Question

In case of ideal current sources, they have ____.

• A. zero internal resistance
• B. low value of voltage
• C. large value of current
• D. infinite internal resistance

Hint:

Contrast this with voltage sources: - Ideal Voltage Source: Zero internal resistance (connected in series). - Ideal Current Source: Infinite internal resistance (connected in parallel).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
An ideal current source is a theoretical circuit element that maintains a constant current regardless of the voltage across its terminals or the load resistance connected to it.

Step 2: Detailed Explanation:
Internal resistance in a source is considered to be in parallel with the current.
• If the internal resistance were low or zero, the current would bypass the external load and flow through the internal path (short circuit).
• For a source to deliver $100\%$ of its current to any external load, its internal parallel path must be a complete "open circuit."
• An open circuit has infinite resistance ($\infty\,\Omega$). Therefore, an ideal current source must have infinite internal resistance to ensure the current remains independent of the external circuit.

Step 3: Final Answer:
Ideal current sources have infinite internal resistance.

Answer 2

An ideal current source is best understood by contrasting it with its dual, the ideal voltage source, and checking what property each option would imply about the source's internal parallel resistance.

1. Zero internal resistance: A zero internal resistance placed in parallel with the source would provide a direct short-circuit path for the current to bypass any external load entirely, meaning none of the current would actually reach the load. This is in fact the defining property of an ideal voltage source's internal series resistance, not a current source's internal parallel resistance.
2. Low value of voltage: The terminal voltage of an ideal current source is not fixed at all, it adjusts automatically depending on whatever external resistance is connected, so as to always deliver the same fixed current. There is no sense in which it has a defining low voltage.
3. Large value of current: The magnitude of the current a source supplies is just a rating, it could be 1 A or 100 A, and it says nothing about the internal structure that makes the source behave ideally. This is not a defining characteristic of ideal behaviour.
4. Infinite internal resistance: For all of the source's current to reach the external load regardless of the load's resistance, the internal parallel path must offer no alternative route at all, that is, an open circuit, which corresponds to infinite resistance. This is exactly what makes the source's output current independent of the external circuit, the defining property of ideal.

Contrasting with the ideal voltage source, zero series resistance, confirms that the current source's ideal condition instead requires infinite parallel resistance.

Therefore, the correct answer is infinite internal resistance.