92047 · Energy in physical systems

• A physical system is a line you draw

  A physical-system is a defined space that holds an object or interacting objects. You draw a system-boundary so that you can say what is inside and what is not. Official examples include a vacuum flask, an insulated home, a simple circuit, a falling object and a lifted object. Once the line is drawn, you describe the system by the properties and behaviour that relate to energy: height, motion, temperature, whether a spring is wound. The same physical event can be labelled differently if you move the line. Air resistance that is a transfer out of a backpack-only system becomes an internal thermal store once the nearby air is inside the boundary. The fall itself has not changed.

• Five stored forms, and no more

  An energy-form is a way energy is stored in a system. This standard limits the stored forms associated with matter to five. Kinetic energy is present because something in the system is moving. Gravitational potential energy is present because of an object's height in Earth's gravity; gravitational energy is the same form. Elastic energy is stored when something is stretched, compressed or bent and can spring back. Thermal energy is stored in the random motion of the particles of a substance, the total rather than the average. Chemical energy is stored because of composition, as in a battery, food or fuel. If a word is not on that list, it is not a sixth store.

• Pathways are not extra stores

  Mechanical energy is a grouping name for kinetic plus gravitational potential energy, and sometimes elastic as well. It is not a sixth form on the closed list. Electricity, sound and electromagnetic radiation, including light, carry energy across a boundary. They are pathways, not extra bars on a final chart. When a paper says electrical energy was provided by a motor, read that as energy transferred electrically from a chemical store, not as a new stored form sitting in the wires. The catalogue of transfer processes is a later page. Here you only need the store-versus-pathway contrast so a bar chart stays honest.

• Conservation is an account, not a slogan

  conservation-of-energy means energy is neither created nor destroyed. It changes form, or it crosses the system boundary, and the account must still add up. Stating the slogan is not the same as applying the law. Applying it means naming the system, the two states, the forms present, and the joules you were given. If a backpack starts with 50 J of gravitational potential energy and, halfway down, has 25 J gravitational, 20 J kinetic and 5 J thermal, the total is still 50 J. The 5 J was not destroyed. A leftover that is “whatever is left” is not an independent measurement: every error in an input sits inside it.

• Name the change, then the two states

  A change to a physical system is any change to its physical properties or behaviour: a pot is lifted, a band is released, a drink cools, a backpack falls. Read the change that is actually named. Describing a fall when the stem is a lift is the wrong event. To represent the energy change you identify the system, then the initial and final states, then the forms associated with each state. Halfway through a fall, both gravitational potential energy and kinetic energy are present. Naming only the form that is growing is an incomplete description of that state.

• Bar charts show stores at two snapshots

  An energy-bar-chart shows the stored forms in a named system at an initial state and a final state so conservation can be checked. Each bar is a store at one snapshot. Draw a transfer in or out only when your chosen boundary requires it. A closed system whose bars add to the same total at both states is a model, and you should label it as one. The chart cannot show a pathway, only the stores before and after. A shorter useful-motion bar is a different distribution of the same total, not energy destroyed. Adding a light bar or a sound bar as a final store, or counting the same joules as a store and as a transfer, is a misread of the representation.

• Mechanical energy is a grouping, not a sixth bar

  Kinetic plus gravitational energy — and elastic, if a spring or a stretched band is in the system — can be called mechanical energy. That grouping is a reading of the closed list, not a sixth stored form sitting beside the five. A bar chart that draws a “mechanical” bar and also draws the kinetic and gravitational bars double-counts the same joules. If the mechanical total is 80 J, that 80 J is already the gravitational bar, or the kinetic bar, or both together. The grouping name is useful in a sentence. It is not an extra rectangle.

• A residual is not a measurement

  Conservation constrains the total. It does not pick a unique split, and it does not turn a leftover into an independent reading. After two named bars, “whatever is left” carries every error in those bars: a 2 J slip in the gravitational number sits inside the residual. Redrawing the system-boundary relabels a transfer as an internal thermal store, or the reverse. The physical event has not changed and no joules have been created. Conserved does not mean the object keeps rising, keeps rolling, or stays bright. The total is still there after the change stops; it is usually thermal.

• Transfer is a process, not a sixth store

  A transfer carries energy from one object or system to another. The stored forms stay the five you already know. Electricity in a flex, light from a lamp, sound from a speaker and heating across a flask wall are how energy moves, not extra bars to park on a final chart. If you catch yourself writing “sound energy stored in the radio” or “the wires hold electrical energy”, you have turned a pathway into a store. Name the process that is crossing the boundary, then keep the account of stores on the forms page.

• Four processes, recognised by what crosses the boundary

  Doing work is a transfer by a force moving something: a hand on a crank, a cable lifting a crate, a shaft turning a generator. Heating is a transfer driven by a temperature difference: energy leaving tea in a vacuum flask, or leaving a lounge through walls and windows. Electricity is a transfer along a circuit, not a stored form and not a voltage calculation. Waves carry energy as sound or as electromagnetic radiation — light from a lamp, radiation arriving at a solar panel. Mechanism names such as conduction or radiation may label which process is occurring. This page does not first-teach those mechanisms, and it does not teach wavelength, frequency or any other wave behaviour.

External · grade-score marked.

This is an external achievement standard. Whetū offers original exam-style questions marked on the NCEA grade-score scale. Grades are estimates, not official results.

A mock paper is available in Whetū: Mock paper · Energy in physical systems.

5 authored Learn units for this standard.

• Energy forms and conservation

  This standard asks you to understand a physical system by using energy ideas. A physical system is a defined space with an object or interacting objects — a backpack on a hut deck, a pull-back toy, a battery lantern, a flask of tea. This page first-teaches how to name that system, how to name the change, and how to keep an energy account. Power, efficiency, circuit maths and heat-transfer mechanisms belong on later pages.

• Transfer, power and efficiency

  Energy does not jump from one object to another by magic. Energy transfer is a process that carries energy across the edge of a physical system by doing work, heating, electricity, or waves — sound and electromagnetic radiation. Those four are how energy moves. They are not extra stores sitting beside kinetic, gravitational, elastic, thermal and chemical energy. A lamp does not “contain light energy” in the way a raised crate has gravitational energy; light is radiation leaving the lamp.

• Mechanical energy

  A hay bale winched into a loft, a waka hauled up a boat ramp, and a crate that slips from a packhouse mezzanine are the same kind of problem. Something in a named physical system changes height, speed, or both. This page is about applying the three mechanical formulae printed on the resource sheet — not recalling them — to describe that change, explain how the energy redistributes, and examine what the numbers force about the real motion.

• Electrical energy

  A bathroom heat-fan, a 12 V work lamp in a farm implement shed, and a soldering iron on a technology bench are the same kind of physical system: a simple-circuit with a few components, and a change you can name. The change might be a dial moved from the 2400 W setting to the 1200 W setting, a halogen lamp swapped for an LED, or a current that has increased. This page is about the energy concepts inside that circuit.

• Thermal energy

  A mug of tea, a Hanmer Springs pool, and the air in a Wellington villa are physical systems whose particles store energy. This page is about that store: thermal energy. You name the system and the change — a temperature rise, a melt, a boil, a cool, or a change in how energy is leaving — and then use energy concepts as evidence for what happened.

36 Practise questions in “Energy in physical systems”. Feedback here is formative and is not an official NCEA grade.

• Five stored forms

  Kinetic, gravitational, elastic, thermal, chemical. Pathways are not extra stores.

• Conservation as an account

  Name the system and the two states. Bar charts are two snapshots.

• Four transfer processes

  Doing work, heating, electricity, waves. Transfer is not a sixth store.

• Power and efficiency

  Name ΔE = Pt. Purpose decides useful output. Efficiency formula in the stem. Duty-cycle average is not the peak.

• Kinetic and gravitational

  Name Eₖ = ½mv² and Eₚ = mgΔh. Height is vertical.

• Work and missing mechanical

  Work is energy transferred by a force. Missing mechanical energy is thermal.

• Voltage, current, resistance

  Electricity is a transfer, not a stored form. Read V and I on a component.

• Electrical calculations

  V = IR, P = VI, chained with ΔE = Pt. A calculation is evidence for the change.

• Temperature is not the store

  Temperature versus thermal energy. Conduction, convection, radiation.

• Heating and change of state

  Name E_thermal = mcΔT and E_thermal = mL. Read a heating-curve plateau.

Sample questions

1. Which stored form is present in the timer-plus-spring system at that moment?
2. Match each stored form to a classroom example of that store.
3. This treats pathways as extra stores. Which changes would fix it? Select all that apply.
4. Halfway down, which description of the stores is complete?
5. Put these steps in order for keeping a conservation account of a named change.
6. Which working line is the strongest evidence that energy has been conserved, not destroyed?
7. Which student names the transfer that is crossing the boundary at the flex?
8. Match each transfer process to an example of energy crossing a boundary that way.

Practise 92047 in Whetū

469 original exam-style questions in Whetū. These are practice papers, not official NZQA assessments.

• Wind-Up Toy · Energy forms and conservation
• Torch Account · Energy forms and conservation
• Hand Warmer · Energy forms and conservation
• Band Cart · Energy forms and conservation
• Hill Climber · Energy forms and conservation
• Five Forms · Energy forms and conservation
• Battery Fan · Energy forms and conservation
• Slingshot Sequence · Energy forms and conservation
• Speaker Sequence · Energy forms and conservation
• Falling Ball · Energy forms and conservation
• Heater Account · Energy forms and conservation
• Bounce Bars · Energy forms and conservation