Electric Forces & Fields
Electric Charges, Millikan's Oil Drop Experiment
DPP # EFF-01

Q1. If an object made of substance $$A$$ is rubbed with an object made of substance $$B$$, then $$A$$ becomes positively charged and $$B$$ becomes negatively charged. If however an object of substance $$C$$ is rubbed with an object of substance $$A$$, then A becomes negatively charged.

What will happen if object $$B$$ is rubbed with object $$C$$?

Q2. Can ever the whole charge of a body be transferred to the other? If yes how? if not why?

Q3. Can two similarly charged bodies ever attract each other ?

Q4. An electron and a Proton initially separated by a distance $$D$$ in air are released. The two particles are free to move. When they collide, are they

a) at midpoint of their initial separation?

b) closer to initial position of proton?

c) closer to initial position of electron?

Q5. Objects $$A , B$$ and $$C$$ are 3 identical insulated spherical conductors. Originally objects $$A$$ and $$B$$ have charges of $$+3\ mC,$$ whereas $$C$$ has charge of $$-6\ mC$$. Objects $$A$$ and $$C$$ are touched and moved apart. Then objects $$B$$ and $$C$$ are touched before they are moved apart.

i) if $$A$$ and $$B$$ are held near each other, they will

a) attract     b) repel     c) have no effect.

ii) if $$A$$ and $$C$$ are held near each other, they will

a) attract     b) repel     c) have no effect.

Q6. Select the correct statement (s) with respect to charges.

a) The additive property of charge is not an obvious property but is related to the fact that charge is a scalar physical quantity.

b) Charge is invariant, that is its value is same in different frames of reference having relative motion.

c) Charge is conserved for an electrically isolated system, this can be concluded from the scalar nature of charge.

d) Charge is conserved for an electrically isolated system, this cannot be concluded from scalar nature of charge.

Q7. Why a third hole in a socket is provided for grounding?

Q8. How many electrons are there in $$1$$ coulomb of negative charge?

Q9. Inside a nucleus ,a neutron can convert itself into a proton and an electron. A third particle known as anti-neutrino is also formed. Can you guess the charge of this particle?

Q10. Three metallic spheres $$A, B$$ and $$C$$ are given, such that on keeping $$A$$ and $$B$$ near, they experience attraction. Similarly, $$A$$ and $$C$$ also experience attraction. What will happen if $$B$$ and $$C$$ are kept in front of each other?

Q11. Five plates numbered $$1$$ to $$5$$ are brought in front of each other in pairs. Pairs $$(1,2) , (2,4)$$ & $$(4,1)$$show electrostatic attraction, while $$(2,3)$$ & $$(4,5)$$ show repulsion. What can be said about charge on plate $$1$$?

Q12. A glass rod is rubbed with silk cloth, the glass rod acquires a charge of $$+ 19.2 \times 10^{-19} C$$.

i) Find the number of electrons lost by glass rod.

ii) Find the negative charge acquired by silk.

iii) Is there any transfer of mass from glass to silk?

Q13. Calculate total positive or negative charge on a $$3.11 g$$ copper penny , if $$N_A = 6.023 \times 10^{23}$$. For copper, atomic mass $$=63.5$$ and atomic number $$= 29$$.

Q14. Figure shows that a positively charged rod is brought near two uncharged metal spheres $$A$$ and $$B$$ clamped on insulated stands and placed in contact with each other.

i) What would happen if the rod is removed before the spheres are separated?

ii) Would the induced charges be equal in magnitude event if the spheres had different sizes or different conductors?

iii) What will happen if the spheres are separated first and then the rod is removed far away? Q15. A table tennis ball covered with a conducting paint is suspended by a silk thread so that it hangs between two metal plates. One plate is earthed. When the other plate is connected to a high voltage generator, what will happen to the ball? Q16. If a drop of mass $$1.08 \times 10^{-14} kg$$ remains stationary in an electric field of $$1.68 \times 10^{5} NC ^{-1}$$, then the charge of this drop is

A. $$6.40 \times 10^{-19} C$$               B. $$3.2 \times 10^{-19} C$$

C. $$1.6 \times 10^{-19} C$$                  D. $$4.8 \times 10^{-19} C$$

Q17. Extra electrons on this particular oil drop (given the presently known charge of the electron) are

A. 4      B. 3      C. 5       D. 8 Q18. The dimensions of $$\left(\cfrac{1}{2}\right) \varepsilon_{0} E^{2}$$ is ($$\varepsilon_{0}:$$permittivity of free space; $$E:$$ electric field)

A. $$\left[M L T^{-1}\right]$$     B. $$\left[M L^{2} T^{-2}\right]$$

C. $$\left[M L^{-1} T^{-2}\right]$$     D. $$\left[M L^{2} T^{-1}\right]$$

Q19. A quantity $$X$$ is given by $$\varepsilon_{o} L \cfrac{\Delta V}{\Delta t}$$, where $$\varepsilon_{o}$$ is the permittivity of the free space, $$L$$ is the length, $$\Delta V$$ is the potential difference and $$\Delta t$$ is the time interval. The dimensional formula of $$X$$ is same as that of

A. Resistance     B. Charge

C. Voltage      D. Current

Q20. Five styrofoam balls are suspended from insulating threads. Several experiments are performed on the balls and the following observations are made:

(i) Ball $$A$$ repels $$C$$ and attracts $$B$$.

(ii) Ball $$D$$ attracts $$B$$ and has no effect on $$E$$.

(iii) A negatively charged rod attracts both $$A$$ and $$E$$.

An electrically neutral styrofoam ball gets attracted if placed nearby a charged body due to induced charge. What are the charges, if any, on each ball? Q21. A spray painting machine has a nozzle that produces droplets of paint in a cone shaped beam. If each droplet is given a positive charge before it leaves nozzle, what effect it will have on beam? Q22. A charged comb attract bits of paper but paper pieces often jump away from the comb after contact, why?

Q23. An electroscope is having two identical gold leaves. It is given a positive charge causing its two leaves to separate. When an object is brought near the metal disc at the top of the electroscope the separation increases even further. We can conclude

A. that the object is positively charged.

B. that the object is negatively charged .

C. that the object is electrically neutral.

D. only that the object is charged.