Energy Stores & Transfers

2. Conservation of energy

The principle

• The principle of conservation of energy states:

Energy cannot be created or destroyed; it can only be transferred from one store to another.

• For any closed system, the total energy across all the stores stays constant
• Energy is never truly lost, but it can be dissipated (spread out) into the surroundings by heating or radiation; once dissipated, it is wasted because the original task cannot reclaim it

Useful and wasted energy

• Every real transfer produces a mixture of:
  • Useful energy in the store the designer wanted (the kinetic store of the moving fan, the thermal store of the food)
  • Wasted energy spread around the surroundings (heating from friction, sound carried away into the air, infrared radiation off hot casings)
• The total of useful + wasted output always equals the input, which is conservation of energy in practice

3. Efficiency

Definition

• The efficiency of a system is the fraction of the energy supplied to it that comes out in the useful form
• Stated as an equation:

efficiency = (useful energy output / total energy input) × 100 %

• Efficiency is a pure ratio and has no units; it may be given as a decimal between 0 and 1 or as a percentage between 0 and 100 %
• A high-efficiency system wastes very little; a low-efficiency system spreads most of its input into the surroundings

Useful and wasted always add to the input

• Because energy is conserved:

total energy input = useful energy output + wasted energy

• This lets you find any one of the three when the other two are known

Sankey diagrams

• A Sankey diagram is a visual way to show an energy transfer:
  • The width of each arrow is drawn in proportion to the amount of energy it represents
  • The left-hand stub is the total input
  • The straight horizontal arrow on the right shows the useful output
  • The arrows that branch downwards show the wasted outputs (often labelled with the loss mechanism, such as heating, sound, or friction)
• A modern LED bulb has a Sankey diagram with a wide horizontal arrow (light output) and only a thin branch (heat waste); an old filament bulb has the opposite shape, with most of the input wasted as heat

4. Conduction, convection and radiation

The three modes of heat transfer

• Three distinct processes carry thermal energy from a hot region to a cold one:
  • Conduction: the main heat transfer in solids; energy passes from particle to particle along the material without the particles moving overall
  • Convection: the main heat transfer in fluids (liquids and gases); whole regions of hot fluid actually move from one place to another, carrying their energy with them
  • Radiation: heat transfer through infrared electromagnetic waves; the only one of the three that can carry energy through a vacuum, because no medium is needed
• A real-world example often uses all three at once. A hot mug of coffee: conducts into the table beneath it, sets up a convection current in the steam rising above the surface, and radiates infrared from the outside of the mug

Conduction in detail

• In any solid, the particles vibrate about fixed positions; the hotter the material, the larger the vibrations
• When one end of a solid is heated, the particles at that end vibrate more strongly. They collide with neighbouring particles, passing the extra vibrational energy along the line

• Metals are especially good thermal conductors because they have delocalised electrons that drift freely through the lattice:
  • the free electrons collide far more often with the vibrating ions than the ions collide with each other
  • they carry energy across the metal much faster than vibration alone
  • this is why a stainless-steel teaspoon left in a hot cup of tea quickly becomes too warm to hold, while a wooden stirrer of the same shape stays cool

Why insulators insulate

• A thermal insulator is a material in which conduction happens only slowly
• Common insulators include wood, plastic, rubber, glass, dry paper, and any material with lots of trapped air
• Trapped air is one of the best insulators:
  • the air molecules are far apart, so very few collisions pass energy along
  • if the air is trapped in pockets (within wool, fibreglass, foam, feathers), it cannot move as a convection current either, so the convection pathway is also shut down

Convection in detail

• When a region of a fluid is heated:
  • the particles in that region gain kinetic energy and push each other further apart
  • the fluid in that region expands and becomes less dense than the cooler fluid around it
  • the less-dense hot fluid rises, while the cooler, denser fluid sinks to take its place
• The rising and sinking sets up a closed loop of moving fluid called a convection current

• Convection currents drive everyday heating: radiators warm rooms by setting up a current of air around the room; the sun warms the atmosphere by setting up large convection cells; ocean currents move warm and cool water around the globe

Radiation in detail

• Every object at any temperature above absolute zero emits infrared (thermal) radiation; the hotter the object, the more it emits per second
• Thermal radiation is a transverse electromagnetic wave, so it carries energy through a vacuum (the way sunlight reaches Earth across 150 million km of empty space) as well as through transparent media
• The colour and finish of a surface strongly affects how well it emits and absorbs thermal radiation:

• Practical consequences:
  • solar panels and solar-heating absorbers are painted matt black to absorb as much sunlight as possible
  • emergency space blankets are made of shiny silver foil to reflect a person's body-heat radiation back inwards
  • houses in hot climates are often painted white to reduce both absorption from the sun and emission of heat back outwards
  • vacuum flasks have silvered inner walls to cut radiation losses (alongside the vacuum that blocks conduction and convection)

5. Core practical: investigating thermal energy transfers

Three sub-experiments

The single Edexcel core practical covers three separate investigations, each one isolating a different mode of thermal energy transfer.

Experiment A: rates of conduction in different metals

• Aim: compare the rate of thermal conduction in several metals (copper, aluminium, brass and iron) by timing how long it takes a wax-attached marker to fall off the cold end of each rod when the hot ends are heated equally
• Independent variable: the metal of the rod
• Dependent variable: the time taken for the wax to melt and the ball bearing (or pin) to fall
• Control variables: identical rod dimensions, identical wax blobs, identical ball bearings, identical heating from the central flame

Method:

1. Stick an identical small ball bearing to the cold end of each of the four rods using a small piece of wax
2. Clamp the conduction wheel to a stand so the central hub sits over a Bunsen burner, with the rods spreading horizontally
3. Light the burner so the central hub of all four rods receives the same heat
4. Start the stopwatch the moment the flame steadies under the hub
5. As the wax on each rod melts, the ball bearing drops; record the time for each rod
6. Allow the rods to cool fully before repeating

Analysis:

• The shorter the time before the ball drops, the faster the conduction in that metal
• The expected ranking, fastest first, is copper → aluminium → brass → iron
• Plot a bar chart of mean time against metal to display the ranking

Experiment B: convection in water using potassium permanganate

• Aim: see a convection current set up in water by following the path of a coloured dye crystal that dissolves where the water is heated

Method:

1. Fill the beaker about three-quarters full with cold water and place it on the tripod above the unlit burner
2. Carefully pick up a single potassium permanganate crystal with the forceps and drop it gently to the bottom of the beaker, slightly to one side rather than the centre
3. Light the Bunsen burner directly below the crystal
4. Observe how the purple trail moves through the water

Analysis:

• Where the crystal sits, the water heats, expands, becomes less dense, and rises, carrying the purple dye upwards in a clearly visible plume
• Cooler water from the rest of the beaker flows in to take its place, completing the convection loop
• The visible purple loop is direct evidence that bulk fluid motion (not just particle vibration) is transferring the heat
• Safety: potassium permanganate stains skin, clothing and surfaces; handle only with forceps and rinse spills immediately

Experiment C: thermal radiation from differently coloured surfaces

• Aim: compare how quickly water inside differently coloured but otherwise identical containers cools, and so investigate how surface colour affects emission of infrared radiation
• Independent variable: the colour and finish of the container (matt black, dull grey, matt white, shiny silver)
• Dependent variable: the water temperature recorded at fixed time intervals
• Control variables: identical container shape and size; identical starting volume of water; identical starting temperature; identical room temperature; identical lid arrangement

Method:

1. Set the four cans side by side on the bench at least an arm's length apart so they do not radiate to each other
2. Fill each can from the kettle with the same volume of just-boiled water; replace the lid and slot the thermometer through the hole
3. Note the starting temperature in each can (all four should match within ~1 °C)
4. Take a temperature reading from each can every 30 s for 10 min
5. Plot a graph of temperature (y-axis) against time (x-axis) for each can on the same axes

Analysis:

• The cans cooled fastest are the best emitters of infrared radiation
• The expected order, fastest cooling first, is matt black → dull grey → matt white → shiny silver
• All four cans lose roughly the same amount of heat by conduction (through the bench) and convection (out of the lid hole), so any difference between the cooling curves must be due to radiation differences

Sources of error and safety

• Experiment A: ensure all rods are at room temperature before starting; if one rod is still warm from a previous run, its wax will fall too quickly. Avoid handling the wax with your fingers because the heat from your hands also pre-warms the wax
• Experiment B: drop the crystal gently; agitating the beaker will create currents that look like convection but are mechanical. The room should be still
• Experiment C: read each thermometer at eye level to avoid parallax; minimise the size of the lid hole; ideally use a data logger to take all four readings simultaneously
• General safety: wear goggles when using a Bunsen burner; do not lean over hot apparatus; run any burns under cold water for at least five minutes

6. Reducing energy loss

Why bother?

• Real appliances waste energy in three common ways: conduction through casings, convection currents leaving hot regions, and radiation from hot surfaces. The energy is conserved, but the wasted share costs money and adds to greenhouse-gas emissions whenever the source is fossil-fuelled
• Reducing the wasted output raises the efficiency of the system: more of the same input ends up in the useful store

Reducing conduction

• Use materials with low thermal conductivity (good thermal insulators) wherever heat needs to stay in (or out)
• Make the insulating layer thicker, because heat takes longer to conduct through a thicker barrier
• Use materials that trap pockets of air, since air is itself a very poor conductor and pockets stop convection currents from forming inside the insulator
• Everyday examples:
  • double glazing sandwiches a thin layer of air or argon between two glass panes
  • loft insulation made of fibreglass mats placed between the joists
  • woolly jumpers and quilted jackets trap air between the fibres

Reducing convection

• Stop the fluid (usually air) from moving and you stop convection currents from forming
• Practical methods:
  • Cavity wall insulation: foam blown into the gap between the inner and outer walls of a house traps the air that would otherwise circulate as a current

- Curtains and draught excluders cut down warm-air loss from windows and gaps around doors - A lid on a hot drink prevents warm air rising from the surface and being replaced by cool air

Reducing radiation

• Use surfaces that emit infrared poorly: polished, shiny silver
• Practical methods:
  • The inner walls of a vacuum flask are silvered on both sides; the inner silver reflects infrared back inside, the outer silver reflects infrared from outside back out
  • Foil-backed loft insulation reflects infrared from a warm room back downwards
  • Emergency space blankets wrap a person in shiny silver to retain body heat in a cold environment

A vacuum flask in one sentence per pathway

• A vacuum flask combines all three tricks:
  • the double-walled vacuum between the inner and outer flask stops both conduction and convection (no medium for either)
  • the silvered surfaces of the inner walls stop radiation
  • the insulating stopper at the top stops conduction and convection through what would otherwise be an open neck

Common-language traps to avoid

• "Heat rises": what actually rises is hot fluid that has become less dense. Heat itself is not a substance with a direction
• "Shiny things reflect heat": shiny surfaces reflect infrared radiation; they cannot reflect conduction or convection because neither bounces
• "Black things absorb heat": black surfaces absorb infrared radiation; they do not absorb conduction or convection separately
• A good exam answer almost always mentions more than one of conduction, convection and radiation