In electricity and electronics, the farad (F) is the SI unit of which physical quantity?

Introduction / Context:
Many physical quantities in electricity have specific SI units, such as coulomb for charge, ohm for resistance and farad for another important quantity. Recognising these units is essential for understanding circuit diagrams, component ratings and formulas. This question focuses on the farad, denoted by the symbol F, and asks which quantity it measures.

Given Data / Assumptions:

• The farad (F) is an SI derived unit used in circuit analysis.
• Other related quantities include electric charge, conductance and reactance.
• We assume conventional definitions from basic physics and electronics.
• Capacitors in circuits are often marked with values such as microfarads (µF) or picofarads (pF).

Concept / Approach:
Capacitance is the property of a system that describes its ability to store electric charge per unit potential difference. For a simple capacitor, capacitance C is defined as C = Q / V, where Q is the magnitude of charge stored and V is the potential difference across the plates. The SI unit of charge is coulomb (C), and the SI unit of potential difference is volt (V), so the unit of capacitance is coulomb per volt, which is named the farad. Reactance, electric charge and conductance each have their own distinct units.

Step-by-Step Solution:
Step 1: Recall the definition of capacitance: C = Q / V. Step 2: Identify that Q is measured in coulombs and V in volts, so C has units of coulomb per volt. Step 3: Recognise that this derived unit is named the farad (F). Step 4: Compare with electric charge, whose unit is coulomb (C), not farad. Step 5: Recall that conductance, the reciprocal of resistance, has the unit siemens (S). Step 6: Note that reactance, like resistance, is measured in ohms (Ω), not in farads.

Verification / Alternative check:
Look at any capacitor data sheet or the markings on a small capacitor. You will typically see values like 100 nF, 10 µF or 220 pF. All of these abbreviations refer to nanofarads, microfarads and picofarads, confirming that capacitance is measured in farads. Circuit analysis formulas such as Q = C * V and energy stored in a capacitor E = (1/2) * C * V^2 further confirm this relationship between capacitance, charge and voltage.

Why Other Options Are Wrong:
Reactance: This is the opposition offered by a capacitor or inductor to alternating current and, like resistance, is measured in ohms (Ω).

Electric charge: Charge is measured in coulombs (C), not in farads.

Electric conductance: Conductance has the SI unit siemens (S), which is the reciprocal of ohm.

Common Pitfalls:
New learners sometimes mix up farad with coulomb because both appear in the formula C = Q / V. It helps to remember that C is the symbol for both charge and capacitance in different contexts, so you must pay attention to the formula and units. The farad is often a large unit in practice, which is why actual capacitors are rated in microfarads or nanofarads rather than whole farads.

Final Answer:
The farad (F) is the SI unit of capacitance.