Physics definitions

• Density is mass per unit volume.
• Scalar quantities have size only.
• Vector quantities have size and direction.
• Mass refers to the amount of matter in an object.
• Weight is the measurement of the pull of gravity on an object.
• Hooke's Law: Force is directly proportional to extension, provided the elastic limit is not exceeded.
• Elastic limit is the point beyond which a spring will not return to its original length after unloading.
• Moment is the turning effect of a force.
• The principle of moments: At equilibrium the total clockwise moment is equal to the total anti-clockwise moment.
• At equilibrium, the total downward forces are equal to the total upward forces.
• Center of gravity is the point where the entire weight of an object seems to act and where it balances.
• Pressure is the force acting per unit area.
• Pascal's principle: Pressure applied to any part of a liquid is transmitted equally to all parts of the liquid.
• Work is done when a force acts on an object and the object moves in the direction of the force.
• The principle of conservation of energy: Energy changes form, but cannot be created or destroyed.

Types of Energy

• Non-renewable energies cannot be replenished quickly, e.g., Coal, Oil, Natural Gas, and Nuclear Energy.
• Renewable energies are replenished quickly, e.g., Solar, Wind, Geothermal, Hydroelectric, Tidal and Wave energy.
• Efficiency is the ratio of useful energy delivered by a dynamic system to the energy supplied to it.
• Power measures how much work is done in a certain time.
• Kinetic energy is the energy of motion.
• Potential energy is the energy of height.
• Heat is the flow of energy caused by a change in temperature.
• Expansion occurs when a substance's temperature increases, causing molecules to move faster and force each other apart.
• Evaporation requires particles near a liquid's surface to travel fast enough to overcome attractive forces.
• Specific heat capacity is the energy (in Joules) needed to raise the temperature of 1kg of a substance by 1°C.
• Internal energy is the energy of the particles that make up matter and relates to particle movement.
• Conduction is heat flow through matter from particle to particle, from higher to lower temperature areas.
• Conductors allow heat flow due to free-moving electrons (e.g., copper, aluminum, brass).
• Insulators impede heat flow due to bound electrons (e.g., glass, stone, air).
• Convection currents are fluid flows (liquid or gas) driven by temperature changes, carrying heat energy.
• Radiation involves infra-red rays able to pass through a vacuum.

Celestial Bodies

• A planet is a spherical rock/gas ball orbiting the Sun in an elliptical path and is considered a celestial body -- a natural mass visible in space.
• Celestial bodies include stars, planets, moons, and satellites.
• A planet orbits the Sun, has a nearly round shape and has "cleared the neighborhood" around its orbit.

Stars vs. Planets

• Stars are huge and distant, while planets are smaller and nearer.
• Stars emit their own light (luminous), while planets reflect sunlight (non-luminous).
• Stars do not move; planets orbit stars.

Galaxies and Light Years

• A galaxy is a system of stars, dust, and gas held together by gravity.
• A light year is the distance light travels in one year.

Linear Motion

• Acceleration is the rate of speed change.
• Distance traveled can be found by calculating the area under a velocity-time graph.
• Acceleration is the gradient of a velocity-time graph.
• Speed is the gradient of a distance-time graph.
• Variables: S (distance in meters), U (initial velocity in m/s), V (final velocity in m/s), a (acceleration in m/s²), t (time in seconds).
• Thinking distance is the distance a car travels from when the driver sees an obstacle until braking starts.
• Braking distance is the distance the car travels from braking until it stops.
• Reaction time is the time from when the driver sees an obstacle until braking begins.
• Terminal Velocity: The maximum constant speed when resultant force is zero.
• Momentum is the product of mass and velocity.
• The principle of conservation of momentum: In a collision, total momentum before equals total momentum after, without external forces.

Laws of Motion

• Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with constant velocity unless acted upon by a resultant force.
• Inertia is an object's resistance to changing its state of motion.
• Newton's Second Law: If an unbalanced force acts on a body, it accelerates; acceleration is directly proportional to the unbalanced force and inversely proportional to the body's mass.
• Newton's Third Law: For every action, there is an equal and opposite reaction.

Light Waves

• Reflection is the bouncing of light; the angle of incidence equals the angle of reflection. Incident ray, reflected ray, and normal all lie in the same plane.
• Refraction occurs when light passes from one medium to another transparent medium, changing direction and speed/wavelength. Incident ray, refracted ray, and normal all lie in the same plane.
• Refractive Index: The amount light bends entering a denser medium, slowing down and has no units.
• Total Internal Reflection: Occurs when light hits a medium's surface and reflects back inside, when light travels from a denser to a less dense medium and the incidence angle exceeds the critical angle.
• Dispersion is white light's separation into the spectrum's seven colors.
• Lenses - Magnification is how much an image is enlarged or diminished.
• Focal length is the distance between the principal focus and the optical center of the lens.
• A convex lens converges rays of light to a point (focus).
• A concave lens spreads out rays of light.

Waves

• Transverse Wave: Particle displacement is perpendicular to the direction of travel (e.g., light, water, X-rays, gamma rays).
• Longitudinal Wave: Particle displacement is parallel to the direction of travel (e.g., sound and ultrasound waves).
• Periodic Time: Time for one complete wave.
• Wavelength: Distance between two successive crests or troughs.
• Frequency: Number of waves in one second.
• Velocity: Distance moved per second.
• Crest: The upper part of a wave.
• Trough: The lower part of a wave.
• Amplitude: Height of a crest or depth of a trough above rest position.
• Displacement: Overall change in object's position and direction.
• Diffraction: A wave changes shape only when passing through a narrow or wide gap.

Electromagnetic Waves

• Common properties: all transverse waves with no charge, travel through a vacuum at 3 x 10^8 m/s, and can be reflected, refracted, diffracted, and carry energy.

Electricity

• Polythene strips become negatively charged when electrons move from the duster to the strip.
• Perspex/acetate strips become positively charged as electrons move from the strip to the duster.
• Like charges repel, unlike charges attract.
• Current is the rate of charge flow.
• Charge is the amount of electrons flowing.
• Voltage is the work per unit charge.
• Resistance is the opposition to current flow.
• Power is the energy supplied per unit time.
• Ohm's Law: Current is directly proportional to potential difference if temperature remains constant.
• Conductors easily allow current flow with free-moving electrons.
• Insulators block current flow with bound electrons.
• Ohmic conductors obey Ohm's Law, showing a linear V-I graph through the origin.
• Non-Ohmic conductors do not obey Ohm's Law and have a curved V-I graph.
• Semi-conductors conduct electricity under specific conditions.
• LEDs allow current to pass in one direction only.
• LDR resistance decreases with increased light.
• Thermistor resistance decreases with increased temperature.

Direct vs Alternating Current

• Direct current (DC) flows in one direction only.
• Alternating current (AC) changes direction every half cycle.
• Rectification is changing AC to DC.
• A fuse is a wire that melts with high current, breaking the circuit to protect appliances.
• An earth wire prevents electric shock by connecting to the metal case of an appliance.
• A live wire carries electricity from the power grid.
• A neutral wire returns electricity to the power grid.

Magnets

• Properties: Attracts magnetic materials, has poles, and when suspended, its north pole points to the Earth's magnetic North.
• Magnetic field: The area around a magnet where magnetic force is felt; field lines point out of the North and into the South.
• Properties of Iron: Easily magnetized and demagnetized, used where temporary magnetism is needed.
• Properties of Steel: Hard to magnetize and demagnetize, used where permanent magnetism is needed

Magnetizing/Demagnetizing Methods

• Magnetizing: align a bar with Earth's magnetic field (N-S direction) while hammering, heating and cooling, or winding a wire around it and connecting to a DC current.
• Demagnetizing: hammering in a West-East direction, heating and allowing it to cool in a West-East direction, or winding a wire around the bar and connecting it to an AC current.
• Electromagnetism - An electromagnet is a temporary magnet (iron) as long as current flows through its coil.
• A relay is a mechanical switch used for safety.
• The right-hand grip rule helps find the north pole of a magnet or electromagnet.
• Fleming's left-hand rule helps find the direction of force on a current-carrying conductor in a magnetic field.
• Faraday's Law: Induced current is directly proportional to the rate at which magnetic field lines are cut.
• Lenz's Law: Induced current flows in a direction that opposes the change that produced it.
• A transformer increases or decreases alternating voltage.
• An ideal transformer has no energy losses.
• Energy losses in a transformer include coil resistance, eddy currents, and field line leakage.
• Transformers work with alternating current because it produces a continuously changing magnetic field, inducing an emf in the secondary coil.
• Emf - Electromotive force is the energy given by the battery to each coulomb of charge.
• Ways to improve transformer efficiency: use pure copper wire to minimize resistance and build the iron core with thin, insulated sheets to prevent eddy currents.

Magnet Materials

• Soft iron can be easily magnetized and demagnetized.
• Ensure the primary and secondary coil are wound closely together to avoid field line leakage.

Radioactivity

• Nucleon number is the number of protons and neutrons in the nucleus.
• Proton number is the number of protons in the nucleus.
• Ion: a charged atom.
• Isotopes: Atoms of the same element with the same proton number, but a different nucleon number.

Detecting Radioactivity

• Use Gm tubes, ratemeters/counters, and photographic film.
• Background radiation is a natural radiation form caused by air, rocks, and cosmic rays.
• Background count rate is the count rate due to background radiation.
• Corrected count rate is the count rate from the source only.
• Half-life is the time for half of the atoms to decay.
• Radioactivity is used for measuring material thickness, carbon dating, cancer treatment, smoke alarms, and tracing underground leakage.

Types of Radioactive Particles

• Alpha particle
  • Consists of a Helium nucleus (α)
  • Has a positive charge
  • Range in air is 5 cm
  • Stopped by paper, leaves, smoke, skin
  • High ionizing power
• Beta particle
  • Consists of fast-moving electrons (β)
  • Has a negative charge
  • Range in air is a few meters
  • Stopped by 3 mm sheet of aluminum
  • Low ionizing power
• Gamma radiation
  • Consists of electromagnetic wave (γ)
  • Has a neutral charge
  • Range in air is infinite
  • Stopped by several cm of lead
  • Negligible ionizing power