Lesson A · 4.1–4.5
By the end you should name the eight stores, pick the pathway, apply conservation, calculate efficiency, and read a Sankey diagram.
How I start this topic
Energy is not a thing that sits in a battery like juice. It is a way of keeping score: if one store goes down, another store (or several) must go up by the same number of joules. The examiner wants the store names and the pathway, not “the energy goes into the kettle.”
4.1 · Units
Specification 4.1 is not a list to memorise once. Every calculation later in this topic uses these units. Convert before you substitute.
| Quantity | Symbol | SI unit | What I say in class |
|---|---|---|---|
| Mass | $m$ | kilogram (kg) | $g \div 1000$. A 250 g brick is $0.25\,\text{kg}$ |
| Energy / work | $E$, $W$ | joule (J) | $1\,\text{kJ}=1000\,\text{J}$. Never leave kJ in $mgh$ |
| Distance / height | $d$, $h$ | metre (m) | cm $\div 100$. Height of a desk is about $0.75\,\text{m}$ |
| Speed | $v$ | m/s | km/h $\div 3.6$ if you ever meet it |
| Acceleration | $a$ | m/s² | $g=10\,\text{N/kg}$ (or $10\,\text{m/s}^2$) unless told otherwise |
| Force | $F$ | newton (N) | Weight $W=mg$. A 2 kg brick weighs $20\,\text{N}$ |
| Time | $t$ | second (s) | minutes $\times 60$. Power needs seconds |
| Power | $P$ | watt (W) | $1\,\text{W}=1\,\text{J/s}$. $1\,\text{kW}=1000\,\text{W}$ |
Convert before you calculate
$2.4\,\text{kJ}=2400\,\text{J}$. $1.5\,\text{kW}=1500\,\text{W}$. $400\,\text{g}=0.40\,\text{kg}$. Leaving kJ in $KE=\tfrac{1}{2}mv^2$ gives a speed that is nonsense, and you often will not notice.
4.2 · Eight stores
A store is where the energy is accounted for. The examiner has eight names and no others. “Heat energy” is sloppy — say thermal store. “Movement energy” is sloppy — say kinetic store.
| Store | What it means | Classroom example |
|---|---|---|
| Chemical | Energy in chemical bonds / fuels / food / cells | Battery, petrol, a sandwich, muscle fuel |
| Kinetic | Energy of a moving object | A falling brick, a skateboard, a spinning motor |
| Gravitational | Energy due to position in a gravitational field | A book on a shelf, a roller-coaster at the top |
| Elastic (strain) | Energy stored when an object is stretched or compressed | A stretched spring, a drawn bow, a compressed ball |
| Thermal | Energy in the motion of particles — hotter means a larger thermal store | Hot water, a warm brick, the surroundings after friction |
| Magnetic | Energy in a magnetic field / two magnets attracting or repelling | Two bar magnets pulled apart |
| Electrostatic | Energy in an electric field / charged objects | A charged balloon, charges on a capacitor |
| Nuclear | Energy stored in the nucleus | A radioactive source, the Sun’s core — name it, do not generate electricity with it on this course |
There is no “electrical store”
Electricity is a pathway, not a store. A kettle empties the chemical store of the power-station fuel (or the chemical store of a battery) and fills the thermal store of the water. The energy travels electrically through the cable. Write “transferred electrically” — do not invent an electrical store.
4.2 · Four pathways
A pathway is the mechanism of the transfer. You only need four phrases, and I want them word-for-word on the paper.
Mechanically
A force moves something — pushing, lifting, stretching, friction. Work is done. A falling brick transfers gravitational → kinetic mechanically.
Electrically
A current in a circuit. Kettle, lamp, motor. The cable is the pathway, not a store.
By heating
Energy moves because of a temperature difference — conduction or convection. A hot pan heating soup.
By radiation
Light or sound (and infrared). A lamp transfers electrically then by radiation (light) into the surroundings; a loudspeaker transfers by radiation (sound).
2-mark “describe the energy transfer”
Name the store that decreases [1] and the store that increases, plus the pathway [1]. Example: “The chemical store of the cell decreases. Energy is transferred electrically to the thermal store of the resistor.”
Practise these four stories until they are automatic:
4.3 · Conservation
The principle of conservation of energy: energy cannot be created or destroyed, only transferred from one store to another. In a closed system the total is constant. In every real machine some energy is wasted — it still exists, but it is not in the store you wanted.
Wasted energy almost always ends in the thermal store of the surroundings: friction, electrical heating of wires, sound that dies away as thermal energy. That is why a room warms up when you run a laptop.
“Lost” is a dangerous word
If you write “energy is lost” the examiner may think you believe it vanished. Write “transferred to the thermal store of the surroundings” or “dissipated as thermal energy.” The joules are still there — they are just spread out and less useful.
4.4 · Efficiency
The specification writes “total energy output.” Treat that total as the energy supplied to the device — input = useful + wasted. You can also be asked for a decimal: $0.80$ instead of $80\%$. Read the question. If it says “calculate the efficiency” and the options are $0.2$, $0.8$, $20$, $80$, look at the units they want.
Board example — kettle
A kettle is supplied with $2000\,\text{J}$. $1600\,\text{J}$ is transferred to the thermal store of the water. The rest is wasted.
Useful $=1600\,\text{J}$, total input $=2000\,\text{J}$, wasted $=400\,\text{J}$.
$$\text{efficiency} = \frac{1600}{2000}\times 100\% = 80\%$$Sense-check: most of the energy did go into the water, so $80\%$ is plausible. A filament lamp is often around $10\%$–$20\%$ because most of the energy is thermal, not light.
Do not invert the fraction
$2000/1600 = 1.25$ is $125\%$. Efficiency cannot be greater than $100\%$ — that would create energy. If your answer is over $100\%$, you have swapped useful and total.
4.5 · Sankey diagrams
A Sankey diagram is an energy-flow picture. One arrow comes in from the left (the input). It splits into a useful arrow (usually straight on) and one or more wasted arrows (usually down). The width of each arrow is proportional to the energy it represents. Input width = useful width + wasted width.
What the examiner awards on a sketch
Input arrow labelled with the energy supplied [1]. Useful and wasted arrows labelled [1]. Widths roughly in proportion [1]. A statement that input = useful + wasted if they ask you to “use the diagram” [1].
Watch
1.
A motor is supplied with $800\,\text{J}$. $600\,\text{J}$ is transferred to the kinetic store of a flywheel. Calculate the efficiency and the wasted energy.
2.
Describe the useful energy transfer in a kettle. Name the stores and the pathway.
3.
A Sankey diagram shows an input of $60\,\text{J}$ and a useful light output of $12\,\text{J}$. What is the wasted energy and the efficiency?