First law of thermodynamics (A-level only) Flashcards – AQA A-level Physics (7408)

What conservation principle does the first law of thermodynamics express?

Conservation of energy for a system undergoing energy transfer by heating and work.

What is the first-law equation relating heating $Q$, change in internal energy $\Delta U$, and work done by the system $W$?

$Q = \Delta U + W$

In the first-law sign convention, when is heating $Q$ positive or negative?

Positive for heating into the system; negative for heating out of it.

In the first-law sign convention, when is work $W$ positive or negative?

Positive for work done by the system; negative for work done on it.

What do positive and negative values of $\Delta U$ indicate?

Positive: internal energy increases. Negative: internal energy decreases.

What unit is used for $Q$, $\Delta U$ and $W$ in the first law of thermodynamics?

Joules ($\mathrm{J}$).