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## Problems from Electricity

- The parallel plates of an air filled capacitor with area of area 1.1 x 10^8 m^2 . What must be the width of the plate be if the Capacitance is 1 Farad?
- Four capacitor each of 2microfarad are connected in such a way that total capacitance is also 2microfarad. Show what combination gives this value. Also try to illustrate the diagram.

(Fari posted these)

Answer

1.Using the formula:

and substituting , C= 1 F, A = 1.1 x 10^{8}, = 8.8 x 10^{-12},

we get , d =9.68×10^{-4} metres

## Passenger in a moving train

A passenger in a moving train tossed a coin which falls behind him .State the type of motion of the train ?

**Answer:**

The train is in accelerated motion.

**Explanation**

If the train were moving with uniform velocity, the coin would fall back to the passenger’s hand (This is due to the inertia of motion of the coin. When it was thrown ‘vertically’ up, it had the same horizontal velocity as that of the train. Therefore, it will move alongwith the train and fall back to the thrower’s hand)

If the train is accelerated, the horizontal velocity of the train would be more than that of the coin (which remains constant through out its flight) by the time the coin returns. Therefore, the train travels a greater horizontal distance than the coin and the coin falls **behind the passenger.**

## A problem from Electrostatics

Two parallel plates A and B of infinite extent are spaced 1 cm apart. a steam of electrons is projected at a velocity corresponding to an accelerating voltage of 2kV into space betn the plates through a hole in plate A at an angle of 30 deg to it. find the value and polarity of the potential reqd betn A and B such that electrons just graze B.

Ganesh Asked

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