Topic explanation

Standard explanation

An isobaric process occurs at constant pressure. Heat transfer can change internal energy and produce expansion work, so Q = ΔU + PΔV.

Key points

• Pressure remains constant: P₂ = P₁.
• Work done by the gas is W = PΔV.
• The first law becomes Q = ΔU + PΔV.
• During heating, both volume and internal energy may increase.
• The P–V graph is a horizontal line called an isobar.
• The rectangular area under an isobar equals work.

Labelled diagram