Current, voltage and measuring instruments

The same charge per second may pass through two devices, yet one receives more energy. How can we distinguish the amount of charge passing from the work per coulomb, and how do we measure each?

Imagine a counter for charge passing through a wire. To measure the lamp current I in amperes, the same flow must pass through the counter and the lamp. We therefore insert an ammeter into a break in that branch, in series with the lamp. If there are no branches between them and the current is steady, charge does not accumulate: equal amounts pass both cross-sections in the same time. A parallel connection would create another path and measure the current in that path.

Equal currents do not tell us how much energy a section receives. The electric field does work A as charge q moves between its ends. To compare sections independently of how many coulombs pass, we find the work per unit charge. This quantity is voltage U. It refers to a pair of points: saying 'voltage at a point' leaves out the second, reference point. Work A is measured in joules.

Divide the total work by the transferred charge: U = A/q. In simple calculations we use consistent positive magnitudes; 1 В = 1 Дж/Кл. At U = 6 В, the field does 6 J of work per coulomb. Transferring twice the charge at the same U doubles the work but leaves the ratio unchanged. Zero work for a nonzero transferred charge means zero voltage. More coulombs alone do not increase voltage.

A voltmeter compares the two ends of a lamp, so its wires connect to both terminals, in parallel. As little extra current as possible should flow through it, to avoid changing the circuit being studied. It therefore strongly opposes current: we say it has a high resistance. In the ideal model its resistance is infinite and no current flows through the meter; a real voltmeter only approaches this condition.

An ammeter has the opposite task: inserted into the shared path, it should hardly obstruct the current. Its resistance is therefore low. Connecting it directly across a source creates an easy bypass without the intended load, and the current may become dangerously large. The connections differ because the ammeter carries the flow being measured, whereas the voltmeter compares two ends while drawing as little current as possible.

Before switching on a classroom circuit, check that you have selected the correct mode, current or voltage, and a range that can handle the expected value. For direct current, observe polarity; on an analogue meter, check zero and the scale interval. The wrong mode can change the circuit itself and damage the meter. Assemble connections with the low-voltage classroom source switched off, then check the circuit before measuring.

The field does 24 J of work as 4 C passes through a lamp. Find the voltage across the lamp.

We need the field's work per coulomb between the lamp terminals, so U = A/q. The stated 24 J is the work for transferring 4 C.

U = 24/4 = 6 Дж/Кл = 6 В. Check the meaning: four one-coulomb portions correspond to four portions of 6 J of work.

To measure this 6 V, connect the voltmeter to both lamp terminals, in parallel with it. A and q alone, without time, do not determine the current.

The voltage is 6 V: for each coulomb passing through the lamp, the field does 6 J of work.

Current tells us how many coulombs pass each second; voltage tells us how many joules of work correspond to each coulomb.

An ammeter needs the same charge path; a voltmeter needs the two ends of the section being compared.

The correct mode and connection preserve the meaning of the measurement and must be checked before the circuit is switched on.

The ratio of voltage to current gives the section's resistance. Next we will test Ohm's law.