Internal Resistance. Part II

Internal Resistance. Part IISo we see that internal resistance is an inherent property of any circuit, where there is a current source and a voltage source. Such circuits are often referred to as two-terminal networks. This property is simply a parameter of an abstract two-terminal network, so there is no actual resistor inside the two-terminal network. However, in real galvanic cells internal resistance does exist. It equals to the total resistance of the plus bar, the case itself, as well as that of the electrolyte and hydrogen absorber (in salt cells).In other sources this function is performed by windings and contacts decreasing the characteristics of power sources.If it is possible to identify an input and/or an output in a system, then the following terms are used:- Input resistance is the internal resistance of a two-terminal network at the system's input.- Output resistance is the internal resistance of a two-terminal network at the system's output.The following peculiarities of internal resistance should be emphasized:- it is not possible to eliminate internal resistance from a two-terminal network;- internal resistance is not a stable parameter - its value is dynamic and can change whenever the external environment experiences some changes.The simplest and most common circuit for a two-terminal network consists of a generator and internal resistance.For example, it is impossible to directly change internal resistance by an ohmmeter, because we can not connect multimeter probes to the leads of internal resistance.However, we can use a simple measurement method that does not require additional calculations:- We first have to measure idle voltage;- Then connect a variable resistor as a load. Its resistance is chosen so as to ensure that the voltage on it is half the idle voltage.Now we measure the resulting resistance. This will in fact be the internal resistance of a two-terminal network.In most cases, it would be more logical to talk about considering the negative impact of internal resistance, rather than about applying such internal resistance, since the latter is more of a negative factor.Nevertheless, in some systems internal resistance with nominal rating is a must.

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Science & Technology

Tags:

• electrical engineering
• load
• physics
• resistance