattice and non bravais? explain and give example
c. primitive vactors?
d. brillouine zone? why we must study brillouine zone?
In: Physics
1- what is the relationship between integrity and necessity of enforcement ?
2- what is the relationship between external auditing and window-dressing?
3- At what stage in the design process should prototyping take place? why is prototyping done?
4- Why might design trade-offs be made? Give an example
Essay
1- Do society, employers and employees have shared interests?
2- Are there personal cost involved when one blows the whistle ? and should an individual ever consider doing it despite these consequences?
3- Should there be limits to how far we should go to ensure safety or preserve lives?
In: Physics
There is a tree in the middle of your front yard and you take pictures of a squirrel as it runs around the tree. When you take the first picture, the squirrel is 12.0 m away from the tree in the direction 30.0◦ N of E. Twenty seconds later, you take a second picture of the squirrel when it is 7.50 m away from the tree in the direction 15.0◦ E of S. What is the magnitude and direction of the average velocity of the squirrel between the two pictures?
In: Physics
Check all the correct statements.
| The internal energy of an ideal gas (those that obey PV = nRT) depends only the temperature of the system. |
| The internal energy of any gas depends only the temperature of the system. |
| The heat capacity of an ideal gas does not depend on what molecules the gas is made of. |
| At constant temperature, the internal energy of a real gas increases with increasing pressure because molecules are closer together. We assume that the conditions are such that attractions dominate over repulsions (i.e. the pressure values are not too high) |
| At constant temperature, the internal energy of a real gas decreases with increasing pressure because molecules are closer together. We assume that the conditions are such that attractions dominate over repulsions (i.e. the pressure values are not too high) |
| A cyclic path (initial state = final state) always results in ∆U = 0 |
| ∆U = 0 for any process that does not result in a change in temperature. |
| ΔU= qv only for a monoatomic ideal gas |
In: Physics
A particle of mass m moves under the influence of gravity g on the inner surface of a smooth cone of half-angle α. The axis of the cone is vertical, with its vertex downward. (a) Find the equation(s) of constraint of the system. (b) Find Lagrange’s equations of motion using the method of undetermined multipliers. (c) Determine the condition on the angular velocity ω such that the particle can describe a horizontal circle at a height h above the vertex.
In: Physics
a) In the wave picture of light, what determines the following properties? i. Color of the light ii. Brightness/intensity of the light iii. Energy of the light
b) In the photon picture of light, what determines the following properties? i. Color of the light ii. Brightness/intensity of the light iii. Energy of the light
In: Physics
1. Think about the forces acting on a train engine and the train
cars that it pulls.
a.) Draw a force diagram for the 5 forces (one of the forces is
friction) acting on the engine as it accelerates forward along the
rails.
b.) Draw a force diagram for the 4 forces acting on a train car as
it is pulled by the accelerating engine.
c.) By Newton’s 3rd Law, the train car pulls backward on its engine
exactly as hard as the engine pulls forward on the train car. Since
Newton’s 3rd Law is correct, the force by the engine on the car
should be equal and opposite to the force by the car on the engine.
Therefore, the entire train shouldn’t go anywhere. Using your force
diagrams from parts (a) and (b) above, explain why this analysis is
wrong. That is, show that there is a net force on the engine and
the train car.
In: Physics
a) Draw the energy levels (including the Fermi levels) of a non-degenerate p-n junction with a reverse bias of Vr. Draw the hole and electron concentrations as well. b) If this pn junction is used as a photodetector, what is the threshold wavelength of this detector? Will lower or higher wavelengths be detected by this detector?
In: Physics
A car of mass M and a small truck of mass 3M are travelling along two roads perpendicular to each other as shown.The car and the truck collide at the intersection at O.
The speed of the truck just before collision is VT =15.0m/s. Immediately after the collision, the wreckage moves as a single mass with Vf =12.0m/s as shown.
Using conservation of momentum:
[a] Find the value (in degrees) of θ?
A car of mass M and a small truck of mass 3M are travelling along two roads perpendicular to each other as shown.The car and the truck collide at the intersection at O.
The speed of the truck just before collision is VT =15.0m/s. Immediately after the collision, the wreckage moves as a single mass with Vf =12.0m/s as shown.
[b] What is Vc, the speed (in m/s)of the car just before collision?
In: Physics
Show by direct computation that the impulse momentum theorem and the work-energy theorem are invariant under the Galilean transformation.
In: Physics