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Size of a soap bubble when charged

What will happen to the size of the soap bubble if the bubble is given charges ?


Answer: When a soap bubble is charged, its size increases.

Under normal condictions, a soap bubble is in equilibrium un der two opposing forces – the force of surface tension which tries to compress it and the force due to excess pressure which tries to expand it.

When charged, the force of repulsion among the like charges will try to expand the soap bubble further resulting in an increase in size of the bubble.

Work done in accelerating a proton

The mass of a proton is 1840 times that of an electron. it is accelerated to a potential difference of 1kV. find the amount of work done in process. (Ujjwal Sharma asked)


The work done = the KE acquired = eV=1.6 x 10-19 x 1000 = 1.6 x 10-16 J

How does a microwave oven work?

Now, microwave oven is so popular that majority of houses have one. Many use it mainly to reheat food without loss of nutrient. The advantage is that the microwave heats up the food very fast.

Some observations made on microwave oven are,

  • It does not heat up the vessels directly
  • It is not able to cook food contained inside metal vessels
  • If the turn table does not work, cooking will not be even.


So, it is pertinent to ask how the mechanism of heating in a microwave oven is different from other heating devices?

Here the water molecules, which are polar in nature play the magic. (A polar molecule is one in which the centre of mass of positive charges and the centre of mass of negative charges do not coincide and have a net dipole moment.) When microwaves are passed through food materials, the water molecules contained in them tends to turn and this rapid churning of water molecules develops a lot of heat. The turn-table adds to the churning and randomization developing heat more rapidly.

The food materials contained in a closed metal vessel will not be heated up by the microwave oven as they do not allow microwaves to pass through them. The metals are opaque to microwaves.

Why the food materials heated using microwave dry up and become stiff when cool?

In a microwave oven, the water molecules are heated very fats that the water is converted readily into steam. If kept open, the steam will escape and the material will become hard when cooled due to the decrease in the water content.

Electrostatics Doubts

  1. An infinite number of charges,each of magnitude q,are placed along X-axis at X=1m,2m,4m,8m,16m and so on but the consecutive charges are of opposite sign starting with +q at x=1m. a point charge q’, kept at the origin ,experiences a force of what  magnitude?
  2. four point charges,each +q,are fixed at the corners of a square of side ‘a’. another point charge q’ is placed at a height ‘h’ vertically above the centre of square,assuming the square to be in horizontal plane. what is the magnitude of force experienced by q’?

The total force on charge q on the vertex

Can you please answer the following question?
Consider the charges q,q and -q placed at the vertices of an equilateral triangle. What is the force on each charge?
I actually didn’t understand the total force on charge -q at the vertex C of the equilateral triangle. How is it equal to root3Fn where n is the unit vector along the direction bisecting the angle BCA (Why?).

Thejaswini posted the question


An equilateral triangle. {3/1}
An equilateral triangle. {3/1} (Photo credit: Wikipedia)

What are the limitations of Van de Graff generator?

What are the limitations of Van de Graff Generator?

Saikat asked


One of the limitations of Van de Graff generator is the potential to which the dome can be raised. In normal condition, discharge takes place when the potential reaches 3 x 106 V. The potential can be increased further by  placing the entire system in a container filled with high-pressure gas.

(Are there more? Visitors are requested to post as comments)

Electric field inside and outside the plates of a capacitor

How do you find the direction of electric field on the outer or inner surface of  2 parallel plate of a capacitor ?



The direction of electric field is from the positive plate to negative plate.

The electric field outside the plates is zero as the electric field due to each plate (E=Q/2ε0A) cancel out being equal in magnitude and opposite in direction.

In between the plates the two electric fields add up as they are in same direction. (From positive plate to negative plate).

The magnitude of electric field between the plates is twice the electric field due to either; i.e; E=2 x Q/2ε0A = Q/ε0A

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