- #1
gogos
- 9
- 2
Why does mercury in a thermometer expands when we increase the temperature but it doesn't go down after cooling? Why does it just stay there?
You can only shake the mercury off when you need to measure the temperature again. The time it contracts is very long i suppose since after a week it will still be roughly the same.Drakkith said:As far as I know it does contract. If it didn't, it would only show the highest temperature reached and would be of little to no use in a thermometer.
Can you explain what constriction?Nidum said:In a normal Mercury thermometer the mercury level changes rapidly with the bulb temperature .
I would guess that the time constant is of order of a few seconds for a small thermometer .
There is a type of mercury thermometer that is arranged to record the highest temperature reached . They have a constriction in the neck which slows the fall of the mercury as bulb temperature drops . Some clinical thermometers used to have this feature . Vigorous shaking helped the mercury to fall more quickly .
Nidum said:There is a type of mercury thermometer that is arranged to record the highest temperature reached . They have a constriction in the neck which slows the fall of the mercury as bulb temperature drops . Some clinical thermometers used to have this feature . Vigorous shaking helped the mercury to fall more quickly .
The narrow constriction in a 'Medical Thermometer' allows the column to break because the effect of cohesion of the mercury must be greater than the adhesion to the glass and over the tiny area of the constriction, the top and bottom regions of mercury pull together and away from the nearby glass. I imagine that the space inside the tube will be at low pressure so the two bits of the column will not be pushed together by atmospheric pressure.gogos said:Why does it detach?
Thanks for the answer. I am sorry that I didnt understand the first time. Its just because I am not well taught in the scientific language. I guess ill have to translate it to latvian. Thanks again really, this might earn me a 10/10 in physics.sophiecentaur said:You probably read the term "medical thermometer" and didn't go much further with my post but exactly the same thing is at work in regular 'liquid in glass' thermometers. The adhesion of mercury to glass is much lower than the
cohesion of mercury to itself; that's the comparison between the attraction the molecules have for each other. You can tell this when you look at the meniscus of the top of a mercury in glass column. It is highly convex, which shows the mercury is pulling itself together rather than wetting the glass - which is the opposite of what you get with water against the sides of a glass container (concave meniscus).sophiecentaur said:You probably read the term "medical thermometer" and didn't go much further with my post but exactly the same thing is at work in regular 'liquid in glass' thermometers. The adhesion of mercury to glass is much lower than the cohesion of mercury to itself; that's the comparison between the attraction the molecules have for each other. You can tell this when you look at the meniscus of the top of a mercury in glass column. It is highly convex, which shows the mercury is pulling itself together rather than wetting the glass - which is the opposite of what you get with water against the sides of a glass container (concave meniscus).
Why does it happen in a thermometer? To make a thermometer as sensitive as possible, you need a narrow bore so that the change in the column length is as great as practical for the size of the reservoir in the bulb. It is possible that the thermometer you are using is fairly sensitive (distance between 1° marks) and that will involve a thin bore. Any small disturbance can cause the column to split up in the above way. This 'flaw' is used to advantage in a clinical thermometer as it retains the highest temperature measured.
The other possibility is that your thermometer is not very good and may have small impurities along the column, where the mercury can separate. I have not actually come across this happening with a thermometer that's treated with 'respect' but it's always a potential problem.
They banned mercury for school thermometers years ago and the alternative alcohol thermometers (red column) are also susceptible to physical abuse. The lab techs often spend their time, after lessons, tap tapping the cheapo thermometers that have been dropped and bashed by the kids, to bring the sections of the alcohol columns back together.
Why does it happen in a thermometer? To make a thermometer as sensitive as possible, you need a narrow bore so that the change in the column length is as great as practical for the size of the reservoir in the bulb. It is possible that the thermometer you are using is fairly sensitive (distance between 1° marks) and that will involve a thin bore. Any small disturbance can cause the column to split up in the above way. This 'flaw' is used to advantage in a clinical thermometer as it retains the highest temperature measured.
The other possibility is that your thermometer is not very good and may have small impurities along the column, where the mercury can separate. I have not actually come across this happening with a thermometer that's treated with 'respect' but it's always a potential problem.
They banned mercury for school thermometers years ago and the alternative alcohol thermometers (red column) are also susceptible to physical abuse. The lab techs often spend their time, after lessons, tap tapping the cheapo thermometers that have been dropped and bashed by the kids, to bring the sections of the alcohol columns back together.
gogos said:Failed again, sheesh
I wanted to quote sophiecentaur but i quoted twice.Drakkith said:What do you mean?
Mercury is typically used as the measuring liquid in a thermometer because it has a high boiling point and expands evenly with changes in temperature.
A mercury thermometer works by using the principle of thermal expansion. As the temperature increases, the mercury inside the thermometer expands, causing the liquid to rise up the narrow tube and give a reading on the scale.
Mercury is a toxic substance and can be dangerous if ingested or inhaled. However, modern thermometers are designed with safety features to prevent any mercury from escaping. It is important to properly dispose of old or broken thermometers to avoid any potential harm.
Mercury has a very high coefficient of expansion, meaning it expands significantly with changes in temperature. This makes it ideal for use in thermometers, as small changes in temperature can be accurately measured. Additionally, mercury does not stick to the glass, allowing for consistent readings.
A mercury thermometer is typically used to measure temperatures between -38°C to 356°C. However, special types of mercury thermometers, such as the clinical thermometer, can measure temperatures as low as -20°C and as high as 110°C.