Energy and Matter Transformations
Energy and Matter Transformations
This domain tests forces, motion, energy forms and transfers, waves, sound, light and simple electric circuits. Expect calculations, circuit diagrams described in words, and explanations of everyday devices.
Core rules and concepts
- Energy is conserved but transformed: chemical → electrical → light and heat in a torch. Efficiency = useful output ÷ total input × 100%. 'Energy used up' is imprecise; energy is transferred to less useful stores, often thermal.
- Forces change motion or shape. Weight = mass × gravitational field strength (W = mg); mass is constant, weight varies with g. Unbalanced forces cause acceleration; balanced forces mean constant velocity or rest.
- Waves transfer energy without transferring matter. Frequency × wavelength = speed (v = fλ). Sound needs a medium and travels faster in solids than gases. Light travels in straight lines, reflects with angle of incidence = angle of reflection, and refracts toward the normal when entering a denser medium.
- In series circuits current is the same everywhere and voltage splits; in parallel circuits voltage is the same across branches and current splits. Ohm's law: V = IR; power P = VI.
Worked example
A 12 V battery is connected to a 3 Ω resistor. Find the current and power, then explain the energy transfer. I = V/R = 12/3 = 4 A. P = VI = 12 × 4 = 48 W, or I²R = 16 × 3 = 48 W. Reasoning: each coulomb of charge gains 12 J from the battery and loses it across the resistor, mostly as heat; 48 J is transferred each second. If the resistor were replaced by 6 Ω, current would fall to 2 A and power to 24 W, showing that increasing resistance reduces current for a fixed voltage.
Traps that cost marks
Confusing mass and weight; thinking current is 'used up' in a series circuit; saying sound travels faster in air than in steel; drawing refraction bending away from the normal when light enters glass; forgetting to convert mA to A or cm to m before calculating.