Electrical Circuit
Electricity and Magnetism

Measuring voltages

Physics Narrative for 11-14 Supporting Physics Teaching

The instrument used to measure voltages is the voltmeter

Voltmeters are always connected across circuit components, whether a battery, bulb or some other device.

Voltmeters are connected differently to ammeters (see episode 01). With the ammeter, the instrument is connected into the circuit so that the flow of charge through the circuit can be measured.

Why are voltmeters connected across components?

Why are the voltmeter readings sometimes positive and sometimes negative?

To provide an answers to these questions, we need to be clear about what it is that the voltmeter is measuring.

In the circuit here, the battery is rated at 3 volt and therefore shifts 3 joule of energy as each coulomb of charge passes. What do these measurements mean?

Let's keep things simple: assume all of the energy which is shifted from the battery's store is shifted to the stores of the surroundings by the bulb. Therefore no energy is shifted to the surroundings by the action of the connecting wires. We're assuming that the connecting wires do not heat up at all as a result of the current in them: there is no electrical working in the wires. (In fact, the connecting wires must heat up a little, but the energy involved is negligible compared with that in the bulb).

There is electrical working in the battery and in the bulb. In the battery the action is to empty the chemical store, so the voltage is positive. In the bulb the action is to fill the surrounding thermal stores, so the voltage is negative. You can get the correct readings on the voltmeter by ensuring that the negative terminal is always upstream of the positive terminal, as defined by the current.

Any electrical circuit is a system linking the filling of a store in one place with emptying of a second store in another place. For example, as an electric train pulls away from a station a kinetic store is being filled, whilst (say) a coal-fired power station is burning fuel elsewhere and depleting a chemical store). (This is quite unlike a steam train where the depleted and filled stores are associated with objects at same location).

Making sense of voltmeter readings using a rope loop

The rope loop model, which emphasises electrical working, (so tying in well with the SPT: Energy topic) is the best way to get a for what is happening for flows to or from stores, as signposted by the voltage.

In the electrical circuit, the voltage sets the energy shifting per ampere.

In the rope loop, the force sets the energy shifting per metre / second of rope.

Both larger voltages and larger forces (grip or pull harder) lead to more energy shifting as the charge or rope flows past.

More grip results in a greater retarding force on the loop and so a larger negative voltage (sometimes called a voltage drop) across the resistor or lamp modelled.

More pull results in a greater driving force and so a models a larger voltage across the cell.

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