Class 12 Physics Electrons Notes and Important Questions
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Notes for Class 12 Electrons (Physics) are shown above.
Practice
Important Questions
Explain what is meant by quantization of charge.
In a Millikan‟s oil drop experiment, an oil drop of weight 1.5 x 10 N is held stationary between plates 10mm apart by applying a p.d. of 470V between the plates.
State the condition necessary for the drop to remain stationary. Also, sketch the forces acting on the oil drop.
Calculate the charge on the oil drop.
Explain quantization of charge.
An ion of specific charge 4.40 × 10⁷ C/kg is moving in a circular orbit in a magnetic field of flux density 0.4 T with velocity of 3.52 × 10⁵ m s⁻¹. Calculate the radius of this orbit.
Why are X-rays used in Millikan's oil drop experiment?
Figure below shows the experimental setup of Millikan’s oil drop experiment. Find the expression for a charge of an oil drop of radius $r$ moving with constant velocity $v$ in a downward direction using a free body diagram.
What will be the expression for the charge of an oil drop if the electric force is greater than its weight?
Determine the electric field supplied when the electric force applied between the two horizontal plates just balances an oil drop with 4 electrons attached to it and mass of oil drop is $1.3 \times 10^{-14}\ \text{kg}$.
An oil drop of mass 3 × 10⁻¹¹ g and of radius 2 × 10⁻⁶ cm carries 10 excess electrons. Its terminal velocity when:
i) falling in a region in which there is no electric field. [1]
ii) falling in an electric field of 3 × 10⁵ V m⁻¹ directed downward? [1]
(Viscosity of air = 1.8 × 10⁻⁵ Nsm⁻²)
In Thomson's method for determining specific charge of an electron, why is the electric field kept perpendicular to magnetic field? Justify.
What is the use of X-rays in Millikan's oil drop experiment?
An oil drop carrying a charge q has a mass m kg. It is falling freely in air with terminal velocity v, the electric field required to make the drop move upward with the same speed is
mg / q
2mg / q
mgv / q²
2mgv / q
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Exam
Past Question Analysis
Historical exam-pattern data from past NEB question papers — not a prediction of future questions.
3
Question Items
3
Total Marks
3
Papers Appeared In
| Year | Question Items | Marks |
|---|---|---|
| 2083 | 1 | — |
| 2080 | 1 | — |
| 2078 | 1 | 3 |
Also appears in 3 model questions (not counted above — model sets aren't actual NEB exam history).
This chapter explores the experimental discovery and properties of electrons. It covers Millikan's oil-drop experiment, electron beams, electron motion in electric and magnetic fields, and Thomson's determination of specific charge.
This page covers Electrons, chapter 20 of 25 in the Class 12 Physics syllabus set by the National Examination Board (NEB). 5 important questions for this chapter are available, each with a full solution.
For numerical and derivation-based chapters like this one, working through past NEB questions is usually more useful than re-reading notes alone — try solving each important question above before checking the solution, then compare your working step by step.