The first law is just conservation of energy for a gas: whatever heat goes in either raises the internal energy or comes back out as work. Getting the signs right is the whole skill.
A gas expanding at constant pressure does work W = pΔV. The first law, ΔU = q + W, says the increase in internal energy equals the heat added plus the work done on the gas.
When a gas expands at constant pressure it pushes a piston back, doing work W = pΔV. Energy supplied as heat is then shared: some raises the internal energy, some leaves as this work. The simulation lets you set the heat and the expansion and watch the ledger balance.
The first law of thermodynamics is ΔU = q + W, where q is the heat supplied to the gas and W is the work done on the gas. For an expansion at constant pressure the gas does work, so W = −pΔV. The law then handles the special cases cleanly: at constant volume W = 0; in an isothermal change of an ideal gas ΔU = 0; with no heat exchange (adiabatic) ΔU = W.
Four quick checks on work done by a gas and the first law. Each correct answer earns XP and lights this skill on your star map.
When a gas expands at constant pressure p by a volume ΔV, the work done by the gas is:
In the first law ΔU = q + W, the symbol W stands for:
A gas absorbs 500 J of heat and does 200 J of work as it expands. Its internal energy changes by:
During an isothermal change of an ideal gas, the change in internal energy is:
Watch the sign of W. With ΔU = q + W and W the work done on the gas, an expansion gives W negative (the gas does work) and a compression gives W positive. At constant volume W = 0; in an isothermal change ΔU = 0 so q = −W; with no heat exchange ΔU = W.
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