NEB Class 12 Physics Electrons Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.

Mero Enotes is a simple educational Website Built to help Class 11 and 12 students excel in their studies. We offer Notes, question papers, and important study materials across various subjects for both science and management faculties. Whether you’re looking for Physics, Chemistry, Biology, Accounting, Economics, and Finance, Mero Enotes provides organized PDF resources, embedded via Google Drive for easy access.

We aim to make studying easier for students by giving them the right materials to prepare for exams.

NEB Class 12 Physics Electrons Note Handwritten in PDF

Millikan’s Oil Drop Experiment

Purpose: Determine the charge of a single electron and prove charge quantization.

Setup:

  • Atomizer sprays tiny oil droplets between two horizontal charged plates
  • X-rays ionize the droplets (give them charge)
  • Microscope observes individual drops

Method:

  1. Without electric field: drop falls under gravity at terminal velocity (Stokes law β†’ radius r)
  2. With electric field: adjust E until drop is stationary (electric force = weight)
  3. At equilibrium: qE = mg β†’ q = mg/E
  4. m is calculated from terminal velocity and Stokes law

Result: Charge of every drop was always a multiple of e = 1.6 Γ— 10⁻¹⁹ C

This proved charge quantization β€” charge exists only in discrete units of e.

Motion of Electron Beam in Electric Field

An electron beam entering a uniform electric field E (between parallel plates) follows a parabolic path β€” exactly like projectile motion in gravity.

  • Along electric field direction (y): ay = eE/m, y = Β½aytΒ²
  • Along beam direction (x): vx = constant (no force), x = vxt
  • Eliminating t: y ∝ xΒ² (parabola)

This is the principle behind CRT (Cathode Ray Tube) televisions and oscilloscopes.

Motion of Electron Beam in Magnetic Field

An electron beam entering a uniform magnetic field B (perpendicular to velocity) follows a circular path.

  • Magnetic force provides centripetal force: qvB = mvΒ²/r
  • Radius: r = mv/qB
  • Time period: T = 2Ο€m/qB (independent of speed)

If both E and B are present, for the beam to go straight (velocity selector):

  • qE = qvB β†’ v = E/B (only electrons with this speed go straight)

Thomson’s Experiment Determining e/m

Setup: Cathode ray tube with electric and magnetic fields.

Step 1: Apply only electric field E β†’ measure deflection y₁

Step 2: Apply magnetic field B to cancel deflection β†’ qE = qvB β†’ v = E/B

Step 3: Turn off magnetic field, measure deflection with only E: From parabola geometry: e/m = 2yvΒ²/(ElΒ²) approximately

Thomson’s result: e/m = 1.76 Γ— 10ΒΉΒΉ C/kg

Combined with Millikan’s e = 1.6 Γ— 10⁻¹⁹ C: Mass of electron = e/(e/m) = 1.6 Γ— 10⁻¹⁹ / 1.76 Γ— 10ΒΉΒΉ = 9.1 Γ— 10⁻³¹ kg

Class 12 Science Faculty All Subject Notes

Student Queries:- Class 12 Physics Notes download | Complete Physics Guide (New Syllabus) | Class 12 Physics Book Solution Guide App | Class 12 Physics Book Notes 2081 Guide | Class 12 Physics All Chapter Notes | Kec Publication Physics class 12 solutions | Neb class 12 physics notes pdf free download | Neb class 11 physics notes handwritten | Class 12 Physics Notes PDF | NEB Class 12 Physics Book PDF | Principle of Physics Class 12 pdf Asmita publication | NEB Class 12 Physics syllabus 2080 | Class 12 Physics Book PDF Nepal 2080

NOTE : β€œIf you want to download the notes Offline Contact us in Social media β€œ

Frequently Asked Questions

What is Millikan’s oil drop experiment Class 12?

Millikan sprayed oil drops between charged plates. At equilibrium: electric force = weight β†’ q = mg/E. He found every drop’s charge was always a whole number multiple of e = 1.6Γ—10⁻¹⁹ C. This proved charge quantization electric charge exists only in discrete units and cannot be divided further.

What does quantization of charge mean in Class 12 Physics?

Quantization means electric charge exists only in integer multiples of the elementary charge e = 1.6Γ—10⁻¹⁹ C. A body can have charge 0, Β±e, Β±2e, Β±3e but never 0.5e or 1.7e. Millikan’s experiment proved this. Electrons carry βˆ’e, protons carry +e. Quarks have fractional charges but are never found isolated.

What is Thomson’s experiment and what did it determine?

Thomson applied electric and magnetic fields to cathode rays. From deflection measurements he determined charge-to-mass ratio of electrons: e/m = 1.76Γ—10ΒΉΒΉ C/kg. Combined with Millikan’s value of e = 1.6Γ—10⁻¹⁹ C, electron mass = 9.1Γ—10⁻³¹ kg was calculated. Thomson discovered the electron in 1897.

What path does an electron take in a magnetic field Class 12?

Electron moving perpendicular to magnetic field follows a circular path. Magnetic force provides centripetal force: qvB = mvΒ²/r. Radius r = mv/qB. Time period T = 2Ο€m/qB (independent of speed the cyclotron principle). If velocity has a component along B, the path becomes helical rather than circular.

What is velocity selector in Class 12 Physics?

When electric field E (upward force qE) and magnetic field B (downward force qvB) are crossed, they balance for one specific velocity: qE = qvB giving v = E/B. Only electrons with exactly this speed pass straight through undeflected. Others are deflected. Thomson used this to measure electron speed before calculating e/m ratio.

What path does an electron follow in an electric field?

Electron entering uniform electric field perpendicular to its motion follows a parabolic path exactly like projectile motion in gravity. Along field direction: ay = eE/m (uniform acceleration). Along beam direction: constant velocity. Eliminating time gives y ∝ x² a parabola. This is the principle of cathode ray tube televisions.

What is specific charge of electron Class 12?

Specific charge = charge/mass = e/m = 1.76Γ—10ΒΉΒΉ C/kg for electrons. This was first determined by J.J. Thomson in 1897. It is much larger than specific charge of proton (9.58Γ—10⁷ C/kg) showing electrons are much lighter than protons. Specific charge determines how much a particle curves in electric and magnetic fields.

Which electron topics are most important for Exam?

Most important: Millikan oil drop experiment working and conclusion (4 marks), Thomson’s e/m determination (4 marks), motion of electron in magnetic field circular path with radius formula (2 marks), velocity selector concept (2 marks). Both Millikan and Thomson experiments appear regularly in NEB Class 12 board papers.

✍️ Written by chandnp349@gmail.com

Team Mero Enotes β€” Providing free handwritten notes and question papers for NEB Class 11, 12 and Pokhara University students in Nepal. πŸ‡³πŸ‡΅

Facebook Instagram

Leave a Comment