lecture 9

lecture 9 - 20.9 Internal Resistance So far we've just...

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20.9 Internal Resistance So far we’ve just considered batteries and generators as contributing their emf to a circuit. In reality, they too have some resistance. This is called internal resistance, r . In batteries it’s due to the chemicals, and in generators it’s due to wire resistance. So, if a battery is connected to a load resistor, R , then the internal resistance, r , is in series with the load: Thus, the voltage across the ,g battery (known as the terminal voltage, V T ) is less than the full voltage V , since some is lost across r . otice: nd re in series! Thus, r T V V V = Notice: R and r are in series! V T Terminal voltage Battery voltage Voltage drop across the internal resistance, r

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Kirchhoff’s Rules In many circuits, applying the series or parallel methods is not sufficient to analyze them. There are two other rules we can use called Kirchhoff’s Rules : unction Rule urrent into a junction has to equal current out 1. Junction Rule Current into a junction has to equal current out. It is based on conservation of charge. I flows into junction, and I and I flow out, thus: 1 2 I 2 + I I 1 2 1 2. Loop Rule – Around any closed circuit loop, the sum of the potential (voltage) drops has to equal the sum of the potential rises. It is based on conservation of energy.
Here’s an example with the loop rule. We have a closed circuit loop with multiple batteries. What is the current in the circuit? 1. Choose the direction of the current(s) in each loop. Solution 2. Label the resistors from + to – in the direction of the current flow. Start at point A and go around the loop clockwise, and make a list of the potential drops and rises as we go all the way around.

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lecture 9 - 20.9 Internal Resistance So far we've just...

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