Most probable speed of Aluminium

1. rashi

16
1. The problem statement, all variables and given/known data

Figure 1 shows a type of apparatus for investigating the speed distribution f (v) of molecules in a gas. Molecules emerge into a vacuum chamber from an oven held at a constant temperature T, and are collimated by slits into a parallel beam directed towards a pair of discs rotating at a common angular speed ω, which may be varied. The discs are a fixed distance L apart (L = 262 mm) and contain narrow notches, with the second notch offset from the first by a fixed angle θ = 30° as shown. If a molecule is moving at the correct speed v, it will pass through the notches in both discs to be collected at the detector, which records the rate of arrival of molecules

The oven contains aluminium at 9.00 × 102 °C, which emerges as a monatomic beam. Calculate the speed vmp and the corresponding rotation speed ω at which the largest rate of arrival for aluminium atoms is observed.
(The molar mass of aluminium is 0.0270 kg mol−1.)

2. Relevant equations

Vmp= Square root 2RT/M

3. The attempt at a solution
v = ωL/θ

3. rashi

16
I am working on it. Do I have to change degree to rad?

Staff: Mentor

Yes. Radians are the "natural" units for angles.

16
Thank you!

6. rashi

16

Can any one explain why we always use radian instead of degree?

7. HallsofIvy

41,055
Staff Emeritus
In calculus, at least, the derivative (rate of change) of sin(x) is just cos(x) as long as x is in radians. If you were to use degrees it would be (180/pi) cos(x). The first formula is simpler.

Another way of looking at it (and the reason for that previous formula) is that "360 degrees in a circle" is purely arbitrary. But a circle has a "natural length", it radius. And the radian measures how many radii would fit around the part of the circle's circumference subtended by the angle, so is a "natural" measure.