Determine 
, 
and 
using the superposition method.
Solution
I. Contribution of the 
voltage source:
We need to turn off the current source by replacing it with an open circuit. Recall that we do not turn off dependent sources. The resulting circuit is shown below.
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Determine 
and 
using the superposition method.
Solution
I. Contribution of the 
voltage source:
To find the contribution of the voltage source, other three sources should be turned off. The 
voltage source should be replaced by short circuit. The current source should be replaced with open circuits, as shown below.
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Turning off a source, which is usually used in solving circuits with superposition method, means setting its value equal to zero. For a voltage source, setting the voltage equal to zero means that it produces zero voltage between its terminals. Therefore, the voltage source must insure that the voltage across two terminals is zero. Replacing the source with a short circuit can do that. Thus, voltage sources become a short circuit when turned off.
For a current source, setting the current equal to zero means that it produces zero current. Therefore, the current source must insure that no current flows through its branch. An open circuit can do that. Hence, to turn off a current source it should be replaced by an open circuit.
How about dependent sources? The voltage/current of a dependent source is dependent on other variables of the circuit. Therefore, dependent sources cannot be turned off.
Example I: Turn off sources one by one.
Solution:
I) The voltage source:
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