In a direct current circuit, the current changes direction only when the circuit is switched on or off.

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In direct current, the flow is set by the polarity of the energy source and tends to stay in that single direction while the circuit remains unchanged. When you switch the circuit on, current starts to flow in whatever direction the source dictates; turning the circuit off stops the flow but does not reverse its direction. A reversal only happens if you reverse the source’s polarity or reconfigure the circuit so that opposing forces drive current in the opposite direction. So the statement isn’t generally true: current in a DC circuit does not change direction merely by turning the circuit on or off.