You might have gone through the term open circuit while solving electrical circuits. Lets see what is a open circuit in electrical circuits.
When any element between two terminals is removed then it is called as open circuit between two terminals. The current in an open circuit is always zero regardless of the voltage across it. The voltage across the open circuit depends upon the circuit condition.
The voltage across the open circuit may be zero, positive or negative. The positive or negative sign in voltage denotes the polarity.
Consider a part of a circuit as shown in figure 1 (i). A circuit element is shown in blue box between two points a and b. A open circuit between points a and b take place by removing circuit element. Hence there is no conducting path between points a and b. This is represented by a open line in the circuit as shown in figure 1 (ii). Now the current (I) must be zero. But voltage (V) may have any value depending upon the circuit conditions.
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