In basic electricity, what is the standard SI unit of electrical resistance?

Introduction / Context:
Electrical resistance is a fundamental concept in circuit theory. It measures how strongly a material or component opposes the flow of electric current. Understanding the correct SI unit of resistance is essential for solving basic circuit problems that use Ohm's law and related formulas. This question tests your recall of the correct unit and your ability to distinguish it from units used for other electrical quantities.

Given Data / Assumptions:
- The physical quantity of interest is electrical resistance.
- The options include ohm, farad, henry and weber.
- We assume standard definitions of SI units used in electricity and magnetism.

Concept / Approach:
Ohm's law states that V = I * R, where V is potential difference in volts, I is current in amperes and R is resistance. The SI unit of resistance is the ohm, symbolised by the Greek letter omega. The farad is the unit of capacitance, henry is the unit of inductance and weber is the unit of magnetic flux. Only one of the options matches resistance directly, which is the ohm.

Step-by-Step Solution:
Step 1: Recall Ohm's law: V = I * R and that resistance is measured in ohms. Step 2: Identify that the ohm is the SI unit that relates volts and amperes in this law. Step 3: Recognise that the farad measures capacitance, not resistance. Step 4: Recognise that the henry measures inductance in coils and inductors. Step 5: Recognise that the weber measures magnetic flux. Step 6: Conclude that the correct SI unit of resistance is the ohm.

Verification / Alternative check:
Dimensional analysis also supports this: one ohm equals one volt per ampere. In practice, resistors are labelled in ohms, kilo-ohms or mega-ohms. Capacitors are labelled in farads or submultiples, coils in henries, and magnetic flux in webers. Observing real components and reading their labels confirms which unit applies to resistance.

Why Other Options Are Wrong:
Farad: This is the SI unit of capacitance, used for capacitors in circuits, not for resistors.
Henry: This is the SI unit of inductance, describing how inductors oppose changes in current, not the basic opposition to steady current.
Weber: This is the SI unit of magnetic flux and is used in electromagnetism, not as the unit for electrical resistance.

Common Pitfalls:
Students sometimes confuse farads and ohms because both appear frequently in circuit diagrams. Another common confusion is between resistance and reluctance or reactance, which relate to magnetism and alternating current behaviour. Focusing on the relationship in Ohm's law and remembering that resistors are rated in ohms helps prevent these mix-ups.

Final Answer:
The standard SI unit of electrical resistance is the ohm.