都是传数据，为什么 USB 线会发热而网线很凉快？

Core Viewpoint - The article discusses the reasons behind the heat generation in USB cables compared to Ethernet cables, highlighting the differences in their design and functionality [1][2][3][4]. Group 1: USB Heat Generation - USB interfaces and cables are designed to provide both power and data transmission, leading to significant heat generation, especially during fast charging where current can reach up to 5A [1][3]. - The presence of micro contact resistance at the metal contacts of USB interfaces can increase heat due to oxidation or dirt accumulation over time [1]. - The compact design of USB Type-C connectors makes it difficult for heat to dissipate, causing heat to accumulate at the interface [1]. Group 2: Ethernet Signal Transmission - Ethernet cables do not provide power; instead, they rely on separate power adapters, resulting in minimal heat generation during data transmission [2]. - Ethernet uses differential signaling, which enhances its ability to resist interference, allowing for low voltage swings (around ±1V) and requiring very low current (typically a few milliamps), thus generating negligible heat [2]. Group 3: Power over Ethernet (PoE) - PoE technology allows for both power and data transmission over a single cable, but the maximum current is limited to 2A, significantly lower than USB fast charging, resulting in less heat generation [4]. - The article provides a comparison of different PoE standards, indicating that even under maximum load, PoE devices typically only generate mild warmth [4][5].