Fundamental Electrical and Electronic Principles, Third Edition

14 Fundamental Electrical and Electronic PrinciplesWorked Example 1.8QIf a p.d. of 24 V exists across a 15 Ω resistor then what must be the current flowing through it?AV 2 4 V; R 15 ΩVI Ramp2415I 1. 6 A AnsInternal Resistance (r) So far we have considered that the emf E voltsof a source is available at its terminals when supplying current to a circuit.If this were so then we would have an ideal source of emf. Unfortunatelythis is not the case in practice. This is due to the internal resistance of thesource. As an example consider a typical 12 V car battery. This consists ofa number of oppositely charged plates, appropriately interconnected to theterminals, immersed in anelectrolyte . The plates themselves, the internalconnections and the electrolyte all combine to produce a small but finiteresistance, and it is this that forms the battery internal resistance.An electrolyte is the chemical ‘ cocktail ’ in which the plates are immersed. In the case ofa car battery, this is an acid/water mixture.In this context, finite simply means measurable.Figure 1.9 shows such a battery with its terminals on open circuit (noexternal circuit connected). Since the circuit is incomplete no current canflow. Thus there will be no p.d. developed across the battery ’ s internalresistance r . Since the term p.d. quite literally means a difference inpotential between the two ends of r , then the terminal A must be at apotential of 12 V, and terminal B must be at a potential of 0 V. Hence, underthese conditions, the full emf 12 V is available at the battery terminals.Figure 1.10 shows an external circuit, in the form of a 2 Ω resistor,connected across the terminals. Since we now have a complete circuitAr0.1 ΩE12 VBFig. 1.9