Power and intensity
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The document discusses sound intensity and power. It defines intensity as the power delivered per unit area and provides the formula I=P/A. It also gives an alternative formula for intensity: I=(1/2)pv(w)2S2, where p is mass density, v is wave speed, w is angular frequency, and Sm is amplitude. It notes that the decibel scale is used to measure sound levels logarithmically, with 0 dB corresponding to an intensity of 10-12 W/m2. It provides an example calculation of converting between intensity in W/m2 and decibels.
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【主营项目】
一.毕业证【q微741003700】成绩单、使馆认证、教育部认证、雅思托福成绩单、学生卡等！
二.真实使馆公证(即留学回国人员证明,不成功不收费)
三.真实教育部学历学位认证（教育部存档！教育部留服网站永久可查）
四.办理各国各大学文凭(一对一专业服务,可全程监控跟踪进度)
如果您处于以下几种情况：
◇在校期间，因各种原因未能顺利毕业……拿不到官方毕业证【q/微741003700】
◇面对父母的压力，希望尽快拿到；
◇不清楚认证流程以及材料该如何准备；
◇回国时间很长，忘记办理；
◇回国马上就要找工作，办给用人单位看；
◇企事业单位必须要求办理的
◇需要报考公务员、购买免税车、落转户口
◇申请留学生创业基金
留信网认证的作用:
1:该专业认证可证明留学生真实身份
2:同时对留学生所学专业登记给予评定
3:国家专业人才认证中心颁发入库证书
4:这个认证书并且可以归档倒地方
5:凡事获得留信网入网的信息将会逐步更新到个人身份内，将在公安局网内查询个人身份证信息后，同步读取人才网入库信息
6:个人职称评审加20分
7:个人信誉贷款加10分
8:在国家人才网主办的国家网络招聘大会中纳入资料，供国家高端企业选择人才
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It's based on the first part of this research paper:
The cost of information acquisition by natural selection
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Current descriptions of immersive learning cases are often difficult or impossible to compare. This is due to a myriad of different options on what details to include, which aspects are relevant, and on the descriptive approaches employed. Also, these aspects often combine very specific details with more general guidelines or indicate intents and rationales without clarifying their implementation. In this paper we provide a method to describe immersive learning cases that is structured to enable comparisons, yet flexible enough to allow researchers and practitioners to decide which aspects to include. This method leverages a taxonomy that classifies educational aspects at three levels (uses, practices, and strategies) and then utilizes two frameworks, the Immersive Learning Brain and the Immersion Cube, to enable a structured description and interpretation of immersive learning cases. The method is then demonstrated on a published immersive learning case on training for wind turbine maintenance using virtual reality. Applying the method results in a structured artifact, the Immersive Learning Case Sheet, that tags the case with its proximal uses, practices, and strategies, and refines the free text case description to ensure that matching details are included. This contribution is thus a case description method in support of future comparative research of immersive learning cases. We then discuss how the resulting description and interpretation can be leveraged to change immersion learning cases, by enriching them (considering low-effort changes or additions) or innovating (exploring more challenging avenues of transformation). The method holds significant promise to support better-grounded research in immersive learning.
The binding of cosmological structures by massless topological defects
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
equations sourced by a topological defect, i.e. a singularity of a very specific form, the result is a localized gravitational
field capable of driving flat rotation (i.e. Keplerian circular orbits at a constant speed for all radii) of test masses on a thin
spherical shell without any underlying mass. Moreover, a large-scale structure which exploits this solution by assembling
concentrically a number of such topological defects can establish a flat stellar or galactic rotation curve, and can also deflect
light in the same manner as an equipotential (isothermal) sphere. Thus, the need for dark matter or modified gravity theory is
mitigated, at least in part.
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Power and intensity
- 2. -Intensity, “I”, of a wave is the power delivered per unit area and is defined by I= P/A, where P is the rate at which the wave delivers energy (“W”) per unit area (“m2”). -Intensity can also be defined as I=(1/2)pv(w)2S2 m. Where p = mass density (in kg/m3), v = wave speed, w = angular frequency, and Sm = amplitude of sound wave. When measuring sound levels, it is common to use the unit for logarithmic function “decibel” (dB). Since logarithmic scales are relative scales, we choose a reference intensity level to be 1000 Hz (about 10 W/m2). Therefore 0 dB corresponds to an intensity I0 = 10-12 W/m2. To express any other sound intensity level (B = beta) is “B(I) = 0 dB + 10 log10 (I/I0). Introduction to sound Intensity and Power
- 3. Typical sound intensity levels for some common sources
- 4. Sample Question: Suppose you are being held captive in a cell, being tortured for information by listening to Justin Bieber songs with a sound intensity of 3.24 W/m2. What is the sound intensity in decibels (dB)? (Recall B(I) = 0 dB + 10 log10 (I/I0))
- 5. Solution: Recall that I0 = 10-12 and 0 dB = 10 W/m2 B(I) = 0 dB + 10 log10 (I/I0) Plugging in I = 3.24 W/m2 and I0 = 10-12 W/m2 , we get B(I) = 10 W/m2 + 10 log10 (3.24 W/m2 / 10-12 W/m2) = 125 dB
- 6. If we think about distance and sound intensity, it makes sense that as we move further away from the source, sound intensity decreases. Suppose, then, at some distance (m) we enclose a sphere around the source with radius (r) = distance (m), then P = I(4 π r2) *recall I = P/A P = IA Here we can see that (4 π r2) is the A (area) of a sphere. The equation above states that the power radiated by the source must pass through the surface of the sphere. This is called Isotropic, which means uniformly in all directions.
- 7. Sample Problem #2 Suppose you want to escape the inside of a troll’s spherical stomach with a diameter of 12 m by blasting Justin Bieber songs until he throws up. What is the amplitude of the sound wave given the period of the wave is 0.4 m, the air density in his tummy is 1.20 kg/m3, the power of the wave is 1.14 kW and the speed of the wave is 343 m/s?
- 8. Solution Recall (1) P = I(4 π r2) and (2) I=(1/2)pv(w)2S2 m Where “Sm” is the sound wave amplitude, “p” = density, “v” = wave speed, and “w” = angular frequency. Using equation (1) we can solve for “I” and use it in equation (2) to solve for amplitude (Sm)
- 9. (1) P = I(4πr2) (1) We know that P = 1.14 kW = 1,140 W and that radius (r) = ½ diameter (12m) = 6m. P = I(4πr2) 1,140W = I(4 π (6.0m)2) 1,140/(4π(6.0m)2) = I = 2.52 W/m2
- 10. (2) I=(1/2)pv(w)2S2 m “w” = frequency = 2π /T (“T” = Period), = 2π /0.4 = 5π. “v” = velocity = 343 m/s “p” = density = 1.2 kg/m3 “I” = intensity = 2.52 W/m2 Isolating for Sm we get sqrt [(2I)/(pv(w2))] = Sm Plugging in the values we get sqrt [(2*2.52 W/m2)/(1.2 kg/m3 *343 m/s*(5π)2] = 0.00704 m = Sm