This mixed-methods quasi-experimental study evaluated the impact of Physics Education Technology (PhET) interactive simulations on Grade 11 students\\\' conceptual understanding and engagement in Zambian secondary schools, addressing a critical gap in STEM education within resource-constrained environments characterized by high student-to-teacher ratios and limited laboratory infrastructure. Conducted over four weeks across four government secondary schools— Nelson Mandela and Kafue Day (urban), and Feira and Nyangwena Combined (rural)—the study involved 120 students and 8 physics teachers using a pre-test/post-test non-equivalent control group design. The experimental group (n=60), taught via PhET simulations integrated through a Predict-Observe-Explain (POE) framework, achieved a post-test mean score of 78.5%, significantly outperforming the control group\\\'s 59.8% with a remarkably large Cohen\\\'s d effect size of 1.76 (p < 0.001). Qualitative data from interviews and observations revealed that simulations made \\\"invisible\\\" concepts like electron flow and magnetic flux visible, while teachers demonstrated \\\"Contextual TPACK\\\" by successfully navigating technical barriers such as a 1:15 laptop-to-student ratio and national load-shedding through offline deployment and collaborative \\\"Inquiry Circles.\\\" The study further revealed that rural students achieved nearly identical learning gains to their urban counterparts (+43.8% vs +44.5%), suggesting that PhET simulations act as a \\\"great equalizer\\\" in contexts where physical laboratories are absent. Thematic analysis of teacher interviews identified three key adaptation strategies: resource resilience through collaborative learning, technical agency through offline repositories, and a fundamental pedagogical shift from \\\"lecturer\\\" to \\\"facilitator.\\\" The study concludes that PhET simulations, when embedded within a structured inquiry framework, are a powerful, cost-effective tool for bridging the gap between abstract theory and practical understanding in developing world classrooms. Recommendations include the national scale-up of offline digital repositories, integration of simulation-based inquiry into the national syllabus, specialized teacher training in Contextual TPACK, and revision of examination formats to assess conceptual application rather than factual recall. This research contributes empirical evidence to the limited body of literature on ICT integration in Sub-Saharan African secondary education and provides a replicable model for technology-enhanced science instruction in resource-constrained settings.