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  1. Glossary
  2. /Physics
  3. /First Law of Thermodynamics

What is First Law of Thermodynamics?

In short

The first law of thermodynamics says that the change in the internal energy of a system equals the heat added to it minus the work the system does: ΔU=Q−W\Delta U = Q - WΔU=Q−W.

In simple words

It is the law of conservation of energy for heat and work. Heat QQQ is positive when it flows into the system. Work WWW is positive when the system does work on its surroundings, as when a gas expands. Chemists often write ΔU=q+w\Delta U = q + wΔU=q+w, where www is the work done on the system. State which sign convention you use, because the two give the same physics with different signs.

Example

A gas takes in 500 J500\ \text{J}500 J of heat and does 200 J200\ \text{J}200 J of work in expanding. Its internal energy rises by ΔU=500 J−200 J=300 J\Delta U = 500\ \text{J} - 200\ \text{J} = 300\ \text{J}ΔU=500 J−200 J=300 J.

Also searched as: first law of thermodynamics diagram.

Learn it properly

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Related terms

  • Isothermal Process
  • Adiabatic Process

← All glossary terms

Learn it in

  1. Heat, Work and the First LawClass 11 to 12
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Practise

  1. A sample of a monatomic ideal gas is taken from state A, with volume 1.0 L and pressure…Advanced
  2. Two rigid, insulated vessels are joined by a thin tube with a closed valve, as shown in…Hard
  3. A gas goes clockwise round a triangle ABCA on a pressure-volume graph…Hard
1.5 mol of an ideal gas expands slowly at a steady 320 K from 8.0 to 24 litres…Medium
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Quick questions

  1. One mole of an ideal monatomic gas is at 300 K and 1.0 × 10⁵ Pa in a thermally insulated…Advanced
  2. A gas is taken round a cycle of three steps. From A to B its internal energy increases…Hard
  3. One mole of an ideal monatomic gas is warmed by 100 K in a process for which its molar…Hard

More Physics terms

  1. Acceleration
  2. Acceleration Due to Gravity (g)
  3. Amplitude
  4. Angle of Incidence
  5. Angle of Reflection
  6. Archimedes' Principle
All Physics terms →

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