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Teacher guide: Year 9 Physics

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.

Open the student course

Each lesson works two ways. Students can work through the online tutorial at their own pace (it checks their answers and saves their progress in their browser), or you can teach it to the whole class from the slides, then send students into the tutorial for the Modify and Make steps.

Every lesson guide has plain-language notes, the answers to every check, a solution for every task, and slides and a worksheet for Waypoints Prepare. These guide pages are not linked from student pages, but they are not secret either: keep printed Teacher editions for answers you do not want students to find.

Energy transfer

Using the particle model to explain conduction and convection (AC9S9U04).

Conduction and convection in 5 minutes (read before teaching this unit)

The idea in plain words

Heat is energy moving from hotter to cooler places. The particle model explains two ways it travels through matter.

  • Conduction (mostly in solids): particles at the hot end vibrate harder and bump their neighbours, which then vibrate harder and bump theirs. The energy passes along; the particles stay in place. Metals conduct well (and they have free electrons that carry energy fast, Year 10); most non-metals, and gases, conduct poorly: they are insulators.
  • Convection (only in liquids and gases): the heated part of the fluid expands, becomes less dense and rises; cooler, denser fluid sinks to take its place. The particles themselves move, carrying the energy with them in a loop called a convection current.

Trapped air is an excellent insulator: it conducts poorly and, if it is trapped in small pockets, it can't form convection currents. That is how wool, fur, feathers, foam and double glazing work.

Where the picture breaks

  • The simulation pushes hot particles up directly. In reality hot fluid rises because its particles spread out, making it less dense.
  • Radiation (the third way, which needs no particles) is the next node.

Common misconceptions

  • Heat rises (hot fluid rises; heat goes in every direction).
  • Cold flows into things ("close the door, you're letting the cold in").
  • In conduction the particles travel along the material.
  • Metals feel cold because they are colder than wood at the same temperature.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

  • Does the material of a rod change how fast heat travels along it? Change: the rod (copper, aluminium, steel, glass). Measure: the time until a drawing pin stuck on with petroleum jelly drops off. Keep the same: rod size, water temperature, the pin's distance from the water.
  • Does wrapping a cup change how fast hot water cools? Change: the wrapping (none, foil, bubble wrap, a wool sock). Measure: the temperature drop over 10 minutes. Keep the same: cup, volume, starting temperature. (The best wrappings trap air.)
  • Does hotter water rise faster? Change: the temperature of coloured water released at the bottom of a beaker of cool water. Measure: the time for the colour to reach the top. Keep the same: the beaker, the amount of dye, the depth.

Safety and ethics: use water no hotter than about 60 °C (from a kettle, not boiling), stand beakers on a mat, and report broken glass.

Wave and particle models

Sound and light as waves, electricity as moving charge, and which model suits which phenomenon (AC9S9U04).

Wave and particle models in 5 minutes (read before teaching this unit)

The idea in plain words

The particle model explains how energy moves through matter: conduction and convection (collisions and bulk movement), and electricity (static charge is electrons moved from one material to another; a current is free electrons pushed round a loop by a battery).

The wave model explains sound and light. Sound is a longitudinal wave of compressions and rarefactions that needs a medium; the pitch is set by frequency and the loudness by amplitude. Light is a transverse electromagnetic wave that needs no medium, and sits in a spectrum from radio to gamma: shorter wavelength, higher frequency, more energy. The wave model explains reflection and refraction.

A model is a tool, with a domain of usefulness. Light also behaves as photons when it hands energy to single electrons.

Where the picture breaks

  • Boards slow waves down and show only a few particles; real sound waves have vastly more particles per wavelength.
  • The wave in the spectrum board is a schematic: real wavelengths span about fifteen powers of ten.
  • The current board exaggerates the electrons' drift speed; real drift is a fraction of a millimetre per second.
  • Colours of light and their different bending (dispersion) are not modelled.

Common misconceptions

  • Air particles travel from the speaker to the ear.
  • Sound travels through space or faster in lighter media.
  • Rubbing makes charge, or moves protons.
  • Electrons are used up in the bulb, or the battery makes them.
  • X-rays are faster than light, or microwaves have more energy per wave than X-rays.
  • Light bends because glass pushes it; it speeds up in glass.
  • One of the wave and particle models must be the true one.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

  • Does a longer vibrating ruler make a lower note? Change: the length of ruler over the edge of the desk. Measure: the frequency (a data logger sound sensor), or rank the pitch by ear. Keep the same: the ruler, how far you flick it.
  • Does the angle light hits a glass block change how much it bends? Change: the angle of incidence (0°, 20°, 40°, 60°). Measure: the angle of refraction with a protractor. Keep the same: the block, the ray box. (At 0° the ray goes straight through.)
  • Does rubbing a balloon more give it more charge? Change: the number of rubs on wool (5, 10, 20). Measure: the number of small paper pieces it picks up. Keep the same: the balloon, the cloth, the paper pieces. (Humid days spoil static.)

Safety and ethics: ray box lamps get hot; check for latex allergies before using balloons.

Conservation of energy and efficiency

Energy is never made or destroyed; efficiency is the useful share; Sankey diagrams show where it goes (AC9S9U05).

Conservation of energy and efficiency in 5 minutes (read before teaching this unit)

The idea in plain words

Conservation of energy: energy is never made or destroyed, only transferred or transformed, so total energy in = total energy out. Energy that seems lost has gone to the surroundings, usually as heat and sound.

Efficiency = useful energy out ÷ total energy in × 100%. It is a proportion, and no real device reaches 100%, because some energy always becomes by-products.

A Sankey diagram shows the flows as bands with widths to scale: the useful output goes straight on, wasted outputs bend away, and the outputs add up to the input.

Where the picture breaks

  • The boards show one split from input to outputs; real systems have several stages, each with its own losses.
  • Figures are rounded and typical (filament bulb 10%, LED 40%, car engine 25%, coal station 35%, hydro 90%).

Common misconceptions

  • Energy is used up or disappears.
  • "Conserve energy" in the everyday sense (save it) is the same as the law.
  • The device with the biggest useful output is the most efficient.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

  • Which ball keeps the biggest share of its energy after a bounce? Change: the ball (tennis, ping-pong, rubber, squash). Measure: bounce height ÷ drop height × 100%. Keep the same: drop height, floor. (For one ball, gravitational energy is proportional to height, so this is the efficiency of the bounce.)
  • Does a filament bulb or an LED give off more heat? Change: the bulb, at similar brightness. Measure: a thermometer 2 cm away after 5 minutes. Keep the same: distance, time, power supply. (A warmer thermometer means more by-product heat and a less efficient bulb.)
  • Does a heavier pendulum bob keep swinging longer? Change: the mass of the bob (same size). Measure: swings until the swing is half as wide. Keep the same: string length, starting angle.

Safety and ethics: filament bulbs get hot enough to burn: don't touch them until they have cooled.