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Monthly Archives: January 2014


Sensitivity of Wheatstone’s Bridge

When does the sensitivity of a wheatstone bridge reduce?


Asked Shreya




Sensitivity refers to a large deflection for a small imbalance of the bridge.


Now try to identify the factors contributing to sensitivity!


Wheatstone bridge using Thomson galvanometer


Deduce an expression for velocity of em waves in vacuum

Please find below a detailed version giving the answer to your question

Brown Shadows

Some mornings in our bedroom we get a brown shadow on the ceiling from a hanging light fitting. It is obviously from reflected sunlight and there are also normal grey shadows in different directions. The brown one is the strongest and sharpest. How can it be coloured?

Bill Asked

exceeding EMF safety levels with conventional electronic components

Is it possible to take a standard cellphone or satellite transponder uplink and overdrive the output transmitter power to such a high level that it exceeds the specific absorption ratio (SAR) and thereby becomes a portable harassment weapon? sure, there are power limits on all hardware components since they were only designed for legitimate usage levels, but what if some hacker tested the boundaries and found that they can transmit at high power levels for a day or a week without them burning out…exceed the limits on what is dangerous to transmit…conventional wisdom says that EMF is not dangerous, but this was before people realized how to generate  MVolts/Meter of transmit power… What I want to know is how close we are to enabling this type of future with standard off the shelf components? is it possible today using standard cellphone or satellite transponder components fitting into a nice neat package? A Website online states: RF Effects on the human body Extremely Low frequency RF with electric field levels in the low Kv/m range are known to induce perceivable currents within the human body that create an annoying tingling sensation. These currents will typically flow to ground through a body contact surface such as the feet, or arc to ground where the body is well insulated.[9][10] Canadian safety code 6, also, recommends electric field limits of 100 kV/m for pulsed EMF to prevent air breakdown and spark discharges. Additional rational for EMF restrictions is to avoid auditory effect and energy-induced unconsciousness in rats.[11] Also, see Microwave burn and Electromagnetic radiation and health As a weapon A heat ray is a RF harassment device that makes use of microwave radio frequencies to create an unpleasant heating effect in the upper layer of the skin. A publicly known heat ray weapon called the Active Denial System was developed by the US military as an experimental weapon to deny the enemy access to an area. Also, see death ray which is a heat ray weapon that delivers electromagnetic energy at levels that injure human tissue. The inventor of the death ray, Harry Grindell Matthews, claims to have lost sight in his left eye while developing his death ray weapon based on a primitive microwave magnetron from the 1920s. (Note that a typical microwave oven induces a tissue damaging cooking effect inside the oven at about 2 kV/m.) * Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 kHz to 300 GHz, Canada Safety Code 6, page 63 http://www.euitt.upm.es/estaticos/catedra-coitt/web_salud_medioamb/Informes/informes_PDF/rf/HealthCanada/99ehd237.pdf * Extremely Low Frequency Fields Environmental Health Criteria Monograph No.238, chapter 5, page 121, WHO http://www.who.int/peh-emf/publications/elf_ehc/en/index.html * Noninvasive Radio Frequency for Skin Tightening and Body Contouring, Frontline Medical Communications, 2013 http://www.skinandallergynews.com/fileadmin/content_pdf/san/scms_pdf/SCMS_Vol_32_No_1_Skin_Tightening.pdf


(Posted by Bill)

String tension in case of pulley with mass

  1. In case of a massless string around a massless pulley , tension on either side of the pulley has to be the same. But when the pulley has mass, we consider tensions to be different on each side. Now i got a solution from someone that since pulley has mass it now needs torque to rotate so the tensions cant be same. but what about the string?? the string is massless so wont it have infinite acceleration?
  2. pulley-spring-systemthe diagram is in the file attached. this is a question of simple harmonic oscillation in which time period has been asked. It can be easily done by energy conservation but i also wanted to know how t can be done by analysing the forces method. what will be the tension in the string?

Asked Vasudha Bhandari

From Thermodynamics

I have created a hot box and heated the inside to 140 degrees Farenheit. The hot box has 5 stationary walls (17 inches by 17 inches) made out of
2 inches of polyisocyanurate.
The rated R-value of this material is 13.1. The 6th wall (actually the top) I can take on and off. I have run 3 experiments for 30 minutes (1800 seconds)
each.  The first experiment I ran with the 2 inches of polyisocyanurate. The change in temperature was 38 degrees Farenheit.

Question 1: what is my calculated R-value?

I ran this same experiment with the top being  3.5 inches of air.  The starting tempature was 140 degrees farenheit and the change in tempature was
45 degrees farenheit.

Question 2:  what is my calculated R-value for air?

Finally I ran this with 2 inches of polystyrene and 1.5 inches of air. The starting tempature was 140 degrees farenheit and the change in tempature was
43 degrees farenheit.

Question 3:  what is my calculated R-value fo the poltystyrene?

I have used both q= m c delta T and Newton’s law of Cooling … and I cant seem to get numbers that make sense. Please include all calculations

(Posted by RuthAnne)

How long it will take for electrons to travel through the electron gun?

“You have an electron gun, in which electrons travel through. Find out how long it will take for electrons
to travel through the electron gun. The distance between a charge A and B of the cannon is 6 cm and the voltage between the charges is 360 V. The voltage is divided in half at each charge. ”

Asked Matt

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