Key Principles of Physics: A Concise Reference

The Doppler Effect

The Doppler Effect describes an increase or decrease in the frequency of sound, light, or other waves as the source and observer move toward or away from each other.

• When the sound source is approaching you, the frequency increases.
• When the sound source is moving away, the frequency decreases.

Application: Catching Speeding Motorists

The relative velocity between a police vehicle and a speeding car causes their distance to decrease. This effectively lowers the wavelength of the radar waves, resulting in a higher frequency, which is used to calculate the vehicle's speed.

Conductors and Superconductors

Conductors

A conductor is an object or material that allows the flow of an electrical current in one or more directions.

• Temperature and Conductivity: As the temperature of a conductor increases, its ability to conduct electricity decreases.
• Temperature and Resistance: As the temperature of a conductor increases, its electrical resistance increases.

Superconductors

A superconductor is a substance capable of becoming superconducting at sufficiently low temperatures, exhibiting zero electrical resistance.

Uses of Superconductors

• Magnetic Resonance Imaging (MRI)
• Nuclear Magnetic Resonance (NMR)

Density and the Speed of Sound

The density of a medium is a crucial factor affecting the speed of sound. Generally, the greater the density of the medium, the faster the sound travels through it.

Light Speed and Reflection

The substance through which light travels significantly affects its speed relative to its speed in a vacuum. Light traveling through anything other than a perfect vacuum will scatter off particles present in the medium, causing its speed to be less than the speed of light in a vacuum.

In a vacuum, light travels at a constant, exact value, regardless of who measures it.

Reflection of Light

Reflection occurs when light changes direction upon striking a surface. This can also lead to light being separated into different colors, as seen in phenomena like rainbows or prisms.

Isotopes and Elements

Isotopes

Two different isotopes of the same element differ in their mass. This difference arises from a varying number of neutrons in their atomic nuclei.

Elements

A change in the number of protons in an atom's nucleus will result in the formation of a different element.

Protons, Neutrons, and Electrons

These are the fundamental subatomic particles that constitute an atom:

• Protons: Positively charged particles with a mass approximately 2,000 times larger than an electron.
• Neutrons: Electrically neutral particles with a mass approximately 2,000 times larger than an electron.
• Electrons: Negatively charged particles with a much smaller mass compared to protons and neutrons. The magnitude of their electrical charge is the same as that of protons.

The Four Fundamental Forces

The universe is governed by four fundamental forces, each characterized by its mediating particles, relative strength, and range:

Comparing Blue and Red Light

Blue light carries a higher energy signature than red light. This is because blue light has a shorter wavelength in the visible spectrum compared to red light, and energy is inversely proportional to wavelength.

The Photoelectric Effect

The Photoelectric Effect is the emission, or ejection, of electrons from the surface of a material (typically a metal) in response to incident light. The energy contained within the incident light is absorbed by electrons within the metal, providing them with sufficient energy to be 'knocked' out of, or emitted from, the surface.

According to the classical Maxwell wave theory of light, a more intense incident light should result in electrons being ejected with greater energy. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light.

The Uncertainty Principle

The Uncertainty Principle asserts a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables (such as position x and momentum p), can be simultaneously known.