Question

How many kg of coal are used to supply 1500 Watts to heat a barn for 7 hours? The
energy content of coal is 19 x 106 J/kg.

Answer 1

Answer on Question #40872, Physics, Molecular Physics | Thermodynamics

How many kg of coal are used to supply 1500 Watts to heat a barn for 7 hours? The energy content of coal is $19 \times 10^{6} \, \text{J/kg}$.

Solution:

The heat of combustion $\Delta H$ is the energy released as heat when a compound undergoes complete combustion

$$\Delta H = q m$$

where $q = 19 \times 10^{6} \, \text{J/kg}$ is the energy content of coal.

The power is

$$P = \frac{\Delta H}{t} = \frac{q m}{t}$$

Thus,

$$m = \frac{P t}{q} = \frac{(1500 \, \text{W}) \cdot (7 \cdot 3600 \, \text{s})}{(19 \cdot 10^{6} \, \text{J/kg})} = 1.99 \approx 2 \, \text{kg}$$

Answer. 2 kg.