JEE Physics Physical World Notes Set 01

Science and the Scientific Method

1. Science is the organised study of natural phenomena, carried out so that they can be explained, predicted and, where possible, controlled. Its word root is the Latin scientia, meaning "to know".
2. The scientific method combines careful observation, controlled experiments, qualitative and quantitative reasoning, mathematical modelling, and the testing of predictions (a theory that fails a test must be modified or dropped).
3. Science has no final theory. Better instruments and measurements keep refining it: Kepler used Brahe's planetary data to improve the Copernican picture, and Rutherford's nuclear atom led to Bohr's quantum model of the atom.
4. Natural science has three main branches: physics, chemistry and biology.

What Physics Studies

1. Physics studies the basic laws of nature, matter, energy, force, and motion through space and time. The word comes from the Greek physis, meaning nature.
2. Two broad ways of thinking in physics are unification (explaining many phenomena with a few universal laws, for example gravity governs both a falling apple and the planets) and reduction (explaining the properties of a complex system from its parts, for example temperature as the average kinetic energy of molecules).
3. Physics is useful because simple theories explain a huge range of phenomena, experiments suggest new theories, and its laws are the basis of devices and technology.

Scope and Excitement of Physics

1. Lengths studied range from about 10⁻¹⁴ m (nuclear size) or smaller up to about 10²⁶ m (the observable universe) and beyond; times from about 10⁻²² s to about 10¹⁸ s; masses from about 10⁻³⁰ kg (electron) to about 10⁵⁵ kg (the universe).
2. Classical physics deals with macroscopic phenomena (laboratory, terrestrial and astronomical scales). Modern physics (quantum theory) deals with microscopic phenomena (atoms, molecules, nuclei and elementary particles).

Why Physics Has Progressed

1. Using numbers (quantitative analysis) along with descriptions.
2. Applying the same universal laws in different situations.
3. Breaking a complex problem into simple basic laws (approximation).
4. Concentrating on the essential features of a phenomenon.

Hypothesis, Axiom and Model

Physics and Technology

1. The steam engine drove the industrial revolution of the eighteenth century.
2. Laws of electricity and magnetism made wireless communication possible.
3. Neutron-induced fission of uranium (Hahn and Meitner, 1938) led to nuclear reactors and nuclear weapons.
4. Solar, wind and geothermal energy are converted to electricity using physical principles.

Fundamental Forces of Nature

Everyday forces such as muscular force, friction, spring force, fluid pressure and intermolecular forces are derived forces. They originate from a small number of fundamental forces.
1. Gravitational force: mutual attraction between any two objects because of their masses; universal and of infinite range.
2. Electromagnetic force: acts between charged particles. Static charges give electric attraction and repulsion; moving charges also give magnetic force.
3. Strong nuclear force: holds protons and neutrons together in a nucleus; independent of charge (same between p-p, n-n and p-n); built from the interaction of quarks.
4. Weak nuclear force: appears in certain nuclear processes such as beta decay, in which a nucleus emits an electron and a neutrino (predicted by Pauli in 1931).

Unification of Forces

Conserved Quantities and Conservation Laws

1. A quantity that stays constant in time is a conserved quantity. For a body moving under a conservative force, kinetic and potential energy change but their sum, the total mechanical energy, stays constant.
2. Conserved quantities may be scalars (energy) or vectors (total linear momentum, total angular momentum).
3. A conservation law is a statement drawn from observation and experiment. It cannot be proved mathematically, but it can be tested again and again.
4. Conservation of energy: energy changes from one form to another but the total is constant; this is believed true for the whole universe. It is linked to the fact that the laws of nature give the same results at different times.
5. Conservation of mass: in a chemical reaction atoms are only rearranged, so total mass of reactants equals total mass of products. If the reactants are bound less tightly than the products, the difference is released as heat (exothermic); the reverse gives an endothermic reaction. Einstein's relation \( E=mc^2 \) connects mass and energy.
6. Conservation of linear momentum: follows from the symmetry of the laws of nature under shifts in space. For example, the law of gravitation is the same on the Earth and the Moon even though g on the Moon is one-sixth of g on the Earth.
7. Conservation of angular momentum: follows from the isotropy of space, that is, no direction in space is special.