Energy in systems (thermal, chemical, electrical) Knowledge-Based Study

Study notes

1. Understanding Physical Systems

   In physics, a system is defined as an object or a collection of objects being studied, separated from the rest of the universe (the surroundings) by an identifiable boundary.

   • Closed system: A system across whose boundary energy may be transferred, but matter cannot enter or leave. The total energy inside a closed, isolated system remains constant.
   • Open system: A system that can exchange both matter and energy with its surroundings.

   Energy within a system is held in specific stores (such as thermal, chemical, kinetic, or gravitational potential) and changes form or transfers between stores via mechanisms like mechanical work, heating, radiation, or electrical currents.

   Microscopic Motion and Thermal Stores

   Thermal energy refers to the total internal kinetic and potential energy associated with the random motion and arrangement of atoms or molecules within a substance.

   • Temperature vs. Thermal Energy: Temperature measures the average kinetic energy of the individual particles, whereas thermal energy accounts for both the average kinetic energy and the total mass or number of particles present.
   • Heat Transfer: When a temperature difference exists between two points within a system or between a system and its surroundings, energy flows naturally from regions of higher temperature to regions of lower temperature via conduction, convection, or thermal radiation.
   • Energy Dissipation: In physical processes involving mechanical friction or electrical resistance, useful energy is inevitably dissipated into the surroundings as internal thermal energy.

   Molecular Bonds and Reaction Energetics

   Chemical energy is potential energy stored in the electrostatic attractions between atoms and chemical bonds within molecules.

   • Breaking and Forming Bonds: Breaking chemical bonds requires an input of energy (endothermic step), while forming new chemical bonds releases energy (exothermic step).
   • Net Energy Changes: If the energy released in bond formation exceeds the energy required to break the initial bonds, the net process is exothermic, converting chemical potential energy into thermal, radiant, or electrical energy.
   • Common Chemical Stores: Fuels (such as hydrocarbons), electrochemical cells (batteries), and biological food stores store significant amounts of accessible chemical energy that systems convert into other forms upon reaction.

   Charge Movement and Power Delivery

   Electrical energy is the energy transferred by the movement of electric charges under the influence of an electric potential difference (voltage).

   • Electrical Work: When a charge $Q$ moves across a potential difference $V$, the work done is given by: $$W = Q V = I V t$$ where $I$ is electric current and $t$ is time.
   • Circuit Transformations: An electric circuit functions as an energy transfer pathway. An input source (such as a battery converting chemical energy) establishes an electric field that does work on charge carriers, which then transfer energy to circuit elements such as heating elements (thermal energy), motors (mechanical kinetic energy), or lamps (radiant energy).

   Energy Accounting across System Transformations

   The Law of Conservation of Energy states that energy cannot be created or destroyed; it can only be transformed from one store to another or transferred between systems.

   • Input-Output Balance: For any closed system, $\Delta E_{\text{system}} = E_{\text{in}} - E_{\text{out}}$. If no energy enters or leaves, $\Delta E_{\text{system}} = 0$.
   • System Efficiency: Real systems convert only a portion of the input energy into the intended useful form. Efficiency is quantified as: $$\text{Efficiency} = \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \times 100\%$$
   • Coupled Example: In an electric kettle, electrical work transfers energy into the heating element. Conduction transfers this energy to water, increasing its internal thermal store. Concurrently, thermal energy dissipates into the ambient air, accounting for the system's efficiency deficit.