electricity

« 1.

A positively charged glass rod is bought close to a suspended metal needle.

What can we say about the charge on the needle given that the needle is a) attracted? b) repelled? Answer : The electrostatic force obeys the principle of linear super-position which basically states that force between two particles does not get affected by the charge of the other and hence if the rod is brought close to a suspended metal needle and is getting attracted then needle definitely should possess opposite charge relative to the rod or basically negative charge as opposite charges attract.

(b) In case the needle which is brought near the sus[ended needle and gets repelled that shows that both of them possess same charge and hence positive charge on both objects and will start repelling each other. 2.

How would you determine the sign of a charge on a body using an electroscope? Answer: when a charged object is touched to an electroscope the electroscope automatically takes the same charge as of object.

The electroscope shows the presence of the charge but does not indicate the sign and hence in order to determine the sign we need to bring a known charged body of either positive or negative charge slowly to the head of electroscope and observe the foil motion.

The foils will converge if the charges on both are different and will diverge if the charge on object is same as that of electroscope.

3.

You are given two metal spheres mounted on portable insulating supports.

How would you give them equal and opposite charges? You may use a glass rod rubbed with silk but you may not touch it to the spheres.

Answer: let the spheres be sphere A and sphere B.

before the charging process t here are equal numbers of protons and electrons within the two spheres.

If we bring a positively charged glass rod near sphere A, the presence of positive charge will induce a mass migration of electrons from sphere B into sphere A induced by the principle of opposite attracts.

The negatively charged electrons in both the spheres get attracted towards the positive glass rod.

The electron movement from sphere B to A leaves overall positive charge in sphere B and overall negative charge in sphere A.

4.

You are given two metal spheres mounted on portable insulating supports.

How would you give them identical charges? You may use a glass rod rubbed with silk but you may not touch it to the spheres. Answer: let us take two spehere with neutral charges.

Bring one positively charged glass rod to sphere A.

This repels the +ve charges on the other side of the sphere which in turn induce –ve charge on the sphere B.

Once the negative charge is induced in spehere B it is separated from spehere A.

Hence we have now both sphere A and B having same charge.

5.

Referring to the diagram below.What is the magnitude and direction of the resultant force acting on Q2?. »