# Q & A: thermal equilibrium and thermometers

Q:
i understand that if two objects A and B are in thermal equilibrium with a third object C they are in thermal equilibrium with one another aswell what if the third object is a thermometer.we use it measure the temperature of A so its in thermal equilibrium with A and then do the same with B.what if we get different temperatures?then A and B will both seperately be in thermal equilibrium with the thermometer but wont be in thermal equilibrium with each other.isnt that going agaisnt the law? im having trouble understanding this bit any help clearing this would be much appreciated!!
- Misbah (age 18)
pakistan
A:

I think that the confusion here comes from the thermometer being iin equilibrium with A and B at different times. If one part of the thermometer contacted A and another part contacted B, heat could flow through  the thermometer until A, B, and the thermometer all reached the same temperature. That would be the temperature you would read on the thermometer.

If the thermometer first contacts A, it will reach equilibrium with A and tell you that equilibrium temperatre. Now you remove it from A and let it contact B. It's out of equilibrium for a while. Then it settles in to the new equlilibrium with B, at a new temperature.

Mike W.

p.s. Sorry about the delay in answering. Our computer system occasionally doesn't save answers correctly, and it took a while to realize that we had a blank answer in our file..

(published on 02/28/2014)