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(B) The back $e.m.f.$ $(E_b)$ in a $dc$ motor is given by the relation $E_b = K\omega$, where $K$ is a constant and $\omega$ is the angular speed of t

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Decreases

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(B) The back $e.m.f.$ $(E_b)$ in a $dc$ motor is given by the relation $E_b = K\omega$, where $K$ is a constant and $\omega$ is the angular speed of the armature.The armature current $I_a$ is given by the formula $I_a = \frac{V - E_b}{R}$, where $V$ is the supply voltage and $R$ is the armature resistance.As the speed $\omega$ increases, the back $e.m.f.$ $E_b$ also increases.Since $I_a = \frac{V - K\omega}{R}$, an increase in $\omega$ results in a decrease in the numerator $(V - K\omega)$.Therefore, the armature current $I_a$ decreases as the speed of the motor increases.

When the speed of a $dc$ motor increases, the armature current:

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