| | How does a battery operate in an electric circuit? We can find out simply by using the battery with a number of different resistive elements (resistors) having different resistances (Figure 1, left). It is found that the voltage across the battery drops with increasing current. The characteristic relation is close to linear (Figure 1, right). The voltage measured is close to the rated value of the battery (here: 4.5 V) if there is no current of charge flowing through the circuit. We get this if the circuit is open (or with a resistor having an extremely high resistance); therefore, this value is called the open circuit voltage of a battery. For very low external resistance, the current increases and the voltage drops. The voltage will reach a value of zero if we connect the terminals of the battery by a short, thick wire having hardly any resistance at all. The charge current associated with this point is called short-circuit current.
Interpretation A battery does not establish a fixed voltage, say 4.5 V. Therefore, we have to modify our understanding of a battery (see Figure 2). If we assume that the open circuit voltage is the voltage set up ideally by the chemical reactions, we can understand the decrease of the voltage with increasing current as the result of internal “losses.” Losses are the result of resistive behavior. We know that the electric potential drops across a resistor in the direction of the flow of charge. Therefore, the battery characteristic can be understood as resulting from the interplay of chemical reactions and the flow of charge through an internal resistor (the part of the circuit inside the dashed rectangle in Figure 2). Since the voltage drop across the resistor increases (linearly) with the current through the battery, we now understand the characteristic diagram (Figure 1). Apart from the shape of the curve, it is analogous to that of a water pump (Phenomenon 1.5). A real battery gets warm when operated. This agrees with the fact that resistive elements are heat producing. In this regard too, electrical resistive elements behave just like hydraulic ones (Chapter 1). | |
