University Physics: Energy, Waves, Optics, and Electromagnetism

ENERGY, WORK AND POWER

• Energy Definition: The ability to do work, measured in Joules ($J$).

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed.

• Forms of Energy:

  • Kinetic Energy (KE): Energy due to motion. Formula: $KE = \frac{1}{2}mv^2$.

  • Gravitational Potential Energy (GPE): Stored energy based on height ($h$) and mass ($m$). Formula: $GPE = m \times g \times h$.

  • Other Forms: Chemical, Nuclear.

• Constants: Acceleration due to gravity ($g$) is $9.8\,m/s^2$ (often rounded to $10\,m/s^2$).

• Energy Interchanges: As an object falls, GPE converts to KE. At maximum height, $KE = 0\,J$; just before hitting the ground, $GPE = 0\,J$.

• Work: Done when force ($F$) moves an object over a distance ($d$). Formula: $W = F \times d$. Unit: Joule ($J$).

• Power: The rate of doing work. Units: Watt ($W$) or Joule/second ($J/s$).

  • Formula 1: $Power = \frac{Work}{Time}$.

  • Formula 2: $Power = Force \times velocity$.

WAVES

• Types of Waves:

  • Longitudinal: Particles move in the same direction as the wave (e.g., sound waves, which consist of compressions and rarefactions).

  • Transverse: Particles move perpendicular to wave motion.

• Wave Descriptors:

  • Wavelength (\lambda): Distance between successive identical points (crests, troughs).

  • Amplitude (A): Maximum displacement from rest position.

  • Frequency (f): Number of waves per second, measured in hertz ($Hz$). Shorter wavelength implies higher frequency.

  • Speed (v): Measured in $m/s$.

• The Wave Equation: $v = f \times \lambda$.

• Wave Graphs:

  • Displacement-position: Used to determine amplitude and wavelength.

  • Displacement-time: Used to determine amplitude and period ($T$).

REFRACTION AND LENSES

• Refraction: The bending of light due to changes in speed when entering media of different densities.

  • Entering a denser medium: Light bends toward the normal.

  • Entering a less dense medium: Light bends away from the normal.

• Laws of Refraction:

  • The incident ray, refracted ray, and normal are in the same plane.

  • Snell's Constant: $\frac{\sin(i)}{\sin(r)} = \text{constant}$.

• Refractive Index (n): $n = \frac{\text{speed of light in vacuum}}{\text{speed of light in medium}}$. Speed of light in vacuum is $3 \times 10^8\,m/s$.

• Real and Apparent Depth: Refraction makes objects in water appear shallower than they are.

• Total Internal Reflection: Occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the Critical Angle ($c$). Used in Optical Fibres.

• Mirage: An illusion of water caused by light refraction through air layers of different temperatures near hot ground.

• Lenses:

  • Concave (Diverging): Thin in middle; bends rays outwards.

  • Convex (Converging): Thick in middle; bends rays inwards. Acting as a magnifying glass, it creates a virtual, upright, and magnified image if the object is closer than the focal point.

MAGNETISM

• Magnetic Materials: magnetic materials include Iron, Nickel, Cobalt, and alloys like Steel.

• Properties:

  • Pins/Nails: Like poles repel, unlike poles attract.

  • Soft Magnetic Materials (Iron): Easy to magnetize but lose it quickly (temporary); used in electromagnets.

  • Hard Magnetic Materials (Steel): Difficult to magnetize but retain it (permanent).

• Methods of Induction: Stroking or using an electrical solenoid.

• Demagnetization: Caused by heating, hammering, or using alternating current ($AC$) in a solenoid.

• Magnetic Field: Space around a magnet where force is produced. Lines point from North ($N$) to South ($S$).

STATIC ELECTRICITY

• Electric Charges: Positive (protons) and negative (electrons).

  • Like charges repel; unlike charges attract.

  • Charging occurs via electron transfer during friction (e.g., polythene acquires electrons from wool to become negative).

• Electric Field: Region where a charge experiences force. Field lines start at positive and end at negative charges.

• Gold-Leaf Electroscope: Instrument used to detect charge via leaf repulsion.

• Materials:

  • Conductors: Allow electron flow (e.g., metals, water).

  • Insulators: Resist electron flow (e.g., plastic, rubber, air).

• Electrostatic Induction: Process where a conductor becomes charged when a charged body is brought near it without direct contact.