A particle with a mass of 1 g and a charge of 2.5 × 10^ -4 C is...

The electric force acting on the particle is given by $F = qE$.

Since the particle is released from rest, it moves in the direction of the electric field. The work done by the electric force as the particle travels a distance $d$ is $W = Fd = qEd$.

Substituting the given values:

$W = (2.5 \times 10^{-4} \text{ C}) \times (1.2 \times 10^4 \text{ N C}^{-1}) \times (0.4 \text{ m})$

$W = 3.0 \text{ N} \times 0.4 \text{ m} = 1.20 \text{ J}$