Numford

Heat and thermodynamicsClass 11, Thermal Properties of Matter

Dimensional formula of Thermal resistance

[M-1 L-2 T3 K]

SymbolRth SI unitkelvin per watt (K/W)

Temperature difference per unit rate of heat flow. It is the thermal cousin of electrical resistance.

Derivation

How to get the dimensional formula of thermal resistance

  1. Start from the definition: thermal resistance = temperature / 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: [K] / [M L2 T-3].
  9. Add the powers of each base quantity: [M-1 L-2 T3 K].
Practice

Three questions on thermal resistance

1Which is the dimensional formula of thermal resistance?

2What is the SI unit of thermal resistance?

3Do thermal resistance and coefficient of volume expansion have the same dimensions?

Questions

Thermal resistance: quick answers

What is the dimensional formula of thermal resistance?

The dimensional formula of thermal resistance is [M⁻¹ L⁻² T³ K]. It comes from thermal resistance = temperature / power.

What is the SI unit of thermal resistance?

The SI unit of thermal resistance is the kelvin per watt (K/W).

Is thermal resistance dimensionless?

No. Thermal resistance has the dimensions [M⁻¹ L⁻² T³ K].

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