6

Voltages in a closed circuit

Create a circuit made of a battery or power supply and two lamps (or other resistive elements, or electric motors). The devices will be connected in series forming a single electric circuit (Figure 1, left). Use three volt meters to measure the potential difference across each of the three elements.

It is observed that the sum of the voltages (potential differences) across the lamps (or the other devices) equals the voltage across the battery (or power supply). Alternatively, if we take into consideration the signs of the voltages, we see that the sum of the three voltages equals zero.

Interpretation

Voltages are interpreted as electric potential differences and the electric potential as a kind of electric level comparable to fluid pressure in hydraulics. We can continue to make use of the analogy between electricity and hydraulics and view the electric potential along a circuit as a level in a landscape (Figure 1, right). Going around a circuit we go up and down in this landscape. When we are back at the starting point, we have gone up as far as we have come down. The level differences (voltages) must add up to zero, just as pressure differences in a closed hydraulic circuit add up to zero (Chapter 1, Phenomenon 1.3).

If we orient ourselves according to the direction of flow of electric charge in the simple circuit of Figure 1, the potential goes up when we go through the battery from A to B. In resistive elements or motors, i.e., from C to D and from E to F, the potential goes down. Wires in circuits are typically modeled as ideal conductors where the potential does not change (there is no voltage along an ideal wire).