For each of the basic principles of physics, there are several important laws or theorems that we should all become familiar with. Here’s a breakdown of the key laws and theorems associated with each principle:
1. Motion and Forces
- Newton’s Laws of Motion:
- First Law (Law of Inertia): Explains that an object will maintain its state of motion unless acted upon by an external force.
- Second Law: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (F = ma).
- Third Law: For every action, there is an equal and opposite reaction.
- Law of Universal Gravitation (Newton’s Law of Gravity): Every mass attracts every other mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers (F = G (m1 × m2) / r²).
2. Energy
- Conservation of Energy (First Law of Thermodynamics): Energy cannot be created or destroyed; it can only change forms (e.g., potential energy to kinetic energy).
- Work-Energy Theorem: The work done on an object is equal to the change in its kinetic energy (W = ΔKE).
- Law of Conservation of Mechanical Energy: In a closed system without non-conservative forces (like friction), the total mechanical energy (sum of kinetic and potential energy) remains constant.
3. Work and Power
- Hooke’s Law (for elastic forces): The force needed to extend or compress a spring by a distance x is proportional to that distance (F = -kx).
- Law of Machines: Mechanical advantage (MA) is the ratio of output force to input force, used to calculate efficiency and work in machines.
4. Heat and Temperature
- Second Law of Thermodynamics: Heat naturally flows from hotter objects to colder ones, and processes that convert heat energy to work have limitations (entropy always increases in an isolated system).
- Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
- Third Law of Thermodynamics: As the temperature of a system approaches absolute zero, the entropy of the system approaches a constant minimum.
- Boyle’s Law (Gas Laws): At constant temperature, the pressure of a gas is inversely proportional to its volume (P1V1 = P2V2).
- Charles’s Law (Gas Laws): At constant pressure, the volume of a gas is directly proportional to its temperature (V1/T1 = V2/T2).
5. Waves
- Law of Reflection: The angle of incidence of a wave equals the angle of reflection.
- Snell’s Law (Law of Refraction): Describes how light bends when passing from one medium to another with a different refractive index (n1 sin(θ1) = n2 sin(θ2)).
- Principle of Superposition: When two or more waves overlap, the resulting wave displacement is the sum of the individual displacements.
6. Electricity and Magnetism
- Ohm’s Law: The current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance (V = IR).
- Coulomb’s Law: The electric force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them (F = k(q1q2) / r²).
- Faraday’s Law of Electromagnetic Induction: A changing magnetic field within a closed loop of wire induces an electromotive force (EMF) in the wire.
- Ampère’s Law: The magnetic field in space around an electric current is proportional to the current that flows through a wire.
- Gauss’s Law: The electric flux through any closed surface is proportional to the total charge enclosed within the surface.
7. Light and Optics
- Huygens’ Principle: Every point on a wavefront acts as a source of wavelets that spread out in the forward direction at the same speed as the wave itself.
- Snell’s Law (also mentioned under Waves): Governs the refraction of light when passing between media.
- Law of Reflection: The angle of reflection of light is equal to the angle of incidence.
- Lensmaker’s Equation: Describes the focal length of a lens in terms of its curvature and refractive index.
- Planck’s Law (related to light as electromagnetic radiation): Describes the energy of photons emitted by objects at a given temperature.
8. Sound
- Doppler Effect: The observed frequency of a wave depends on the relative speed of the source and observer (explains pitch changes in moving sources).
- Hooke’s Law (for oscillations): Describes the force in springs and can be applied to simple harmonic motion.
9. Matter and Its Properties
- Archimedes’ Principle: A body submerged in a fluid experiences a buoyant force equal to the weight of the fluid displaced by the body.
- Pascal’s Law: A change in pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid.
- Bernoulli’s Principle: For a fluid in motion, an increase in speed occurs simultaneously with a decrease in pressure or potential energy.
- Boyle’s Law and Charles’s Law (mentioned under Heat and Temperature) also apply here to the behavior of gases.
10. Scientific Method
- Occam’s Razor: When presented with competing hypotheses, the one with the fewest assumptions should be selected.
- Falsifiability Principle (Karl Popper): A scientific hypothesis must be falsifiable, meaning it can be tested and potentially proven wrong.