In terms of units, Planck's constant has units of what?

• A. Joule seconds
• B. Joules
• C. Seconds
• D. Newtons

Answer: (A)

Planck's constant serves as the bridge between energy and frequency through the relation E = h f. Since energy has units of joules and frequency has units of inverse seconds (s⁻¹), the proportionality constant must carry units that multiply those together, giving h units of joule-seconds (J·s). You can also see this from the de Broglie relation p = h/λ: momentum p has units kg·m/s and wavelength λ has units m, so h has units (kg·m/s)·m = kg·m²/s, which is the same as J·s (since 1 J = 1 kg·m²/s², so J·s = kg·m²/s). Thus joule-seconds correctly captures the role of h in both energy–frequency and momentum–wavelength relationships. The other options correspond to only energy, only time, or a different physical quantity (force), which don’t fit the dimensional requirements.