Which of the following is a basic Unit?

1. Analyze the question

The question asks you to identify which of the given options is a basic (fundamental) unit in the International System of Units (SI).

2. Explain the concept

In physics, units are categorized into two types:

• Fundamental (Basic) Units: These are the building blocks of all other units. They are independent and cannot be derived from other physical quantities. There are seven base units in the SI system: Length (meter), Mass (kilogram), Time (second), Electric Current (ampere), Temperature (kelvin), Amount of substance (mole), and Luminous intensity (candela).
• Derived Units: These are units formed by combining fundamental units through multiplication or division. For example, a Joule is a derived unit because it is defined as $kg \cdot m^2/s^2$.

3. Show the solution

Let’s evaluate each option:

• A. Joule: This is the unit of energy. It is a derived unit ($1 \text{ Joule} = 1 \text{ kg} \cdot \text{m}^2/\text{s}^2$).
• B. Volt: This is the unit of electric potential difference. It is a derived unit ($1 \text{ Volt} = 1 \text{ Joule/Coulomb}$).
• C. Ohm: This is the unit of electrical resistance. It is a derived unit ($1 \text{ Ohm} = 1 \text{ Volt/Ampere}$).
• D. Ampere: This is the SI unit for electric current. It is one of the seven fundamental units defined by the SI system.

4. Identify the answer

Since the Ampere is one of the seven fundamental base units, it is the correct choice.

5. Tip for your exam

Remember the acronym "L-M-T-A-K-M-C" to recall the seven fundamental units:

• Length (meter)
• Mass (kilogram)
• Time (second)
• Ampere (current)
• Kelvin (temperature)
• Mole (amount of substance)
• Candela (luminous intensity)

If a unit is not on this list, it is almost certainly a derived unit!