Heat and Hands

Rub your hands together- palm-to-palm, back and forth. What do you notice? You should feel your palms becoming warmer- if you don't you may need to seek medical assistance. This feeling can be ascribed to phenomena discussed in previous posts. Do you think this sensation of warmth (notice how I avoid the word "heat") is due to work or heat? You should be able to recognize that the internal energy of the atoms in your hands is increasing, but how is this increase happening?

temperature pre-rubbing

initiating rubbing

rubbing hands together

finishing rubbing

temperature post-rubbing

Friction is the mechanism that caused this increase in internal energy. It can be seen that the temperature of my palms increased from 23.0 to 23.4 °C, a 0.4 °C difference. To calculate the energy involved in this process, one must first know the specific heat capacity of skin. Specific heat capacity is the energy required per unit mass to raise the temperature of a substance by 1 K or 1 °C. The specific heat of skin is  3.47 kJ/(kg*°C) (source).

Now we need to approximate the mass of the skin involved in the friction force. Knowing that the mass of human skin is 4.1 kg (source: yes, Wolfram Alpha is capable of computing the mass of human skin, don't ask me how) we can reason that skin on our hands is a small fraction of this total mass. Lets say that it is 1/1000, meaning that that the mass of the skin on our hands involved in the process is 4.1g.

We now know all the relevant values required to calculate the energy involved in the interaction:
change in temperature: 0.4 °C
mass: .0041 kg
specific heat capacity: 3.47 kJ/(kg*°C)

The energy transferred is:
mass x change in temperature x  specific heat capacity = (0.0041 kg)*(0.4 °C)*(3.47 kJ/(kg*°C)) = 0.0057 kJ

Our units of mass and temperature cancel appropriately, leaving units of energy kJ (kilojoules). 5.7 joules of energy are therefore transferred when rubbing your hands together.

To try to make sense of this quantity, let's convert it to energy in the form of dietary calories. 5.7 J = 0.00136 Cal (source). Given that your average apple contains 91 dietary calories (source), you could rub your hands together another 67,000 times (91/0.00136). This is assuming your body and digestive system are 100% efficient, which they aren't, but lets let the biophysicists deal with that.