Core Law: Energy cannot be created or destroyed; it can only be transferred from one store to another.
Mathematical Implication: For a closed system, . The total energy remains constant ().
Application: This principle allows us to equate energy loss in one store (e.g., gravitational potential) to energy gain in another (e.g., kinetic), assuming no dissipation to the surroundings.
Pathways describe the process of moving energy between stores. They are the mechanisms of change.
Mechanically: Energy transferred via a force acting over a distance (e.g., pushing, pulling, gravity acting on a falling object).
Electrically: Energy transferred by a charge moving through a potential difference (e.g., current in a circuit).
Heating: Energy transferred from a hotter region to a colder region (via conduction, convection).
Radiation: Energy transferred via waves (e.g., light, sound, infrared).
To describe any physical process, follow this three-step framework:
Identify the Source Store: Where is the energy coming from? (e.g., Chemical store of a battery).
Identify the Destination Store: Where is the energy going to? (e.g., Thermal store of a resistor).
Identify the Pathway: How did it get there? (e.g., Electrically).
Standard Description Format: "Energy is transferred [Pathway] from the [Source Store] to the [Destination Store]."
Store vs. Pathway: A store is a 'noun' (a state of being), while a pathway is a 'verb' (a process of doing). You cannot 'store' energy in 'electrical'—electrical is the transport method.
System Definition: In exams, defining the system prevents infinite regression. You don't need to trace energy back to the Big Bang; only trace back to the start of the specific scenario defined by the question.
Dissipation: In real-world (open) systems, energy is often transferred to the thermal store of the surroundings. This is often termed 'wasted' energy, but it is still conserved, just spread out.
