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# Monthly Archives: September 2011

## Wire Gauge and Resistance problem

I am using 18 gauge nicrome wire having ohm of 17.3 , 220 volt. I want to use 10 gauge wire how to decide the length?

18 gauge corresponds to a diameter of 1.22 mm

Therefore radius r1= 1.22/2=0.61 mm=0.00061m

10 gauge means diameter is 3.25 mm

Radius r2 = 3.25/2=1.625mm=0.001625m

Sine the resistance needs to be the same, we can use the formula

$\LARGE \bg_white R=\frac{\rho l}{A}$
Which gives
$\LARGE \bg_white \frac{l_{1}}{A_{1}}=\frac{l_{2}}{A_{2}}$
As resistivity is same.

$\LARGE \bg_white l_{2}=\frac{l_{1}A_{2}}{A_{1}}=\frac{l_{1}r_{2}^{2}}{r_{1}^{2}}$

Substitute the values and you will get the answer.

## Acceleration due to gravity problem

A body weighs 30 kg on the surface of the earth how much would it weigh on the surface of a planet whose mass is 1/9 the mass of earth and radius is half of that of earth?

Weight on earth = 30 kg wt = 30 x g N

g on new planet, g’ = GM’/R’2=(4/9)g

Therefor weight on the new planet will be equal to 4/9 times its weight on earth

(Whereas the mass will be the same)

## Graphene bubbles can help make auto focus lenses

A group of UK Physicists claim that they have developed graphene bubbles which can change its curvature and hence its focal length and power by applying small voltages. If developed, this can be used in mobile cameras and other such devices. This would better imitate the functioning of Human Eye too.

More details available at http://physicsworld.com/cws/article/news/47250

## Acceleration of a body on a rough surface

A black of mass 1.5kg placed on a rough horizontal surface is pulled by a constant horizontal force of 1.2kg wt. the cofficient of friction between the block and the surface is 0.3.  Find acceleration produced in terms of g.

(Posted by Ankur Srivastava)

Applied force, F=1.2 kg wt = 1.2g N (where g is the acceleration due to gravity)

Friction f=µmg=0.3 x 1.5 x g=0.45g N

The net force = ma

Therefore;

ma= F-f

=> 1.5 x a = 1.2 g -0.45g=0.8 g

or

a = (0.8/1.5)g

## A problem from Dimensional Analysis

if force F us given in terms of timeT and distance X f=AsinCT +BcosDX then the dimensions of A/B is equal to

## Heating up of water due to shaking

Would shaking a bottle of chilled water, cause it to warm up, even slightly, due to some kind of friction?

Yes, This is due to friction (Viscous forces). The different layers of water resist relative motion and the work done in producing motion against the viscous force appears  as heat.

## Download the newly included portion by CBSE for CLASS 12 Physics

CBSE has added some content for class XII. This includes mainly portions from “Combination of mirrors and lenses

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