1. 18.5 Resistance of ammeters, voltmeters and cells Effects of resistance of ammeters and voltmeters I An ideal ammeter should have zero resistance The current I is not affected.

2. 18.5 Resistance of ammeters, voltmeters and cells An ideal voltmeter should have infinitely high resistance No current is drawn.

3. 18.5 Resistance of ammeters, voltmeters and cells  Ammeters and voltmeters always have finite non-zero resistance.  It affects the measurement of the resistance of an object.

4. 18.5 Resistance of ammeters, voltmeters and cells R IVIV IRIR The resistance of the voltmeter is not infinitely high. I V flows through the voltmeter. Current measured = I R + I V > true value I R V measured is true  Resistance calculated < true value R V = I R + I V

5. 18.5 Resistance of ammeters, voltmeters and cells I R I measured is true VAVA VRVR The resistance of the ammeter is not zero. V A across the ammeter Voltage measured = V R + V A > true value V R  Resistance calculated > true value R V R + V A = I

6. 18.5 Resistance of ammeters, voltmeters and cells Effect of resistance of cells  Battery and cells that we use have internal resistance. R I V VrVr r VLVL V = V r + V L V = Ir + V L V L = V – Ir V – voltage of the cell V r – voltage across the internal resistance V L – voltage across the load r – internal resistance

7. 18.5 Resistance of ammeters, voltmeters and cells R VrVr V I r VLVL greater V r smaller V L larger I V L = V – Ir Voltage across the load V L Voltage of the cell V <

8. 18.5 Resistance of ammeters, voltmeters and cells That’s the end of Section 18.5 Check Point Key Ideas Previous Page Exit