I feel stupid to ask but here goes.
So you can sit in a sauna where the thermometer measures the air at 80oc, and it’s fairly comfortable. But if you sit in water which a thermometer measures at 80oc, you die pretty instantly.
So I get that water is denser and so transmits heat faster – but then… what is the thermometer measuring if not heat transfer???
Tia
In: Physics
Thermometer measures objective temperature. It only measures heat transfer if you bring it to a hot sauna and keep a close eye on it to see how quickly it reaches the 80C mark.
Imagine taking a thermometer with you into both the sauna and the water. Suppose the air outside the water or sauna was 30 degrees celsius. The temperature indicated by the thermometer will slowly increase from 30 to 80, but it will increase much faster in the water. When you take the thermometer out, it will not snap back to 30 degrees, it will take some time.
It’s the same way when you are sick, and you put a thermometer under your tongue. You have to hold it there for several minutes until the thermometer stabilizes.
The temperature the thermometer measures is basically how fast molecules are vibrating. It takes time for the molecules in the thermometer to vibrate at the same speed as those of the surrounding air / water.
A thermometer measures its own temperature. Its own temperature would rise or lower to the temperature of the environment faster in water than in air.
Think of temperature like the height of a hill and heat transfer as your speed going down it.
The air and water are two hills that are both 80m tall. One is really wide (shallow slope) and you can walk down it slowly without much worry. The other is super steep and you will end up tumbling down and hurting yourself. An 80m hill has more potential to be “dangerous” than a 40m hill, but the steepness is what really matters.
The thermometer is measuring the height of the hill.
The thermometer is measuring the temperature, not heat transfer. How well heat is tranferring only affects how long it takes for the thermometer to register a change in temperature, not the stabilized reading. The mercury or alcohol expands in higher temperatures. It’s inert. Your body is not a thermometer. Your internal body temperature is regulated. In a sauna, your body sweats and that sweat evaporates off your skin which keeps your body cool. In a hot tub, you’re already in water. sweat cannot evaporate when it’s surrounded by water. And water transfers heat much faster than air. So your internal body temperature is going to go up more in water that is above your internal body temp than air.
Temperature is not the same as heat transfer.
Temperature is the average energy of the molecules. Heat transfer is the exchange of energy that causes changes in temperature.
Water is a very good conductor of heat, where as steam is not that good of a conductor. Try picking up a can of soda from the fridge and something in cardboard. They will feel drastically different because they transfer heat differently, but they have all been sitting in the fridge for hours reaching an equilibrium temperature of the fridge temperature.
A thermometer measures temperature, basically you take a good conductor(steel usually) and put it in a fluid you wish to measure, because it is a good conductor it will quickly exchange heat until they reach an equilibrium temperature. That temperature changes the electronics inside the thermometer(the resistance will change), and that can be translated into a temperature.
The difference between temperature and heat transfer is the reason we have weather indicators like windchill and heat index. Windchill and heat index are the metrics of what the heat transfer is like outside.
For example, if it’s 30 deg F outside and there’s a 10 mph wind, you’ll get colder much faster than if it is 30 deg F and no wind, because the wind blows away the hot air around your body and replaces it with colder air. The rate at which your body cools at 30 deg F with 10 mph wind is the same as if it were 21 deg F and no wind (meaning those two conditions have the same heat transfer). So we define a metric called “wind chill” to say that 30 deg F with 10mph wind “feels like” 21 deg F (and no wind) because those two conditions have the same heat transfer and therefore the same perceived feeling of “cold” to our body.
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