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# Monthly Archives: January 2014

## Sensitivity of Wheatstone’s Bridge

When does the sensitivity of a wheatstone bridge reduce?

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

Now try to identify the factors contributing to sensitivity!

## Deduce an expression for velocity of em waves in vacuum

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?

(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. the 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?

## 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. ”