Electromagnetic field basics in circuit context: an electromagnetic field exists only under which condition?

Introduction / Context:
In introductory electrical engineering, learners often mix up when electric fields, magnetic fields, and full electromagnetic fields appear around conductors. This question targets the everyday circuit context: when does an electromagnetic field (with a magnetic component tied to charge motion) exist around a wire or device? Understanding this is vital for power electronics, signal integrity, and electromagnetic compatibility.

Given Data / Assumptions:

• The discussion is in the context of practical circuits and conductors.
• Conductors are assumed to be surrounded by free space or dielectric materials.
• We focus on the presence of current and its role in producing magnetic fields and thus an electromagnetic field around the conductor.

Concept / Approach:
In circuit practice, a steady current produces a steady magnetic field encircling the conductor (right-hand rule). The combination of the electric potential distribution (due to charges) and the magnetic field (due to moving charges) is commonly referred to as the electromagnetic field surrounding the conductor. While a static voltage establishes an electric field, the distinctive magnetic component that completes the electromagnetic picture appears with current flow.

Step-by-Step Solution:

Verification / Alternative check:
Observe a current-carrying conductor with a magnetic compass; deflection occurs only when current flows, confirming the magnetic component. With only a disconnected high-voltage source and no conduction path, there is an electric field but no surrounding magnetic field due to current.

Why Other Options Are Wrong:

• Voltage: Voltage alone does not guarantee a magnetic field; it primarily establishes an electric field.
• An increasing current / decreasing current: These describe time variation (which changes the field magnitude), but a magnetic field exists even for steady current.
• None of the above: Incorrect because current is the right condition in basic circuit context.

Common Pitfalls:

• Equating any presence of voltage with a full electromagnetic field including a magnetic component.
• Assuming only changing current creates fields; in fact, steady current creates a steady magnetic field.

Final Answer:
current