Calculating charge on a body with excess of electrons

How would I calculate the charge on a small sphere with an excess of 5.0 x 10 14 electrons?

We know that charge on an electron is (-) 1.6 x 10-19 C

If the sphere has 5.0 x 10 14 electrons in excess then the body has a net negative charge and is given by q = ne

Charge on the sphere = 5.0 x 10 14 x (-) 1.6 x 10 -19 = 8 x 10 -5 C

Speed of sound

Why sound travels faster in humid air than normal air as humid air is denser than normal air? (Akash asked)

The speed of sound in a medium is inversely proportional to the square root of its density. Humid air is less dense than dry air. Therefore the speed of sounf in humid air is greater than that in dry air.

Confused?

Are you thinking why dry air is more than that of humid air? This is a common misconception. Students used to think that humid air is denser than dry air. But it is not so. Just like skimmed milk (milk removed off its cream) is more dense than normal milk (with cream). Density of cream is much less than that of the rest of milk. So when it is removed, the residual milk becomes denser.

Humid air contains water vapor content. Water vapor is lighter than air. So when air contains water vapor, its density is less. When the water content is removed, it becomes denser.

For more details refer:
http://www.engineeringtoolbox.com/density-air-d_680.html
http://www.theweatherprediction.com/habyhints/260/
http://www.usatoday.com/weather/wdensity.htm

Motion in a vertical circle and conservation of energy

A stone  tied to a string of length l is whirled around a vertical circle with the other end of the string at the centre. At a certain instant of time the stone is at the lowest position and has a speed u. What is the magnitude of change in its velocity as it reaches a position where the string is horizontal?

Let’s assume that the potential energy at the lowest position be zero. So, when the string is horizontal, the stone has risen by a vertical height l, the length of the string which is also the radius of the vertical circle.

If v is the magnitude of velocity at the horizontal position, then according to the law of conservation of energy,

KE+PE at the lowest position = KE+PE at the horizontal position

$\frac{1}{2}mu^{2}=\frac{1}{2}mv^{2}+mgl$

From the equation above, v-u can be calculated.

$v=\sqrt{u^{2}-2gl}$

The following links will help you for deeper understanding and you can browse through some solved problems from the topic too.

A Numerical from Combination of thin lenses in contact

Two thin lenses of focal lengths f1=20cm and f2=60cm are placed in contact. find the focal length f3 of the combined lenses. find also the focal length f of a third lens placed in contact with these two that would result in overall focal length f11=-40cm. (Asked Young) Incoming search terms:class 9 science sample paper for [...]

How atoms are formed? – Neeraj Asks

Neeraj asks: ”We have read that atoms can neither been created nor been destroyed!I want to know from where atoms came in existence,if they can not been created,also atom contain electron and proton which are negatively and positively charged,i want to know from where these charge came?”   Your question will be answered soon Link to this post!

How atoms are formed? – Neeraj Asks

Neeraj asks: ”We have read that atoms can neither been created nor been destroyed!I want to know from where atoms came in existence,if they can not been created,also atom contain electron and proton which are negatively and positively charged,i want to know from where these charge came?”   Your question will be answered soon Link to this post!

How to prepare for IIT JEE?

Surya Kant Dwivedi asks: “I am always confused in Physics that how can i do my best in Physics. It is my weakest portion in ’PCM’. Please guide me.

Excelling in Physics in IIT JEE requires systematic planned preparation. The first step is to make sure that you can solve all the exercises from NCERT text book (including the solved examples and additional exercises) confidently and completely. This ensures that you have sufficient exposure and you are up to the standard expected of you.

The next step is to get a good collection of questions asked previously in IIT JEE and solve them. If you are not able to solve a problem, you are welcome to post it here.

Recommended Books:

Fundamentals of Physics (Resnick Hallidey)

Concepts of Physics (HC Verma)

Irodov Problem in Physics

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