Cheat Sheet | Topic 4 Energy

Formula grid

$g=10\,\text{N/kg}$. $1\,\text{kJ}=1000\,\text{J}$. $1\,\text{kW}=1000\,\text{W}$. Work done = energy transferred.

Chemical · kinetic · gravitational · elastic · thermal · magnetic · electrostatic · nuclear.

There is no electrical store. Electricity is a pathway.

Mechanically · electrically · by heating · by radiation (light and sound).

Wasted energy usually ends in the thermal store of the surroundings.

	Conduction	Convection	Radiation
Needs	particles (solids best)	a fluid that can move	nothing — infrared
Mechanism	lattice vibrations; free electrons in metals	heat → expand → less dense → rise	EM waves; hotter emits more
Cannot	cross a vacuum	happen in a solid or a vacuum	be stopped by a vacuum

Dull black: best emitter and absorber. Shiny silver: worst emitter/absorber, best reflector.

Loft / cavity foam / double glazing: trapped air is a poor conductor; foam stops convection in the gap.
Draught excluders: stop convection through gaps.
Foil behind radiators: reflects infrared.
Vacuum flask: vacuum (no conduction/convection) + silvering (radiation) + stopper (convection at the neck).

No drag: $mgh=\tfrac{1}{2}mv^2$. With friction: $mgh=KE+W_{\text{friction}}$.

Forgetting to square $v$ in $\tfrac{1}{2}mv^2$.
Writing efficiency as $0.65$ when they asked for $65\%$, or the other way round.
Calling convection in a solid or “heat rises” with no density.
Using $W=Fd$ when $d$ is not in the force direction (or using ramp length instead of vertical $h$ in $mgh$).
Mixing energy (J) and power (W).
Inventing an electrical store; saying energy is “lost” as if it vanished.
Efficiency over $100\%$ — you inverted useful and total.

4.18P–4.19P (generating electricity) are not on this Double Award course.