Question

f a cup of coffee is at 90°C and a person with a body temperature of 36°C touches it, how will heat flow between them?

from the hand to the cup
from the cup to the hand
from the cup to the surrounding air
from the hand to surrounding air

Answer 1

Heat flows spontaneously from the hotter object to the colder one.

- From the cup to the hand: yes. (90 °C → 36 °C) — heat conduction at the contact.
- From the cup to the surrounding air: yes, if the air is cooler than 90 °C (typical). (90 °C → air) — by convection and radiation.
- From the hand to the surrounding air: typically yes, if the air is cooler than 36 °C (36 °C → air) — by convection and radiation. If the air is warmer than the hand, heat would flow from the air to the hand instead.

So under normal conditions (room temperature < 36 °C), heat flows out of the cup to both the hand and the air, and out of the hand to the air as well.