# In w=-p∆v why there is a minus sign before p [closed]

In the 1st law of Thermodynamics $\Delta U=q+w$, $w=-p\Delta v$ why there is a minus sign before $p$?

• Because simple rearrangement maths. – Jan Sep 24 '16 at 23:46
• How can you illustrate – Seemant Shekhar Sep 24 '16 at 23:47