How to play Voltmeter Walk
- Choose a test and read the circuit and where the probes go on the card.
- Follow the potential round the loop: up through cells, down I·R through resistors, level along wires.
- Type your prediction and press Start. The probes walk onto their points and the meter reads red minus black.
- Compare the reading and the landscape with your orange prediction.
Voltage is between two points
Every point in a circuit has a potential, and a voltage is always the difference between two of them. That is why a voltmeter has two probes: it reads the potential at its red probe minus the potential at its black probe, V = V_red − V_black, so swapping the probes flips the sign. Going round a circuit is like walking a landscape: a cell is a ramp that lifts you by its voltage, every resistor or bulb is a ramp down by I·R, and a wire is level ground, because it has no resistance. However you walk, you come back to where you started, so the ups and downs cancel. An open switch stops the current; with no current nothing drops in the bulb, so the whole voltage of the cell appears across the gap. Cells joined + to + push against each other, and the smaller one takes its voltage away from the larger.
A focused learning model
Cells, wires, and the voltmeter are ideal: the cells keep their voltage at any current, the wires have no resistance, and the meter takes no current, so measuring changes nothing. Resistors and the bulb are fixed resistances, and the current is steady at once. The potential is 0 V at the battery's − end; it rises through each cell, falls by I·R through each resistor, and an open switch takes up whatever is left. The probes walk onto their points over 2 s and the meter reads once both touch. The electron dots move at a speed in step with the current, not at the real drift speed. Answers within 2% are right; a reading of 0 must be within 0.05 V.