This is easily and rapidly discharged, so capacitors have good power density (the rate at which they transfer energy, per unit of weight).

比较容易，也比较快速，所以电容器有着良好的功率密度（每单位重量的能量传递速率）。

Now you'll notice that because people didn't usually carry around a standard set of weights they used coins as both the unit of worth and the unit of weight.

现在您会注意到，由于人们通常不会随身携带一套标准的砝码，所以他们使用硬币作为价值单位和重量单位。

Coal contains less energy per unit of weight than weight than natural gas or oil; it is difficult to transport, and it is associated with a host of environmental issues, among them acid rain.

煤的单位能量含量低于石油和天然气，而且会导致包括酸雨在内的一系列环境问题。