12th Physics One Mark Questions and Book Back Answers

About Physics

Medium ~20 h study

Most of Class 12 Physics is about electric charge: what it does when it sits still, what it does when it moves, and what happens when electric and magnetic fields change. Once you understand that, a lot of familiar technology makes sense. A power station generator, a mobile phone signal and an MRI scanner all work on ideas you meet in the first half of Volume 1.

The chapters follow a clear order. You start with Coulomb's law and electric potential, move to current and Kirchhoff's rules, then see how currents produce magnetic fields and how changing magnetic flux produces an emf. That leads into electromagnetic waves and light. Volume 2 then shows where classical physics runs out: the photoelectric effect, de Broglie waves, the Bohr model and the nucleus. The course ends with semiconductors, communication and a short look at current research.

In the board exam you will be asked to derive results, solve numericals and explain applications. Examiners look for clear steps and correct units. When you use a law such as Ohm's law, \(V = IR\), or Faraday's law of induction, say which law you are applying and why it holds in that situation. This habit also helps later in engineering, medical and science entrance exams.

What you'll learn

• Calculate electric and magnetic fields for standard charge and current arrangements.
• Derive expressions for electrostatic potential and induced emf.
• Analyse AC circuits using phasors and phase differences between voltage and current.
• Apply the laws of reflection and refraction, with correct sign conventions, to optics problems.
• Explain the photoelectric effect using Einstein's equation and describe matter waves using de Broglie's relation.
• Describe the characteristics and uses of semiconductor diodes and transistors.
• Build logic gate circuits that satisfy a given Boolean expression.
• Describe how robotics and nanoscience are used in industry, medicine and daily life.

Before you start

• Newton's laws of motion and the idea of gravitational force from Class 11.
• Confident algebra and basic trigonometry, plus simple differentiation and integration.
• Class 11 topics such as work, energy, oscillations and waves.

Physics — the syllabus in depth

What the Class 12 Physics Syllabus Covers

Electricity and Magnetism

Electrostatics begins with Coulomb's law, electric field and electric potential, and ends with capacitors and the energy stored in them. Current electricity deals with drift velocity, resistivity, Ohm's law and circuit networks solved using Kirchhoff's rules, including the Wheatstone bridge and potentiometer. The magnetism chapter connects the two: the Biot-Savart law and Ampere's circuital law give the field of a current, and the Lorentz force explains the moving coil galvanometer and the cyclotron. Practise the force and torque calculations here carefully, because they come back in later chapters.

Electromagnetic Induction and Waves

A changing magnetic flux induces an emf. Faraday's laws tell you how large it is and Lenz's law tells you its direction. From there you study self and mutual induction, AC generators, transformers and AC circuits containing resistors, inductors and capacitors. The electromagnetic waves chapter introduces Maxwell's displacement current, Hertz's experiment and the electromagnetic spectrum. All electromagnetic waves travel through vacuum at the speed of light, \(c = 3 \times 10^8\) m s^-1, which is why radio, microwaves, visible light and X-rays are treated as one family.

Ray Optics and Wave Optics

Ray optics covers reflection at spherical mirrors, refraction at plane and curved surfaces, total internal reflection, lenses, prisms and dispersion. Total internal reflection is the principle behind optical fibres and endoscopes. Wave optics uses Huygens' principle to explain interference (Young's double slit experiment), diffraction and polarisation. These effects cannot be explained by treating light as rays, and they set the limit on the resolving power of microscopes and telescopes.

Modern and Nuclear Physics

The photoelectric effect shows that light behaves like a stream of photons, and de Broglie's hypothesis shows that moving particles such as electrons have a wavelength. The atomic physics chapter covers J.J. Thomson's and Millikan's experiments, the Bohr model and the hydrogen spectral series. Nuclear physics deals with binding energy, radioactive decay, half-life, and the energy released in nuclear fission and fusion. Fusion is the energy source of the Sun and stars, and radioactive decay is the basis of carbon dating.

Electronics, Communication and Recent Developments

The semiconductor chapter explains p-n junction diodes, Zener diodes, LEDs, transistors and logic gates, the basic parts of every digital device. The same unit also introduces communication systems, covering modulation and the propagation of radio waves. The last unit gives a short introduction to nanoscience, robotics, physics in medicine, particle physics and cosmology, including the Higgs boson, black holes and gravitational waves. It is mostly descriptive and shows where the physics you have learnt is being used in research today.

How to study Physics

Start with Volume 1 and give electrostatics and magnetism the most time, since induction, AC and electromagnetic waves all build on them. Write out each derivation yourself at least once, then solve the numerical problems at the end of the chapter. If you understand how stationary charges lead to moving charges and then to induction, the whole volume holds together.

In Volume 2, work through optics and modern physics steadily. Optics needs careful diagrams and sign conventions, and modern physics needs you to remember a few key equations and experiments accurately. For each unit:

• Read the summary and concept map at the end of the unit to check what the main ideas are.
• Solve the worked examples in the text before trying the evaluation questions.
• Use the ICT Corner links and simulations for topics that are hard to picture, such as interference patterns.
• Go through the practical section so you know how each law is verified experimentally.

Keep revising earlier units each week while you move on to new ones, so formulas from the first chapters stay fresh.

Common mistakes to avoid

• Mixing up the hand rules. Use the right-hand thumb rule for the direction of the magnetic field around a current, and Fleming's left-hand rule for the force on a current-carrying conductor in a magnetic field.
• Getting AC phase relationships the wrong way round. In a pure inductor the voltage leads the current by 90°; in a pure capacitor the voltage lags the current by 90°.
• Ignoring sign conventions in optics. Measure all distances from the pole or optical centre, and take distances in the direction of the incident light as positive.
• Confusing mass number and atomic number in nuclear reactions. Check that both totals balance on each side of a decay equation.
• Using classical formulas for electrons and other tiny particles when the question needs the de Broglie wavelength or quantised energy levels.

Frequently asked questions

How do I find the direction of induced current?

Use Lenz's law: the induced current flows in a direction that opposes the change in magnetic flux that caused it. For a straight conductor moving through a magnetic field, Fleming's right-hand rule gives the direction of the induced current directly.

What is the difference between ray optics and wave optics?

Ray optics treats light as travelling in straight lines. That is enough to work out images formed by mirrors, lenses and prisms. Wave optics treats light as a wave, which you need to explain interference, diffraction and polarisation, such as the bright and dark fringes in Young's double slit experiment.

Why do we study semiconductors?

Diodes and transistors are made from semiconductors. A diode lets current flow mainly in one direction, and a transistor can amplify a signal or act as a switch. Every calculator, computer, mobile phone and solar panel depends on these devices.

What are logic gates used for?

Logic gates are the building blocks of digital circuits. The basic gates, AND, OR and NOT, act on signals that are either 0 or 1. NAND and NOR are called universal gates because any other gate can be built from them. Combinations of gates make up the processors in computers and programmable appliances.

How does a transformer help in power transmission?

A transformer works on mutual induction. A step-up transformer raises the voltage and lowers the current, so less energy is lost as heat in long transmission lines. A step-down transformer near homes brings the voltage back to a safe level. The frequency of the AC does not change.

What is meant by the dual nature of radiation and matter?

Light shows wave behaviour in interference and particle behaviour in the photoelectric effect. De Broglie proposed that material particles such as electrons also have a wave nature, with wavelength \(\lambda = h/p\). Electron diffraction and the electron microscope confirm this.

Why is Gauss law useful?

Gauss law says the total electric flux through a closed surface equals the net charge enclosed divided by \(\varepsilon_0\). For symmetric charge distributions, such as a long charged wire, an infinite charged plane or a charged spherical shell, it gives the electric field in a few lines, where Coulomb's law would need long integration.

Where is physics used in medical technology?

MRI works on nuclear magnetic resonance, and CT scans use X-rays. Endoscopes use optical fibres based on total internal reflection, and lasers are used in precise surgery. The recent developments unit also mentions nanorobots that could deliver drugs to specific cells.