Numford

Induction, AC and EM wavesClass 12, Alternating Current

Dimensional formula of Quality factor

[M0 L0 T0]

SymbolQ SI unitnone, a pure number

ωL/R at resonance. Both parts are in ohm, so the ratio is a pure number.

Derivation

How to get the dimensional formula of quality factor

  1. Start from the definition: quality factor = reactance / resistance.
  2. velocity = length / time, so velocity is [L] / [T] = [L T-1].
  3. energy = mass × velocity2, so energy is [M] × [L T-1]2 = [M L2 T-2].
  4. electric charge = electric current × time, so electric charge is [A] × [T] = [T A].
  5. electric potential = energy / electric charge, so electric potential is [M L2 T-2] / [T A] = [M L2 T-3 A-1].
  6. reactance = electric potential / electric current, so reactance is [M L2 T-3 A-1] / [A] = [M L2 T-3 A-2].
  7. electrical resistance = electric potential / electric current, so electrical resistance is [M L2 T-3 A-1] / [A] = [M L2 T-3 A-2].
  8. Put in the dimension of each part: [M L2 T-3 A-2] / [M L2 T-3 A-2].
  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 quality factor with the others in the full grid.

Practice

Three questions on quality factor

1Which of these has the same dimensions as quality factor?

2Which of these has no dimensions at all?

3Which one has different dimensions from the others?

Questions

Quality factor: quick answers

What is the dimensional formula of quality factor?

The dimensional formula of quality factor is [M⁰ L⁰ T⁰]. It comes from quality factor = reactance / resistance.

What is the SI unit of quality factor?

Quality factor has no unit, because it is a pure number.

Is quality factor dimensionless?

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

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