Voltage

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The idea

Why does current flow in a circuit at all? The battery drives it, like a pump drives water around a loop. How strongly it drives is told by the voltage. Without a drive nothing flows, even when the circuit is closed. As with the water picture: the picture explains, but it has limits.

What to remember

Voltage: The voltage says how strongly the power source drives the current. Its symbol is $U$ and it is measured in volts (V).

Voltmeter: A voltmeter measures the voltage. You connect it in parallel: with its two terminals at the two ends of the part you want to measure.

Typical voltages: - AA battery: 1.5 V - flat battery: 4.5 V - car battery: 12 V - socket: 230 V

A rectangle of wires with a battery on the left and a lamp at the top. Next to the lamp a loop branches off with a circle with the letter V, connected at both ends of the lamp. — The voltmeter is **in parallel** with the lamp: it measures the voltage across the lamp. Labels: 1 battery, 2 lamp, 3 voltmeter.
Source: https://github.com/lernapps/elektrizitaet. Licence: CC0-1.0.

Even without current

There is voltage between the poles of a battery even when the circuit is open. But current only flows in a closed circuit.

Safety

Only experiment with batteries, never with the socket: 230 V can kill.

Example

Two AA batteries

A torch holds two AA batteries one behind the other. Each has 1.5 V, together they drive the current with 3 V. A voltmeter across both batteries shows 3 V.

Practice

Task no. 1

The battery has 9 V. Connect the voltmeter so that it shows 6.75 V.

The four points: 1 before the first resistor (10 Ω), 2 between the resistors, 3 after the second (30 Ω), 4 in the bottom wire.

Solution

The voltmeter at point 2 and point 3, that is, in parallel with the second resistor (30 Ω): 9 V · 30 Ω : (10 + 30) Ω = 6.75 V.

Exercise

Solution

In parallel with the lamp, with both terminals at the ends of the lamp. right answer
Voltage exists between two points, here the two ends of the lamp.
In series with the lamp, in the middle of the wire.
That is not how a voltmeter is connected. That is how an ammeter is connected.
Only to one pole of the battery.
Voltage is always measured between two points. With only one terminal there is nothing to measure.

A voltmeter is in parallel with the part whose voltage you want to measure.