Ib Physics 5.2 Apr 2026
[ V_t = \varepsilon - Ir ]
[ P = IV ]
These are defined such that an AC circuit dissipates the same average power in a resistor as a DC circuit with (I_{\text{rms}}) and (V_{\text{rms}}). Thus, (P_{\text{avg}} = I_{\text{rms}}^2 R = V_{\text{rms}} I_{\text{rms}}). This concept is essential for understanding household electricity: a 230 V AC mains supply means (V_{\text{rms}} = 230) V, with a peak voltage of about 325 V. The heating effect is harnessed in resistive devices like kettles, ovens, and incandescent bulbs (which operate at high temperatures, emitting visible light as a byproduct of heat). However, it also poses challenges. In long-distance power transmission, heating losses ((P_{\text{loss}} = I^2R)) are minimized by stepping up voltage (thereby reducing current) using transformers—a concept linking Topic 5.2 with Topic 5.4 (Magnetic Effects). Furthermore, circuit breakers and fuses rely on the heating effect: excessive current melts a fuse wire or triggers a bimetallic strip, breaking the circuit and preventing fire. Conclusion Topic 5.2 reveals that the heating effect of electric currents is not a mere accident but a predictable consequence of the conversion of electrical potential energy into internal thermal energy via collisions in a resistive medium. By mastering the relationships (P = IV), (P = I^2R), and (P = V^2/R), along with the real-world complication of internal resistance and the statistical equivalence of AC and DC via rms values, students gain a powerful toolkit. This knowledge not only explains why devices warm up but also underpins the design of efficient power systems and safe electrical installations—demonstrating how a microscopic collision of an electron with an atom scales up to light a city or charge a phone. Ib Physics 5.2
Since energy ((E)) is power multiplied by time, the electrical work converted into heat over time (t) is (E = IVt). [ V_t = \varepsilon - Ir ] [
[ P = \frac{V^2}{R} ]
The lost volts ((Ir)) are dissipated as heat inside the source. This explains why batteries become warm during heavy use and why a car battery’s voltage drops when starting the engine. The maximum power transfer theorem (often a HL extension) states that to extract maximum power from a source, the load resistance must equal the internal resistance, but this condition results in 50% efficiency—half the power is wasted as heat inside the source. The heating effect behaves differently under DC and AC. With DC, the current is constant, so the power dissipation is steady: (P = I^2R). With AC, the current varies sinusoidally. Since heating depends on (I^2), the average power is not zero (even though the average current over a cycle is zero). IB Physics introduces the root-mean-square (rms) values for AC: The heating effect is harnessed in resistive devices
