# Homework Help: How stirring a liquid affects temperature measurements.

1. Nov 29, 2011

### dJordiB

1. The problem statement, all variables and given/known data
"Some people say stirring a hot liquid with a thermometer gives a better measurement and some say it yields a worse measurement. Explain with physics how one is actually better"

2. Relevant equations
None that I know of.

3. The attempt at a solution
I know that stirring a liquid generally cools it, but I was also thinking that the increased kinetic energy due to the stirring may cause an increase in temperature. Also, stirring would evenly distribute the liquid so that the average temperature would be more measurable, but if it changes the temperature through the previously stated ways, then wouldn't it be worse?

2. Nov 30, 2011

### bbbeard

Gee, my immediate reaction is that stirring gives you a more accurate measurement, but that's because I've had a couple of heat transfer courses. If you don't stir, the heat transfer is limited by natural convection, that is, a film of cool liquid forms around the end of the thermometer and gives an artificially low reading. Stirring disrupts the film and provides much better "forced convection" heat transfer. It exposes the tip of the thermometer more directly to the bulk fluid. I'm not sure what principles of physics you could quote for that, although I have some textbooks with a number of methods for calculating film coefficients of heat transfer under natural and forced convection....

BBB

3. Nov 30, 2011

### dJordiB

That's great thank you so much! I was leaning toward that answer but didn't have much of a justification.