((d )/dt) ∫_( V^( 3) ) ρ_q (r,t)dV=−∮_( ∂V) J_q (r,t)∙da+∫_( V^( 3) ) S_q (r,t)dV
∫_( V^( 3) ) ((∂ρ_q (r,t))/∂t) dV=−∫_V^( 3) ▽^→ ∙J_q (r,t)dV+∫_( V^( 3) ) S_q (r,t)dV
∫_( V^( 3) ) [ (∂ρ/∂t)+▽^→ ∙J^ (r,t)−S(r,t)]dV=0
∴((∂ρ_q (r,t))/∂t)+▽^→ ∙J_q (r,t)=S_q (r,t)
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