Question

Internal resistance of ideal current source is

• A. Zero
• B. Infinity
• C. Unity
• D. 100

Hint:

To remember source characteristics easily, think of their ideal configurations: - An ideal voltage source has zero internal resistance (connected in *series*), preventing internal voltage drops. - An ideal current source has infinite internal resistance (connected in *parallel*), preventing internal current leakage.

Answer 1

The Correct Option is B

Concept: An electrical source can be modeled as either a voltage source or a current source. An ideal current source is a theoretical circuit element that delivers a completely constant current to its connected external load, regardless of the load resistance or the voltage across its terminals. In practical applications, a real current source always exhibits an internal power loss, which is modeled by placing an internal source resistance ($R_{\text{in}}$) in parallel (shunt) with an ideal current source ($I_s$). This arrangement is known as a Norton equivalent circuit. Let us analyze the current distribution using a basic parallel circuit model: Suppose an external load resistor $R_L$ is connected across the terminals of this source. According to the current division rule, the total current $I_s$ splits between its internal parallel resistance $R_{\text{in}}$ and the external load resistance $R_L$: \[ I_L = I_s \cdot \left( \frac{R_{\text{in}}}{R_{\text{in}} + R_L} \right) \] Step-by-step Evaluation for an Ideal Source: An ideal source must deliver 100% of its generated current directly to the load without any internal current leakage, meaning we want $I_L = I_s$ for any arbitrary load value $R_L$. Let us take the mathematical limit of the current division equation as $R_{\text{in}}$ approaches infinity: \[ \lim_{R_{\text{in}} \to \infty} I_L = \lim_{R_{\text{in}} \to \infty} \left[ I_s \cdot \left( \frac{1}{1 + \frac{R_L}{R_{\text{in}}}} \right) \right] = I_s \cdot \left( \frac{1}{1 + 0} \right) = I_s \] This shows that if the internal parallel resistance is infinitely large ($R_{\text{in}} = \infty$), it acts as an open circuit. As a result, zero current leaks internally, and the full current flows entirely through the load. Therefore, an ideal current source must have an internal resistance of infinity ($\infty$), matching Option (2).