Portable boiling elements that can boil small quantities of water are useful when the right voltage of electricity is available.
如果有相配的电压，可以煮少量水的便携式热水装置是有用的。
The useful power in ac circuit also depends on the current and voltage in the circuit.
交流电路中的有用功率还取决于该电路中的电流和电压。
This is useful for detecting when a signal gets stuck at a particular voltage.
这是非常有用的对于检测一信号什么时候滞留在一特定电压。
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