Energy on the move. The particle model explains how heat travels through solids and fluids; then energy is followed through whole systems, where it is never lost but not all of it is useful. Concept tutorials with particle simulations, a roller coaster and Sankey diagrams.
Using the particle model to explain conduction and convection (AC9S9U04).
Conduction passes energy from particle to particle by collisions, with the particles staying in place. Convection carries energy as the heated fluid itself moves. The particle model explains both, and why metals conduct and trapped air insulates.
Sound and light as waves, electricity as moving charge, and which model suits which phenomenon (AC9S9U04).
Sound is a longitudinal wave: a vibrating source makes compressions (particles squashed together) and rarefactions (particles spread out) that travel through a medium, with each particle only vibrating to and fro in place. Frequency sets the pitch and amplitude sets the loudness. Without particles, as in a vacuum, there is nothing to carry the wave.
Everything is made of atoms with positive protons that stay put and negative electrons that can move. Rubbing two materials moves electrons from one to the other, leaving one positive and one negative. Opposite charges attract, like charges repel, and a spark is the extra electrons rushing back to even things out.
In a conductor such as copper each atom lets go of an electron, so there are free electrons that can move. A battery pushes them round the circuit: this flow of charge is the current, and more volts push harder. In an insulator the electrons stay tied to their atoms, so there is no current. The electrons are not used up; the bulb transfers the battery's energy.
Light is a transverse wave, and it is one small part of a family of electromagnetic waves: radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. They all travel at the same speed in a vacuum (300 000 km/s) and differ in frequency and wavelength. Higher frequency means a shorter wavelength and more energy, which decides what each is used for and how careful we must be with it.
The wave model explains two things you can see. In reflection, wave fronts bounce off a surface and the angle of reflection equals the angle of incidence. In refraction, light slows down in a denser material such as glass or water, so the wave fronts bunch up and the direction bends towards the normal; light going straight in at 90° does not bend.
A model is a tool, not a photograph. The particle model explains conduction, convection and static charge; the wave model explains sound, reflection, refraction and the electromagnetic spectrum. Light shows both wave-like and particle-like behaviour, so neither model alone explains it all: each is useful for some phenomena and weak for others.
Energy is never made or destroyed; efficiency is the useful share; Sankey diagrams show where it goes (AC9S9U05).
Energy is never made or destroyed: it only moves and changes type. This is the law of conservation of energy. When energy seems to disappear (a ball bounces lower, a car slows down), it has gone into heat and sound in the surroundings. The total is always the same.
Efficiency is the share of the energy put in that comes out as useful energy: efficiency = useful energy out ÷ total energy in × 100%. No real device is 100% efficient, because some energy always becomes by-products, usually heat. A device can give out more useful energy and still be less efficient.
A Sankey diagram shows energy flowing through a system as bands whose widths are proportional to the amounts. The input splits into the useful output (straight on) and the wasted outputs (bending away). You can read the efficiency off it and see at a glance where energy is wasted, which is how engineers compare real systems.
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