Paddlo

Meet Paddlo, a paddle wheel who warms water just by stirring it.

In the 1840s James Joule set out to show that heat and work are two forms of the same thing. He hung weights on strings wound around a spindle. As the weights fell, they spun paddles inside an insulated tank of water, and the churning water became very slightly warmer.

The work done by the falling weights equals the heat gained by the water, W = mcΔT. A 10 kg weight falling 2 m does about 10 × 9.81 × 2 ≈ 196 J of work. Spread through 1 kg of water, with c = 4186 J/(kg·K), it raises the temperature by only about 0.05 K.

Joule needed very careful thermometers and many repeated drops to see such small changes.

His result that a fixed amount of work always gives the same amount of heat helped establish the conservation of energy, and the SI unit of energy, the joule, is named after him.