Expert: Bobby Soltani - 2/6/2005

Question
A circuit was set up to investigate the current through a 15 ohm resistor with applied pd .a 6 volt supply  with   negligible resistance was used  . A rheostat  was used which  had a maximum resistance of 45 ohms . what would be  the maximum  and minimum reading at which the  pd across the resistor could be set .


Ok this question has  a diagram but of course I cant send it  . ill describe it
The circuit is set  up with the variable resistor used as  a rheostat

To get a  full variation of voltage from 0v to 6v  with this apparatus could I use the variable resistor as  a potential divider instead  ????


Answer
Hi Andrew,

I'm having a little trouble understanding exactly what you are trying to describe.  If you have a 6V power supply with a 15 ohm resistor in series with a rheostat, then the total resistance can vary between 15 ohms and 15 + 45 = 60 ohms.  So, if the resistor is on the positive side, the voltage at the divider point will be zero is the rheostat is at 0 ohms.  The voltage will be at 6*45/60 = 4.5 volts when the rheostat is at 45 ohms.  You can figure out using the same method what the voltage would be if the rheostat is placed on the positive terminal( it would vary between 1.5 and 6 V).  Also, in a series circuit, the current will be the same through both resistors, I = V/(total resistance), V = 6V.

I hope this helps your understanding.  If you have any questions, or if I didn't understand you question, please let me know.  Good luck.

Bobby