Operating at 40 kV , an X-ray tube has an electric current of 10 mA...

The total electrical power supplied to the X-ray tube is given by $P = VI$.

Substituting the given values:

$P = 40 \times 10^3 \times 10 \times 10^{-3} = 400 \text{ W} = 400 \text{ J/s}$

This represents the total kinetic energy of the electrons striking the target per second.

Since $1\%$ of the total kinetic energy is converted into X-rays, the remaining $99\%$ is converted into heat.

Heat produced per second $= \dfrac{99}{100} \times 400 = 396 \text{ J}$

Thus, the amount of heat produced in the target every second is $396 \text{ J}$.