Which statement correctly describes Norton versus Thevenin equivalents?

• A. Norton uses a voltage source in series with resistance; Thevenin uses a current source in parallel with resistance.
• B. Norton and Thevenin are completely different; they cannot be converted.
• C. Norton uses a current source in parallel with resistance; Thevenin uses a voltage source in series with resistance.
• D. Norton uses a current source in series with resistance; Thevenin uses a voltage source in parallel with resistance.

Answer: (C)

Think about how a linear two-terminal network can be represented from the outside. There are two standard forms that give the same behavior at the terminals: Norton form uses an ideal current source in parallel with a resistor, while Thevenin form uses an ideal voltage source in series with a resistor. These two are interchangeable for the external circuit: the resistor value is the same in both representations, and the relationship between the two sources is Vth = In × Rth (or In = Vth / Rth). This is why you can transform between them by a simple source transformation: a voltage source in series with a resistor can be converted to a current source in parallel with the same resistor, and vice versa. That makes the description of Norton as a current source in parallel with resistance and Thevenin as a voltage source in series with resistance the correct pairing. The other descriptions mix up series and parallel or incorrectly swap the roles of the sources, which breaks the equivalent behavior at the terminals.