Engineering Questions: Punch of the Week (14-Dec-09)

by Deepa Mittal

What is “Punch Of The Week”?

Every week Learnhub’s team will post one question each on Physics, Chemistry and Mathematics of the IIT-JEE, AIEEE and BITSAT Test Preparation

Members can post their answers along with explanations for the next one week.

After one week, Learnhub’s team will post the official answer along with the explanation.Besides answering the previous week’s questions, Learnhub’s Team will come up with the next set of questions for that week as well.

What are the rules of “Punch Of The Week”?

Every user must refrain from posting sarcastic comments on the answers posted by others.

Once the Learnhub’s team posts the official answer along with the explanation, no comments must be made regarding those questions. In case of further clarification please send a personal message to the respective team member

If you think that there is something wrong with the explanation of the Learnhub’s Team, which seldom happens, please send a personal message to the respective team member informing him/her the same. Posting the same here will not be considered and will be deleted.

Please bear in mind that only Learnhub’s Team members have the privilege of posting questions here. Questions posted by other members will not be entertained and hence will be deleted immediately.

“Punch Of The Week” Team Members

This Week’s Punches

Physics

|A sheet of aluminium foil of negligible thickness is placed between the plates of a capacitor. What effect has it on the capacitance if

(i)The foil is electrically insulated, and
(ii)The foil is connected to the upper plate with a conducting wire.

||

Chemistry

Write Phosphorus on reaction with NaOH gives PH_3 as one of the products. This is a

(A) Dimerization Reaction
(B) Disproportionation Reaction
(C ) Condensation Reaction
(D) Precipitation Reaction

Math

If one of the lines of my^2 + (1-m^2) xy - mx^2 = 0 is a bisector of the angle between the lines my = 0 , then $m</math? is :$

(A) 1
(B) 2
(C ) $\frac{1}{2}$
(D) -2

%{font-size:16px}"*Click here for Previous week’s Punch of the Week%*":http://bitsat.learnhub.com/lesson/16423-engineering-punch-of-the-week-08-dec-09

Posted on 18-Dec-09

So, folks here are the correct answers

Physics

Solution:

(i) The arrangement is equivalent to two capacitors connected in series. Each such capacitor has plate separation d/2

and hence capacitance 2C. Total capacitance is $C_S = \cfrac{2C \times 2C}{2C + 2C}= C$

i.e. the capacitance remains unaffected.

(ii) The capacitance becomes twice the original capacitance because d becomes d/2.

|<>{border-color:white;font-size:14px}.%{color:red}*Chemistry*%
The correct answer is B.

Solution:

$P_4 + 3NaOH + 3H_2o \rightarrow 3Na H_2 PO_2 + PH_3$ |

|<>{border-color:white;font-size:14px}.%{color:red}*Math*%
The correct answer is A.

Solution:

The equation of the angle bisectors between the co-ordinate axes are y = x and y = -x.
$\therefore m + 1 -m^2 - m = 0$
$\Rightarrow m^2 = 1$
$\Rightarrow m = ^+_-1$ |