physics potentiometers

A student connects a potentiometer across a 6V supply and adjusts it to give an output voltage measured with a digital voltmeter, of 1.5V. He then connects a 1.4 V torch bulb across the output and is very surpised when it only glows dimly. Explain why this happens.

HELP!

Reply 1

elpaw

the bulb will have a relatively small resistance (unlike the DMM which has a high resistance), and will be in parallel with the potentiometer resistance. and so the combined resistance will be lower, and therefore there will be a lower output voltage

Reply 2

ryan750

elpaw

is that because when you have two resisitors in parallel the total resistance is lower than the individual resistances and because V = IR - the lower the resistance - the lower the output voltage will be. I was revising this today and found it confusing. Another thing i find really confusing is which equations to use for total power output - or power dissipated? :confused:

Reply 3

john !!

P = IV = RI^2

and yes you are right about the others, two resistors in parallel will give the electrons more ways to travel round the circuit so the combined resistance is lower... think of it as a river, if you have two paths for the water to go the water can go faster.