IB HL Physics Laws

Newton’s first law of motion

A body that experiences a zero resultant force will remain at rest or continue to move at a constant velocity.

Newton’s second law of motion

The acceleration of a body depends on the mass of the body and the resultant force acting on the body.

Newton’s third law of motion

For every action (force) there is an equal and opposite reaction (force).

The Law of Conservation of Energy

Energy cannot be created or destroyed, only transferred.

The First Postulate of Special Relativity

The laws of physics are the same and can be stated in their simplest form in all inertial frames of reference.

The Second Postulate of Special Relativity

The speed of light in a vacuum (denoted as c) is constant for all observers in inertial frames of reference, regardless of the motion of the light source or the observer.

Stefan-Boltzmann Law

tTe total radiant heat energy emitted per unit surface area of a black body (a perfect absorber and emitter) is directly proportional to the fourth power of its absolute temperature.

Wien’s displacement law

The wavelength at which a black body emits its peak radiation is inversely proportional to its temperature.

First law of thermodynamics

The thermal energy entering a closed system is equal to the sum of the change in internal energy of the system and the work done by the system.

Second law of thermodynamics

Clausius statement: Thermal energy cannot spontaneously be transferred from a cold body to a hot body. Kelvin statement: In a cyclic process, it is impossible to completely convert heat into work.

Ohm’s law

The electric potential difference across a conductor is directly proportional to the current flowing through it, at constant temperature.

Snell’s law

An equation that describes the angle of incidence and angle of refraction as light (or other waves) is refracted through a boundary between two media that have different refractive index.

Kepler’s first law

Planets orbit in ellipses with the sun at one focus.

Kepler’s second law

Planets sweep out equal areas in equal time.

Kepler’s third law

The period for one orbit (T) squard divided by the length of the major axis (M) cubed is a constant for all planets. 

Newton’s universal law of gravitation

F=GMm/r²

Coulomb’s law

F=kQq/r²

Millikan’s experiment

How it Worked

1. Atomization:

   An atomizer produced a fine mist of oil droplets, which fell into a chamber containing two charged, parallel metal plates. 

2. Ionization:

   X-rays were used to ionize gas molecules, and some of these electrons attached to the oil droplets, giving them a negative charge. 

3. Electric Field:

   A voltage was applied across the plates, creating an electric field. The top plate was typically positive and the bottom negative, causing the negatively charged oil droplets to experience an upward electric force. 

4. Balancing Forces:

   The experimenters adjusted the voltage to balance the upward electric force with the downward gravitational force (plus a small buoyancy force from the air). When the forces were equal, the oil droplets would remain suspended. 

5. Determining Mass:

   To find the mass of a droplet, the electric field was turned off. The droplet then fell at its terminal velocity, limited by air resistance. This terminal velocity allowed Millikan to calculate the droplet's radius and, using the known density of the oil, its mass. 

6. Calculating Charge:

   With the mass known, the charge on the oil droplet could be calculated by equating the electric force (F = EQ, where E is the electric field strength and Q is the charge) and the gravitational force (F = mg). 

Key Findings

• Quantization of Charge:

  The most significant finding was that the calculated charges on the oil droplets were always integer multiples of a specific smallest unit of charge. 

• Electron Charge:

  This fundamental unit of charge was identified as the charge of a single electron, with a magnitude of approximately 1.6 × 10⁻¹⁹ Coulombs. 

• Fundamental Nature of Matter:

  The experiment provided strong evidence for the existence of electrons and demonstrated that electric charge is not continuous but comes in discrete packets. 

Faraday’s law of induction

A change in the magnetic flux through a circuit induces an electromotive force (EMF), or voltage.

Geiger-Marsden-Rutherford experiment

Experiment Setup

• Alpha Particle Source: A radioactive source emitting alpha particles (helium nuclei). 

• Thin Gold Foil: A thin sheet of gold, approximately 100 atoms thick. 

• Fluorescent Screen: A screen coated with a material like zinc sulfide that would produce a flash of light (scintillation) when struck by an alpha particle. 

• Microscope: Used by Geiger and Marsden to observe the scintillations on the screen and measure the angles of deflection. 

Key Observations

• Most particles passed through undeflected:

  This indicated that atoms are mostly empty space, contradicting the plum pudding model. 

• Some particles deflected at large angles:

  The large-angle deflections were unexpected and could only be explained by a strong repulsive force. 

• A few particles bounced directly back:

  The most surprising finding was that some alpha particles were reflected almost 180 degrees. 

Einstein’s explanation of the photoelectric effect

Light consists of discrete packets of energy called photons, where each photon's energy is proportional to the light's frequency (E=hf). The effect occurs when a photon collides with an electron, transferring its energy to overcome the electron's binding energy (work function, 𝜙) to escape the metal. The remaining photon energy becomes the electron's kinetic energy (E=ℎ𝑓−𝜙).
 

The de Broglie wavelength

The corresponding wavelength of a particle in which the wavelength is inversely proportional to its momentum.

Compton Scattering

The observation that the wavelength of a high energy photon increases when it collides with a charged particle.