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Internal Resistance

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**1 volt is 1 joule per coulomb**

Batteries and Cells Cells and batteries come in all shapes and sizes. They all have their ‘voltage’ marked on the side. This is called the e.m.f. ( E ) of the battery or cell. e.m.f. stands for electromotive force. If a voltmeter is connected to the terminals of a cell, it will measure the e.m.f. V e.m.f. = 1.5 V The e.m.f. tells you how many joules of energy the battery will supply to every coulomb of charge which passes through it. 1 volt is 1 joule per coulomb

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Internal Resistance Unfortunately, cells and batteries ( and all other voltage supplies ) also have INTERNAL RESISTANCE. It is a nuisance. We use the symbol r to denote internal resistance and it is often drawn as a small resistor inside the voltage supply. V e.m.f. = 1.5 V r E The problem with internal resistance is that it causes the voltage supplied by the supply to drop when you start to use it to supply anything with current.

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Lost Volts We call this drop in the voltage of the cell the LOST VOLTS. Remember that it only occurs when a current starts to flow. V 1.5 V r E load resistor A

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Lost Volts We call this drop in the voltage of the cell the LOST VOLTS. Remember that it only occurs when a current starts to flow. V When the switch is closed …. 1.5 V r E …the current starts to flow … load resistor A 0.2 A

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Lost Volts We call this drop in the voltage of the cell the LOST VOLTS. Remember that it only occurs when a current starts to flow. V When the switch is closed …. t.p.d. = 1.2 V r E …the current starts to flow … …and the battery voltage drops. load resistor A 0.2 A The voltmeter reading is no longer the e.m.f. It is now called the t.p.d. This stands for terminal potential difference. The lost volts = 0.3 V

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**The Terminal Potential Difference**

This is not always the same. It depends on the size of the lost volts which, in turn, depends on the current in the circuit. 1.2 V r E R 0.2 A 1.2 V The voltage across the load resistor ( R ) will also be equal to the t.p.d.

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**The Equations E = t.p.d. + lost volts**

Ohm’s law (V = I x R) can be applied in three ways. For the whole circuit E = I x RT For the Internal Resistance lost volts = I x r For the Load Resistor t.p.d. = I x R

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