Ib Physics 5.2 95%

Alternatively, substituting (I = V/R) gives:

[ P = IV ]

In the macroscopic world, we often observe that electrical devices—from a simple toaster to a supercomputer—become warm during operation. This phenomenon is not merely a nuisance or a byproduct of inefficiency; it is a fundamental manifestation of energy transfer governed by the principles of electromagnetism and thermodynamics. IB Physics Topic 5.2, "Heating Effect of Electric Currents," explores the precise relationship between electrical work and internal energy, introducing core concepts such as electrical power, resistance, Ohm’s law, and the distinction between direct current (DC) and alternating current (AC) in practical applications. The Origin of Heating: Resistance and Collisions At the heart of the heating effect is electrical resistance . When a potential difference (voltage) is applied across a conductor, it establishes an electric field that accelerates free electrons. However, these electrons do not move unimpeded; they continuously collide with the fixed, vibrating positive ions of the metallic lattice. Each collision transfers kinetic energy from the electron to the ion. Consequently, the amplitude of vibration of the ions increases, which is macroscopically observed as a rise in temperature—an increase in the internal energy of the material. Thus, resistance is the property that converts organized electrical work into disordered thermal energy. Power and Energy: The Fundamental Equations The rate at which this heating occurs is defined as electrical power ((P)). The IB syllabus emphasizes that for any component, the power dissipated (as heat or light) is the product of the voltage ((V)) across it and the current ((I)) through it: Ib Physics 5.2

[ P = \frac{V^2}{R} ]

These three equations are not interchangeable in all contexts. The form (P = I^2 R) is the most fundamental for heating because it explicitly shows that for a given current, heating increases linearly with resistance. Conversely, (P = V^2 / R) shows that for a fixed voltage (e.g., mains supply), a lower resistance produces more power—which explains why a short circuit (very low (R)) causes dangerously high power and fire. A critical refinement in Topic 5.2 is the concept of internal resistance ((r)). No real source of emf (electromotive force, (\varepsilon)), such as a battery or generator, is perfect. Internal resistance represents the inherent opposition to current flow within the source itself. When a current (I) flows, the terminal voltage (V_t) is less than the emf: Alternatively, substituting (I = V/R) gives: [ P

[ P = I^2 R ]

[ V_t = \varepsilon - Ir ]