In nuclear physics, nuclear sizes are conventionally expressed using which special unit?

Introduction / Context:
In nuclear physics, scientists deal with extremely small dimensions, much smaller than atomic sizes. Ordinary units like metre or centimetre are not convenient for expressing nuclear radii, so special submultiples are used. This question checks whether you know the standard unit used for nuclear sizes and can distinguish it from units used for atomic dimensions or physical quantities like force and magnetic field strength.

Given Data / Assumptions:

• The question asks specifically about nuclear sizes, that is, the typical dimension or radius of an atomic nucleus.
• Options include Fermi, Angstrom, Newton, and Tesla.
• We use standard definitions as accepted in physics.
• No calculation is needed; this is a conceptual recall question.

Concept / Approach:
Nuclear radii are of the order of 10^-15 metre. A handy unit for such lengths is the fermi, named after the physicist Enrico Fermi. One fermi is equal to 10^-15 metre. In contrast, the angstrom is 10^-10 metre and is more suitable for atomic and molecular dimensions, not nuclear radii. Newton is the SI unit of force, and Tesla is the SI unit of magnetic flux density or magnetic field. Therefore, only one of the listed units is used to express nuclear sizes directly.

Step-by-Step Solution:
Step 1: Recall the approximate size scale of nuclei, which is about 10^-15 metre. Step 2: Remember that the unit fermi (fm) is defined as 1 fermi = 10^-15 metre, which matches the nuclear size scale. Step 3: Recall that 1 angstrom (Å) equals 10^-10 metre, suitable for atoms and molecules, but this is much larger than a typical nucleus. Step 4: Recognise that Newton is a unit of force and Tesla is a unit of magnetic field, so neither expresses length. Step 5: Conclude that the correct unit used to express nuclear sizes is the fermi.

Verification / Alternative check:
Consulting standard nuclear physics relations, nuclear radius R is often given by R = R0 * A^(1/3), where R0 is around 1.2 to 1.4 fermi. This formula explicitly uses fermi as the length unit. Additionally, nuclear scattering experiments and nuclear models quote radii directly in fm. This independent theory and data use confirms that fermi is the appropriate unit. Other length units like picometre (10^-12 metre) or nanometre (10^-9 metre) may appear in advanced contexts, but for classical descriptions of nuclear size, fermi is the conventional choice.

Why Other Options Are Wrong:
Angstrom: Used mainly for atomic and molecular dimensions, approximately 10^-10 metre, too large for nuclear radii.
Newton: Unit of force in the SI system, not a unit of length at all.
Tesla: Unit of magnetic flux density or magnetic field strength, also not a unit of length.

Common Pitfalls:
A frequent error is confusing angstrom with fermi because both are used for very small distances. Students may also ignore what physical quantity a unit represents and choose Newton or Tesla by mistake. To avoid such confusion, always connect each unit with the physical quantity it measures. For nuclear sizes, remember the scale 10^-15 metre and the name fermi associated with Enrico Fermi and nuclear physics.

Final Answer:
Nuclear sizes are typically expressed in the unit Fermi (1 fermi = 10^-15 metre).