## Change in Internal Energy by Performing Work: Nail Hammering

### Experiment number : 1705

• #### Goal of experiment

This experiment shows the temperature increase when a nail is hammered into a piece of wood.

• #### Theory

The first law of thermodynamics states that:

$\Delta U\,=\,W\,+\,Q,$

where ΔU is the change in internal energy of the system, W is the work supplied to the system and Q is the supplied heat. The first law of thermodynamics in this form allows a change in internal energy of any thermodynamic system (provided that the system does not exchange particles with the surroundings) by only two ways – through a heat exchange and by performing mechanical work. In our case the work is performed by the hammer. The friction between the nail and the wooden board results in excitation of particles on the contact surfaces of the metal and the wood, thereby increasing the temperature of the two materials.

• #### Tools

Thermal imaging camera, wooden board, hammer, nail (Fig. 1).

• #### Procedure

We simply hammer a nail into a board. At the same time we use the thermal imaging camera to observe the temperature increase at the point where the nail penetrates the wood. A place in which the temperature increases, according to the infrared image, is the place where the nail penetrates the wood. The nail is almost invisible on the video; the nail temperature during the experiment is similar to the ambient temperature.

• #### Sample result

The video below illustrates a successful performance of this experiment.

In this video a thermal imaging camera FLIR i7 was used. The temperature range of the colour scheme was chosen in the interval 24 °C to 30 °C, the emissivity was ε = 0.95.

• #### Technical notes

Generally, it is convenient to choose nails with matte finish. This helps us avoid the problems with different emissivity of wood and metal, which may lead to false reading of the nail temperature.

• #### Pedagogical notes

It is good to point out that in addition to the temperature increase in the place where the nail penetrates the wood, the temperature also increases along the nail inside the wood; however, it cannot be detected by the thermal imaging camera.