Numford

Heat and thermodynamicsClass 11, Thermal Properties of Matter

Dimensional formula of Emissivity

[M0 L0 T0]

Symbole SI unitnone, a pure number

Power radiated divided by the power a perfect black body would radiate, so it is a pure number.

Derivation

How to get the dimensional formula of emissivity

  1. Start from the definition: emissivity = power / power.
  2. velocity = length / time, so velocity is [L] / [T] = [L T-1].
  3. acceleration = velocity / time, so acceleration is [L T-1] / [T] = [L T-2].
  4. force = mass × acceleration, so force is [M] × [L T-2] = [M L T-2].
  5. displacement = length, so displacement is [L] = [L].
  6. work = force × displacement, so work is [M L T-2] × [L] = [M L2 T-2].
  7. power = work / time, so power is [M L2 T-2] / [T] = [M L2 T-3].
  8. Put in the dimension of each part: [M L2 T-3] / [M L2 T-3].
  9. Add the powers of each base quantity: [M0 L0 T0].
Same dimensions

Quantities with the dimensions [M⁰ L⁰ T⁰]

Equal dimensions do not make them the same quantity. Compare emissivity with the others in the full grid.

Practice

Three questions on emissivity

1Which of these has the same dimensions as emissivity?

2Which of these has no dimensions at all?

3Which one has different dimensions from the others?

Questions

Emissivity: quick answers

What is the dimensional formula of emissivity?

The dimensional formula of emissivity is [M⁰ L⁰ T⁰]. It comes from emissivity = power / power.

What is the SI unit of emissivity?

Emissivity has no unit, because it is a pure number.

Is emissivity dimensionless?

Yes. Emissivity is a ratio of like quantities, so every power in its dimensional formula is zero: [M⁰ L⁰ T⁰].

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