English
Menu

Thermal properties of materials

A metal plate and a wooden board are left long enough at the same room temperature (about 20℃). When touched by hand, the metal feels colder. What is the main reason?

  1. AMetal's specific heat per unit mass far exceeds wood's, so it draws a lot of heat from the hand and stores it
  2. BMetal has a far larger effusivity √(κρc) than wood, so the contact surface ends up near the metal's temperature
  3. CEven at room temperature, heat escapes easily from the metal surface, so it is colder than the surface of the wood
Show answer and explanation

Answer: B. Metal has a far larger effusivity √(κρc) than wood, so the contact surface ends up near the metal's temperature

Right after two bodies at different temperatures touch, the temperature of the contact surface is an average weighted by their thermal effusivities e = √(κρc). Metal has a thermal conductivity hundreds of times or more that of wood and a far larger effusivity, so the contact surface drops to near 20℃, close to the metal's temperature, heat is drawn quickly from the skin, and the metal feels cold. With wood, the contact surface stays close to body temperature. Dry wood (about 1.7 J/(g·K)) actually has a larger specific heat per unit mass than aluminum (about 0.9 J/(g·K)), so specific heat cannot explain it. Left long enough at the same room temperature, both surfaces are at about room temperature, with no difference.

Report an error

Practice this topic Back to topic

More Thermal properties of materials questions

  1. In the laser flash method, a pulse of light was applied to the front of a disk of thickness L = 2.0 mm, and the time for the temperature rise of the back face to reach half its maximum was t½ = 0.050 s. Which value is closest to the thermal diffusivity found with Parker's equation α = 0.1388L²/t½, which neglects heat loss?
  2. What is the main cause of solids expanding when heated (thermal expansion)?
  3. Which ordering of linear thermal expansion coefficients near room temperature, from largest to smallest, is correct?
  4. Which statement correctly explains why the linear thermal expansion coefficient of Fe-36%Ni alloy (Invar) is very small near room temperature, about 1×10⁻⁶ K⁻¹?
  5. According to the Grüneisen relation, the volume expansion coefficient of a solid is roughly proportional to its constant-volume heat capacity. Which statement correctly describes the linear expansion coefficient of an insulator at low temperatures near absolute zero?