Heat flow between two blocks

Touch a hot block to a cold one and predict the final temperature.

Heat flows from hot to cold

Interactive

A90°C

B20°C

heat moved0kJ

final temperature26.8°C

Block A material

Block B material

A's mass1 kgB's mass1 kgA's starting temperature90 °CB's starting temperature20 °C

The math, with your numbers

Q = m c\,\Delta T \qquad C_A = 1 \times 450 = 450\ \text{J/K} \qquad C_B = 1 \times 4186 = 4186\ \text{J/K}

T_f = \frac{C_A T_A + C_B T_B}{C_A + C_B} = \frac{450 \times 90 + 4186 \times 20}{4636} = 26.8\ ^\circ\text{C} \qquad Q = 28.4\ \text{kJ}

Heat lost by the hot block = heat gained by the cold one. The one with the bigger heat capacity (m·c) changes temperature less.

Try this

→Put 1 kg of iron at 90 °C against 1 kg of water at 20 °C. Why does the final temperature end up so close to 20 °C?
→Make both blocks the same material and mass. Where does the final temperature land?
→Can you make heat flow from the colder block to the hotter one?

Want to know why it behaves like this?

The lesson “Heat Flow & Equilibrium” explains it step by step: Why does heat always flow from hot to cold?