A small decorative 50 mW LED lamp is connected to a 3.0 V DC button cell. The circuit is left open, with a 2.0 cm gap between two exposed copper terminals inside a terracotta diya. The LED is mounted at the top of the diya, while the button cell is hidden at the bottom.
Person X proposes pouring ordinary tap water into the diya so that the water bridges the gap between the two terminals. He says that distilled water is generally considered safe but is of no use here.
Person Y says tap water causes a short circuit. So, pouring it would burn the circuit.
Who is correct?
Answer
Usually, it is considered that water and electricity do not play well together, and water is a no-no as far as electricity is concerned. This perception has a lot to do with high voltages, including domestic supply voltages, and the dangers posed by water during the rainy season or in bathrooms. In this problem, however, we are dealing with a low-voltage, small-current situation.
Tap water is not a short circuit in the usual electrical sense. It contains dissolved minerals and ions, so it conducts electricity, but it also has significant electrical resistance.
With the assumed 2.0 cm electrode gap, the tap water can provide a conductive path and may allow enough current for the LED to glow. Distilled water has much higher resistance, so it may conduct too little current for the LED to illuminate noticeably.
A short circuit would be a very low-resistance path, such as the two copper terminals being directly connected by a wire. Tap water is not equivalent to that.
Person X is correct.