Energy Transfers

Edexcel awards a mark for the correct unit alongside a numerical answer. For energy transfers you'll need:

• mass in kg
• energy in joules (J)
• distance in m
• velocity in m/s
• acceleration in m/s²
• force in newtons (N)
• time in seconds (s)
• power in watts (W = J/s)

Larger energies often quoted in kJ ($10^{3}$ J) or MJ ($10^{6}$ J). Convert to J BEFORE using $P = W/t$ to avoid index errors.

Eight named stores. Edexcel expects you to recognise and use these names:

• Chemical — fuel, food, batteries.
• Kinetic — moving objects.
• Gravitational (potential) — objects raised against gravity.
• Elastic (potential) — stretched / compressed springs, tensioned bows.
• Thermal (internal) — random KE of particles.
• Magnetic — magnets close to magnetic materials.
• Electrostatic — separated electric charges.
• Nuclear — stored in atomic nuclei.

Four named transfer pathways.

1. Mechanically — a force does WORK on an object (e.g. pushing a box → kinetic energy increases).
2. Electrically — an electric current carries energy (e.g. battery → motor).
3. By heating — conduction/convection transfer thermal energy between objects at different temperatures.
4. By radiation — energy carried by EM waves (light, IR) or sound waves.

Worked descriptive example. A torch:

• Chemical store (battery) → electrical transfer to bulb → small useful transfer to LIGHT (radiation) + larger wasted transfer to THERMAL store of bulb + surroundings.

Edexcel often asks you to name the stores AT EACH STAGE. Use the named stores/transfers from spec 4.2 verbatim.

Statement. Energy cannot be created or destroyed; it can only be TRANSFERRED from one store to another (spec 4.3).

Consequences.

• A pendulum's GPE at the top = KE at the bottom (neglecting air resistance).
• The total energy IN to a device = total energy OUT (useful + wasted).
• A 100% efficient device transfers all input to useful output — impossible in practice because some always goes to thermal store of surroundings.

Mark-scheme pitfall. Never say energy is 'lost' or 'used up'. The correct vocabulary is 'wasted', 'transferred to thermal store of surroundings' or 'dissipated to the surroundings'.

Numerical use. If 1000 J enters a motor and 850 J leaves as useful kinetic energy, then 1000 − 850 = 150 J was transferred to thermal store of the motor / surroundings. This is the basis of efficiency calculations.

Formula.

$\eta = \dfrac{E_\text{useful}}{E_\text{total}} \times 100\%$

A 60 W LED bulb that produces 54 W of light has efficiency $= 54/60 \times 100\% = 90\%$. A 60 W filament bulb producing 4 W of light has efficiency $\approx 7\%$.

Both energy ratio AND power ratio work. The formula can use joules or watts on the top and bottom — but you MUST be consistent. Don't mix joules and watts.

Sankey diagrams (spec 4.5). A visual representation of efficiency:

• One arrow per energy form, drawn horizontally.
• Arrow WIDTHS to scale with the energy values.
• USEFUL output continues to the right; WASTED energy bends downward (or upward).
• Label each arrow with the energy form and the value in joules.

Typical Sankey for a filament bulb (60 J input): input 60 → useful 5 J light + wasted 55 J thermal.

Sankey for an electric motor with no friction losses (impossible — but a textbook exercise): input 1000 J → useful 1000 J kinetic. A 100% efficient device has only one outgoing arrow.

Sankey for a real car engine (100 J of fuel): typically 25 J useful KE + 70 J wasted thermal + 5 J wasted sound. Efficiency = 25%.

Conduction (solids, especially metals). Particles vibrate in place. When one end of a solid is heated, particles there vibrate more vigorously and collide with their neighbours, passing on KE. In metals, FREE ELECTRONS also carry KE rapidly through the solid → metals are the best conductors. Non-metallic solids (wood, plastic) and gases conduct poorly.

Convection (fluids: liquids + gases). Fluid in contact with a hot surface heats up, EXPANDS and becomes LESS DENSE → rises. Cooler, denser fluid flows in to replace it → a convection current circulates.

Spec 4.7 examples:

• Heating water in a kettle. Water near the heating element rises; cooler water from the top sinks to take its place. The whole pot heats from the bottom up.
• Sea breezes by day. Sun heats land more than sea. Air over land warms, rises; cooler air flows in from the sea → onshore breeze.
• Land breezes at night. Reverse: sea retains heat longer; air over the sea rises; air flows out from the land → offshore breeze.
• Hot-air balloons. Burner heats air inside the envelope; less dense than surroundings → balloon rises.

Radiation (all warm objects; works through a vacuum). Every object emits infrared electromagnetic radiation. Hot objects emit more, and at shorter wavelengths. Radiation passes through air, glass, or vacuum; absorbed at surfaces it strikes.

Why this matters: the Sun's heat reaches Earth across 150 million km of empty space via INFRARED RADIATION. Conduction and convection cannot work through a vacuum.

Spec 4.8 rule. Dark, dull (matt) surfaces are GOOD emitters AND good absorbers of thermal radiation. Light, shiny (polished) surfaces are POOR emitters AND poor absorbers (but good reflectors).

Implications.

• Radiators are painted matt black or dark colours to EMIT IR effectively. (Modern domestic radiators are sometimes painted white for aesthetics — convection is the dominant transfer route in any case.)
• Saucepans have a polished metal exterior — when not in use, they retain heat (poor emitter).
• A teapot painted white reflects IR back into the tea → tea stays hotter for longer.
• Houses in hot countries are often painted white or have reflective roofs to REFLECT solar IR.
• Survival blankets are made of shiny aluminised plastic to REFLECT body radiation back to the wearer.

Required practical (spec 4.9). Investigate emission with the LESLIE'S CUBE: a hollow cube with four differently-finished faces (matt black, matt white, shiny silver, dull silver) filled with hot water. Use an infrared detector held at a fixed distance from each face → readings prove matt black emits most strongly.

Investigate absorption by aiming an IR lamp at a thermometer painted matt black versus shiny silver — the matt thermometer warms up faster.

Spec 4.9 names three investigations — examiners may ask about any of them.

(1) Conduction. Take rods of similar shape made of different materials (copper, brass, iron, glass). Coat the ends with WAX (or attach drawing pins with wax). Heat one end of each rod uniformly. Time how long the wax melts on the far end of each rod. Best conductor → wax melts fastest.

(2) Convection. Half-fill a beaker with cold water. Drop a single crystal of potassium permanganate (purple dye) at one edge. Heat the water gently directly below the crystal. Observe the purple streak rise vertically, then loop across the top and descend on the other side → tracks the convection current.

(3) Radiation. Two identical metal cans, one painted matt black, one polished. Fill each with boiling water. Place a thermometer in each. Record temperature every 30 s for 10 min. Plot $T$ vs $t$. The black can's curve falls steeper → emits IR faster.

Reducing random error. Repeat each experiment 3 times and take the mean. Control variables: same volume / same starting temperature / same room temperature / same air-flow / identical apparatus apart from the variable being tested.

Most domestic-physics insulation methods work by TRAPPING AIR (a poor conductor) and PREVENTING CONVECTION CURRENTS:

• Loft insulation (fibreglass / mineral wool). Thick fluffy mat in the loft TRAPS pockets of air. Air is a poor conductor → reduces CONDUCTION upwards through the roof.
• Cavity-wall insulation (foam). Foam fills the gap between the inner and outer brick walls. Without foam, convection currents form in the cavity. With foam, both CONDUCTION (air is poor) and CONVECTION (no bulk movement possible) are reduced.
• Double-glazed windows. Two panes of glass separated by a sealed gas (sometimes argon) or partial vacuum. Reduces CONDUCTION (gap is a poor conductor) and CONVECTION (sealed gap can't develop currents). Some windows have low-emissivity coatings that reflect IR back into the room (reducing RADIATION).
• Carpets / curtains. TRAP air; reduce CONDUCTION through the floor and CONVECTION through window gaps.
• Draught excluders. Block air gaps around doors and windows → stop CONVECTION carrying warm air outside.
• Reflective foil behind radiators. Reflects IR RADIATION back into the room.

Cost-effectiveness considerations. Examiners sometimes give energy-saved data and ask candidates to calculate payback time (cost ÷ annual saving). Loft insulation typically pays back in 2-3 years; double-glazing in 10-15 years.