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Welcome to Class 12 Physics

Class 11 taught you how forces move things. Class 12 asks where electric and magnetic forces come from, how they make light, and what light and matter are made of.

1. Where Class 11 left you

Class 11 Physics was mostly about mechanics: describe motion with vectors, explain it with Newton's laws, and account for it with energy and momentum. You met one force law in detail — Newton's law of gravitation, $F=Gm_1m_2/r^2$ — and you learned how oscillations and waves carry energy from place to place.

Class 12 Physics follows the other great force of nature, the electromagnetic force. It holds atoms together, makes every electric circuit work, turns motors and generators, and — surprisingly — is light: a travelling ripple of electric and magnetic fields. The year ends with the physics of the twentieth century: light arriving in packets (photons), electrons behaving as waves, the structure of atoms and nuclei, and the semiconductors inside every phone. Every one of these topics is examined in the board exam, in JEE Main and Advanced, and in NEET.

📦 Class 11 ended with…

$$F=G\frac{m_1m_2}{r^2},\qquad y=a\sin(kx-\omega t)$$

🚀 Class 12 begins with…

$$F=\frac{1}{4\pi\varepsilon_0}\frac{|q_1q_2|}{r^2},\qquad c=\frac{1}{\sqrt{\mu_0\varepsilon_0}}$$

2. A story that needs all of Class 12

Look at the phone in your hand. Its touchscreen senses your finger because your body changes the charge stored between transparent electrodes — electrostatics and capacitance (Chapters 1–2). Its battery drives currents through circuits that obey Ohm's and Kirchhoff's rules (Chapter 3). Its speaker is a coil in a magnetic field (Chapters 4–5), and if it charges wirelessly, a changing magnetic field in the pad induces an emf in a coil inside the phone (Chapters 6–7).

The phone talks to the tower with electromagnetic waves (Chapter 8). Its camera focuses light with tiny lenses (Chapter 9); the coatings that stop reflections on the screen work by interference (Chapter 10). The camera sensor turns each photon of light into electrons — the photoelectric idea of Chapter 11 — and the light from the screen comes from LEDs, semiconductor devices whose colour is set by the energy gap of the material (Chapters 12 and 14). The lab below lets you move along the whole electromagnetic spectrum, from gamma rays to radio waves, and see which chapter each part belongs to.

Move along the electromagnetic spectrum (the bar uses a logarithmic scale: each step is ten times the last). The readouts give the frequency $\nu=c/\lambda$ and the energy of one photon $E=h\nu=hc/\lambda$, computed with $hc=1240$ eV nm. Press Play to sweep across the whole spectrum; the wave above is a cartoon — its drawn wavelength shrinks as $\lambda$ does.

wavelength:500 nm

wavelength $\lambda$—

frequency $\nu$ (Hz)—

photon energy (eV)—

region—

✏️ Example 1 — One photon of green light

Green light has wavelength $500$ nm. Find its frequency and the energy of one photon, in joules and in electronvolts. Use $c=3.0\times10^8$ m/s, $h=6.63\times10^{-34}$ J s and $1\ \text{eV}=1.6\times10^{-19}$ J.

💡 Show solution▶

Frequency $\nu=\dfrac c\lambda=\dfrac{3.0\times10^8\ \text{m/s}}{500\times10^{-9}\ \text{m}}=6.0\times10^{14}$ Hz.

Energy $E=h\nu=(6.63\times10^{-34})(6.0\times10^{14})\approx4.0\times10^{-19}$ J $=\dfrac{4.0\times10^{-19}}{1.6\times10^{-19}}\approx2.5$ eV.

Shortcut With $hc=1240$ eV nm: $E=\dfrac{1240\ \text{eV nm}}{500\ \text{nm}}=2.48$ eV. ✓ This shortcut appears again and again in Chapters 11–14.

Meaning A few electronvolts is the scale of energies of electrons in atoms and semiconductors — which is why visible light can free electrons from some metals (Chapter 11) and why LEDs emit visible light when electrons cross a gap of about $2$–$3$ eV (Chapter 14).

3. The fourteen chapters at a glance

The course follows the chapter order of the current rationalised NCERT Class 12 Physics textbook (Part I: Chapters 1–8, Part II: Chapters 9–14).

1–2 · Electrostatics→3 · Current→4–5 · Magnetism→6–7 · Induction & AC→8 · EM waves→9–10 · Optics→11–13 · Modern physics→14 · Electronics

Chapter 1

Electric Charges and Fields

Charge, Coulomb's law, superposition, the electric field, dipoles, flux and Gauss's law.

$\oint\mathbf E\cdot d\mathbf A=q_\text{enc}/\varepsilon_0$

Open chapter → Chapter 2

Electrostatic Potential and Capacitance

Potential and potential energy, equipotentials, conductors, dielectrics and capacitors.

$C=\varepsilon_0A/d$

Open chapter → Chapter 3

Current Electricity

Drift velocity, Ohm's law, resistivity, cells and internal resistance, Kirchhoff's rules.

$I=neAv_d$

Open chapter → Chapter 4

Moving Charges and Magnetism

Magnetic force, circular motion of charges, Biot–Savart and Ampère's laws, galvanometers.

$\mathbf F=q\mathbf v\times\mathbf B$

Open chapter → Chapter 5

Magnetism and Matter

Bar magnets as dipoles, magnetisation, and dia-, para- and ferromagnetic materials.

$\mathbf B=\mu_0(\mathbf H+\mathbf M)$

Open chapter → Chapter 6

Electromagnetic Induction

Faraday's and Lenz's laws, motional emf, inductance and the AC generator.

$\varepsilon=-d\Phi_B/dt$

Open chapter → Chapter 7

Alternating Current

Phasors, reactance, the series LCR circuit, resonance, AC power and transformers.

$\omega_0=1/\sqrt{LC}$

Open chapter → Chapter 8

Electromagnetic Waves

Displacement current, Maxwell's equations, the nature of EM waves and the spectrum.

$E_0=cB_0$

Open chapter → Chapter 9

Ray Optics and Optical Instruments

Mirrors, refraction, total internal reflection, lenses, prisms, microscopes and telescopes.

$\dfrac1v-\dfrac1u=\dfrac1f$

Open chapter → Chapter 10

Wave Optics

Huygens' principle, interference and Young's experiment, single-slit diffraction.

$\beta=\lambda D/d$

Open chapter → Chapter 11

Dual Nature of Radiation and Matter

The photoelectric effect, photons and the de Broglie wavelength of matter.

$h\nu=\phi_0+K_\text{max}$

Open chapter → Chapter 12

Atoms

Rutherford's nuclear atom, line spectra and Bohr's model of hydrogen.

$E_n=-13.6\ \text{eV}/n^2$

Open chapter → Chapter 13

Nuclei

Nuclear size and composition, mass defect, binding energy, fission and fusion.

$E=\Delta m\,c^2$

Open chapter → Chapter 14

Semiconductor Electronics

Energy bands, intrinsic and doped semiconductors, the p–n junction and rectifiers.

$n_en_h=n_i^2$

Open chapter →

🗺️ How the chapters connect

Charges at rest (Ch. 1–2) produce electric fields; the field and the potential are the two languages for describing them, and capacitors store their energy.
Charges in steady motion (Ch. 3–5) are currents. Currents are driven by potential differences and they produce magnetic fields; magnetic fields push on moving charges.
Changing fields (Ch. 6–8): a changing magnetic field makes an electric field (induction, generators, transformers), and a changing electric field makes a magnetic field. Together they travel as electromagnetic waves at the speed of light.
Light (Ch. 9–10) is one of those waves. Ray optics is the short-wavelength approximation; wave optics explains interference and diffraction.
Quanta (Ch. 11–13): light also comes in photons, electrons also behave as waves, atoms have quantised energy levels, and nuclei store energy $E=\Delta m\,c^2$.
Electronics (Ch. 14) puts the band picture of solids to work in diodes and rectifiers.

Threads to watch for: inverse-square laws (Coulomb, Biot–Savart), superposition (of forces, fields, potentials and waves), conservation of charge and energy in every circuit, and the constants $\varepsilon_0$, $\mu_0$, $c$, $h$ and $e$ that tie the whole subject together.

✏️ Example 2 — How many electrons charge a phone?

A phone charger supplies $2.0$ A at $5.0$ V for one hour. How much charge flows, how many electrons is that, and how much energy is delivered?

💡 Show solution▶

Charge $Q=It=(2.0\ \text{A})(3600\ \text{s})=7200$ C (Chapter 3).

Electrons $n=\dfrac{Q}{e}=\dfrac{7200}{1.6\times10^{-19}}=4.5\times10^{22}$ electrons — charge is quantised (Section 1.1), but the number is so large that current seems continuous.

Energy $P=VI=10$ W, so in one hour $W=Pt=(10\ \text{W})(3600\ \text{s})=3.6\times10^4$ J $=10$ W h.

🧭 How to use this course

Read with a pen. Try every worked example before opening its solution, then compare step by step. Always carry units and check them at the end.
Play with the labs. Each section has two interactives that compute what they draw — field lines from Coulomb's law, rays from the lens formula. Change the settings until you can predict what will happen before you press ▶ Play.
Use the 🎯 Exam corner. Every section has original problems tagged Board JEE Main JEE Advanced NEET, written in the style of those papers, with full solutions and an exam tip.
Beyond-the-syllabus boxes are marked. A few topics have been removed from the board syllabus but are still asked in JEE or NEET. They appear only in boxes titled “🚀 Beyond the board syllabus”, whose header names the exams (JEE Main, NEET, JEE Advanced) that list the topic; board students may skip them.
Do the exercises in order — they run from routine to challenging, and every part has a full solution. Then take the five-question quiz: if you score below 4, reread before moving on.
Key terms. Dotted words show their meaning when you hover or tap; the Glossary and Formula Sheet collect everything for revision.

4. Prerequisite diagnostic — are you ready?

Eight quick questions on the Class 11 skills this course uses most. Answer all of them; for each one you miss you will get a link to the Class 11 Physics section to review.

Q1. Two forces of $3$ N and $4$ N act on a body at right angles. The resultant has magnitude

Q2. A $10$ N force makes $30^\circ$ with the $x$-axis. Its $x$-component is

Q3. The Earth pulls a $1$ kg book with a force of $9.8$ N. The force the book exerts on the Earth is

Q4. If the distance between two masses is doubled, the gravitational force between them becomes

Q5. A constant $5$ N force moves a body $2$ m along a line at $60^\circ$ to the force. The work done is

Q6. A particle moves so that $x=3t^2$ (metres, seconds). Its velocity at $t=2$ s is

Q7. A body moves in a circle of radius $0.50$ m at a constant speed of $2.0$ m/s. Its acceleration is

Q8. Which quantity has the dimensional formula $[\text{ML}^2\text{T}^{-2}]$?

When you are ready, start with Chapter 1 — Electric Charges and Fields.

← Class 12 Physics — Contents Chapter 1 — Electric Charges and Fields →