First Law of Thermodynamics

First Law of Thermodynamics

• The first law of thermodynamics concerns the internal energy of a gas and how it changes.

Internal Energy (U)

• Definition: Internal energy (U) refers to the total thermal energy contained within a gas.

• Components:

  • Composed of the kinetic energy of all the particles within the gas.

  • Represents the sum of the kinetic energies of the individual particles.

Changes in Internal Energy

• The internal energy of a gas can change in two primary ways:

  1. Gain or Loss of Heat (Q)

     • If the gas is heated (e.g., above a Bunsen burner), it gains thermal energy:

       • Internal energy (U) increases.

       • Temperature increases; particles move faster.

     • If the gas loses heat, it cools down:

       • Internal energy (U) decreases.

       • Temperature decreases; particles slow down.

  2. Work Done (W)

     • Work Done by the Gas: When the gas expands, it does work (e.g., pushing a piston).

       • This results in a decrease in internal energy.

     • Work Done on the Gas: If work is done on the gas (e.g., compressing it), its internal energy increases.

Equation of the First Law of Thermodynamics

• Fundamental Equation:

  [ Q = \Delta U + W ]

  Where:

  • Q = Heat added to the gas

  • \Delta U = Change in internal energy

  • W = Work done by the gas

• Conservation of Energy: The energy provided to the gas must result in an increase in internal energy or work being done by/on the gas.

Example Application

• If heat is added to the gas, and it does no work (W=0):

  • [ Q = \Delta U ]

  • Internal energy increases as temperature rises.

• In an adiabatic process:

  • When Q = 0 (no heat exchange), the equation simplifies to:

  • [ 0 = \Delta U + W ]

  • Indicates that work done by the gas results in a drop in internal energy.

Practical Insights

• Compression of Air in a Bicycle Pump:

  • Work is done on the air as it is compressed.

  • Resulting increase in internal energy causes the air to heat up.

• Operation of a Fridge:

  • When the fridge turns on, internal energy of the air inside decreases, causing it to get colder.

  • The motor does work to move the refrigerant, thus lowering the internal energy.

Summary of Key Concepts

• The internal energy of a gas can increase either by adding heat or by doing work on it.

• Understanding of terms such as internal energy (\Delta U), heat (Q), and work (W) is crucial for grasping the first law of thermodynamics.