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Circuits: A battery, wires, a light bulb, and ammeters

A small incandescent bulb (or a small motor) is connected to a battery. This is done with two wires. The first goes from the positive terminal of the battery to the lamp, the second goes from the lamp to the negative terminal (Figure 1). It is important that the electric system has a closed circuit. If it is open at a point, if one of the wires is not connected, the lamp will not burn.

We can place one or two ammeters into the circuit. Basically, a wire could be cut in half, and the ends can be connected to the meter. It is observed that two meters, one in the upper part of the circuit in Figure 1 (center) and one in the lower part, show the same readings. The readings are zero if the circuit is not closed.

Interpretation

Batteries or power supplies serve as pumps for electric charge. Here they pump charge through an electric circuit made of the battery, wires, and a lamp. The ammeters measure the flow of electricity, i.e., they quantify charge currents. Since the charge currents are equal in the two branches of the circuit, the charge flowing away from the battery returns to the battery (since there is positive and negative charge, currents of positive charge are those coming from the positive terminal of the battery). The lamp does not use up electricity, just as little as a turbine uses up water. Clearly, in interpreting electric systems and processes, we can make use of a strong analogy with hydraulic circuits and processes (Figure 1, right).