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The dimensions of ratio of energy to Planck's constant are those of

  • A.time
  • B.velocity
  • C.frequency
  • D.linear momentum
  • E.angular momentum

Answer: C

Energy divided by Planck's constant has the dimensions of frequency.

From \(E = h\nu\), \(\frac{E}{h} = \nu\). Dimensionally, h is energy times time (J s), so

\[ \begin{aligned} \frac{[E]}{[h]} &= \frac{[\text{ML}^{2}\text{T}^{-2}]}{[\text{ML}^{2}\text{T}^{-1}]} \\ &= [\text{T}^{-1}] \end{aligned} \]

That is the dimension of frequency. Time is the reciprocal. Planck's constant itself has the same dimensions as angular momentum, which is why angular momentum is a natural trap, but the ratio E/h is not angular momentum. Velocity and linear momentum include length and mass, which cancel here.

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