In electricity and magnetism, the quantity ampere second (A·s) is the unit of which physical quantity?

Introduction / Context:
In basic electrical science, many derived units are formed by combining base units like ampere, second, metre, and volt. Understanding what each combination represents helps in interpreting formulas and circuit calculations correctly. The product ampere second (A·s) appears often when relating current and time. This question asks which physical quantity has ampere second as its unit in the SI system.

Given Data / Assumptions:

• The unit considered is ampere multiplied by second, written as A·s.
• We assume standard SI definitions.
• Possible physical quantities listed include power, energy, conductance, and electric charge.
• We recall the basic relationships between current, charge, and time.

Concept / Approach:
Electric current is defined as the rate of flow of electric charge through a conductor. Mathematically, current I equals charge Q divided by time t, or I = Q / t. Rearranging this gives Q = I * t. If current is measured in amperes (A) and time in seconds (s), then Q is measured in ampere seconds, which is equal to coulombs (C). Therefore, ampere second is the unit of electric charge. Power, energy, and conductance have different unit combinations and do not match A·s.

Step-by-Step Solution:
Step 1: Recall the definition of current: I = Q / t, where Q is electric charge and t is time. Step 2: Rearrange this expression to obtain Q = I * t. Step 3: Note that in SI, current I is measured in amperes (A) and time t in seconds (s). Step 4: Substitute units into Q = I * t to get units of Q as A * s, which is ampere second. Step 5: Recognise that one ampere second is equal to one coulomb, the standard SI unit of electric charge, so ampere second represents charge.

Verification / Alternative check:
Check other quantities. Power P is defined as energy per unit time, and in SI its unit is watt, where 1 watt = 1 joule per second. Energy has units of joules, which can be expressed as volt coulomb or newton metre, but not simple ampere second. Conductance is the reciprocal of resistance and has SI unit siemens, which is ampere per volt. None of these match A·s. This confirms that ampere second is associated with electric charge only.

Why Other Options Are Wrong:
Power: Its unit involves joules per second, not ampere second. Energy: Measured in joules, which correspond to newton metre or volt coulomb, not directly A·s. Electrical conductance: Measured in siemens (A/V) rather than ampere second.

Common Pitfalls:
Learners sometimes see ampere and immediately think of current, forgetting that ampere second arises when integrating current over time. Another confusion is between coulomb and volt, where coulomb measures amount of charge and volt measures potential difference. Keeping track of basic formula I = Q / t and its unit analysis helps avoid these errors and solidifies understanding of derived units.

Final Answer:
The quantity whose SI unit is ampere second is electric charge.