Absolute temperature is measured on a scale that starts at absolute zero, such as the Kelvin scale.

Appliance efficiency is rated from A (most efficient) to G (least efficient). Less efficient devices are being phased out and replaced by more efficient devices, for example LED bulbs (A) instead of filament bulbs (D). More efficient appliances sometimes cost more, but their running costs are lower.

Boiling is the state change from liquid to gas.

The energy in an object’s chemical store changes when atoms or molecules combine or break apart. Associated with batteries, food and fuels.

Condensing is the state change from gas to liquid.

The energy in an object’s elastic store changes when it is stretched or squashed.

Electrical working is an energy pathway that happens when a current passes through an object.

The energy in an object’s electric store changes when brought closer or further away from another charged object.

Energy is a quantity/number that is always conserved in a complete system. It can quantify how much work has been done on an object, or the amount of work that it has the potential to do – therefore giving us an indication of what can happen in that system. Objects can store energy in different ways, and objects can have multiple energy stores at the same time (a flaming arrow possesses energy in a kinetic, gravitational, thermal, and even a chemical store). The SI unit for energy is joules.

Energy pathways are the physical process by which one energy store empties and another fills. As energy pathways measure a change in energy in a given amount of time, it may be more appropriate to  measure some changes using power. The SI unit for power is watts. We will consider the following pathways.

Energy stores is the term used to describe the amount of energy associated with one property of an object. The amount of energy in a store is measured in joules and are a scaffold for future calculations.

Evaporation is also a state change from liquid to gas, but rather than being caused by heating, it is caused by the particles having random speeds spread over a distribution. There will be a small number of particles at any one point that are near the surface and are also moving fast enough that they can escape. Evaporation is therefore a slow process. As it takes away the fastest particles, evaporation will cool a liquid. Increasing a liquid’s temperature will increase the rate of evaporation.

Fossil fuels such as oil, coal and gas were created by the decay of organic matter over thousands or millions of years. They are all inherently non-renewable.

In a gas the particles are free to move around, and are much more spread out. They move quickly, and randomly collide with each other and the container walls.

The energy in an object’s gravitational store changes when its position changes compared to a massive object (e.g., an apple being lifted up on Earth).

Heating by particles is an energy pathway that occurs between two objects which are at different temperatures and touching one another.

Heating by radiation is an energy pathway that occurs when an object emits or absorbs an electromagnetic wave (such as infra-red).

Heat as a noun is deprecated, see thermal store of energy.

A heat pump transfers thermal energy between the inside and outside of a house, which can significantly reduce the amount of electricity used in heating/cooling your home.

Heat is the process by which an object is warmed.

A kilocalorie is a measure of energy that is used to quantify the chemical store of energy in food. is how much energy is required to raise the temperature of  of water by , and is about . A Mars Bar contains around  (). To add confusion, nearly all the time that people working with food or exercise refer to a calorie, they are actually referencing a kilocalorie!

A kilowatt-hour () is the unit used to measure energy for a domestic customer. It is exactly as described – it is the amount of energy used by a power source of  in .

The energy in an object’s kinetic store changes when it changes speed.

In a liquid the particles are free to move, but are still touching each other. A liquid is less ordered than a similar solid. They will fill a container from the bottom up.

The energy in an object’s magnetic store changes when brought closer or further away from another magnetic object. Some teachers and schemes of learning combine electric and magnetic stores because they are intrinsically linked together, as explained at university level by Maxwell’s Equations.

Mechanical working is an energy pathway that occurs when a resultant force acts to move an object.

Melting is a state change from solid to liquid.

Non-renewable energy resources cannot be replaced and are running out significantly faster than they are being used, including oil, coal, gas and nuclear power.

The energy in an object’s nuclear store changes when nuclei are changing, for example combining or breaking apart.

We use the term particles as a catch all term that includes atoms, ions and molecules.

Payback time is the time taken before an appliance’s cost has been recovered by reduced running costs – usually measured in years. 

See state change.

See states of matter.

Renewable energy resources are ones that can be replenished as they are being used. They can be replenished by a natural process (the wind keeps flowing, the sun keeps shining, or hydro-electric dam fills with water) or with human action (more crops are collected and then planted for the biodiesel or biomass).

These state changes are all reversible physical changes, where the original form of matter can be restored.

In a solid the particles are in fixed locations, but they vibrate. They are tightly packed together.

Solidfying is the state change from liquid to gas (called freezing when the substance is water).

The specific heat capacity of an object is the amount of energy that is needed to increase one unit of mass by one degree of temperature. In GCSE physics we normally measure specific heat capacity in .

The specific latent heat of fusion is the amount of energy that is needed to turn one unit of mass from solid to liquid. In GCSE physics we normally measure specific latent in .

The specific latent heat of vapourisation is the amount of energy that is needed to turn one unit of mass from liquid to gas. In GCSE physics we normally measure specific latent in .

A phase change or state change occurs when a substance changes from one state to another.

At KS3 and KS4 we consider there are three different phases or states of matter (solid, liquid and gas). There are other interesting states that are considered elsewhere, such as plasma (which is an ionised gas that can conduct https://www.britannica.com/science/plasma-state-of-matter) and Bose-Einstein condensates (predicted in 1942 and only created for the first time in 1995, see more here).

Sublimation is the state change from a solid directly to a gas without passing through the liquid state. Solid carbon dioxide is an example substance that at standard temperatures and pressures will sublime, rather than melt (it is used in some types of fog machines).

Temperature is a measure of hotness or coldness. Usually expressed in degrees Celsius or kelvin.  When two objects at different temperatures are placed next to one another, the hotter object will try to heat up a cooler object – until an equilibrium is reached.

The energy in an object’s thermal store changes when it changes temperature. It can also change as it changes state.

The energy in an object’s vibrational store changes when an object is oscillating about a point. Many teachers and schemes of learning ignore this as a store. One example where it may be useful is when describing a just-plucked guitar string: the string is alternately moving quickly and then stretching many of times a second; energy is constantly shifting between elastic and kinetic stores, it is simpler to describe it as a single vibrational store of energy.

Work done is the measure of energy transferred when a force moves an object through a distance (in the direction of that force).